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Battery Pack Design

Battery Pack Design
The purpose of a battery pack is to provide a convenient integrated power source for portable applications. The advantages of using custom designs are outlined in the section on Benefits of Custom Packs. The pack may fulfil several functions:-
It enables higher voltage or higher capacity batteries to be built up from low voltage, low capacity cells.
It houses a cell or a bank of cells together with the associated interconnections in a single convenient pack.
It accommodates any necessary monitoring and electronic protection devices or circuits within the pack.
It can accommodate additional circuitry such as indicator lights, heaters, cooling ducts and solar panels.
It matches and meshes with the cavity in the product which the battery is intended to power providing both electrical and mechanical interfaces.
It can provide unique electrical and mechanical interfaces to ensure compatibility both of the battery with the intended product and the charger with the battery.
Two examples of battery packs from Axeon Power are shown below.
The pack on the left is a 12 Volt 30 Ah Lithium Ion battery used for seismic instrumentation. It uses 32 pouch cells in a 4 series, 8 parallel configuration. The pack incorporates heaters which enable it to work down to -30°C and a solar panel which keeps the battery charged.
The pack on the right is a 3.6 Volt 800 mAh battery employing three Nickel Metal Hydride cells used in mobile phones. The gold plated connector is moulded into the plastic frame.

See also Cell Construction
Capacity and Voltage
With a simple series chain of cells, the battery capacity in AmpHours is the same as the capacity of the individual cells since the current flows equally through all the cells in the chain.
High battery voltages are achieved by adding more cells in a series chain. The voltage of the battery is the voltage of a single cell multiplied by the number of cells in the chain. This does not increase the AmpHour capacity of the battery , but it increases the WattHour capacity, or the total stored energy, in proportion to the number of cells in the chain.
Battery capacity can increased through adding more parallel cells. This increases the AmpHour capacity as well as the WattHour capacity without increasing the battery voltage. For batteries with parallel chains the capacity of the battery is the capacity of the individual chain multiplied by the number of parallel chains.
Whereas cell voltage is fixed by the cell chemistry, cell capacity depends on the surface area of the electrodes and the volume of the electrolyte, - that is, the physical size of the cell. If at all possible the number of cells in a pack should be minimised to simplify the design and to minimise potential reliability problems. Fewer cells require fewer support electronics. Thus parallel chains should be avoided by specifying the highest capacity cells available. Design issues for multi-cell batteries are considered further in the section on Cell Balancing.
NOTE Cells with different capacities or cell chemistries should not be mixed in a single battery pack.
Pack Design Options
The design of the outer package or housing of the battery depends to a great extent on the components it has to accommodate and the physical protection it has to provide for them. These components are not just the cells, but also protection devices, electronic circuits, interconnections and connectors which must all be specified before the final battery case can be designed. For high power, high energy batteries robust packaging is required for safety reasons.
Cell Configuration
The ultimate shape and dimensions of the battery pack are mostly governed by the cavity which is planned to house it within the intended application. This in turn dictates the possible cell sizes and layouts which can be used. Prismatic cells provide the best space utilization, however cylindrical cells provide simpler cooling options for high power batteries. The use of pouch cells provides the product designer more freedom in specifying the shape of the battery cavity permitting very compact designs.
The orientation of the cells is designed to minimise the interconnections between t he cells.
Battery Electronics
Besides the cells many battery packs now incorporate associated electronic circuits. These may be protection devices and circuits, monitoring circuits, charge controllers, fuel gauges, and indicator lights. Electronics for high power multi-cell packs also include cell balancing and communications functions.
The packs may also be designed to deliver more than one voltage from the basic cell combination, although applications requiring multiple voltage sources are more likely to make provision for this within the application. See Multiple Voltages
In addition to the basic battery support electronics the battery pack may include other functions such as heaters to extend the lower working temperature or solar cells to keep the battery fully charged. These circuits in turn have their own control circuits.
Space, fixing points and methods and interconnections need to be allocated for all these electronic circuits.
Sofware is a major component of Lithium batteries, particularly for automotive applications. See the section on Battery Management Systems (BMS) . Control systems are required to keep the cells within their specified operating range and to protect them from abuse. Fuel gauging needs complex algorithms to estimate the state of charge (SOC). Communications with other vehicle systems are needed for monitoring the battery status and controlling energy flows.
Internal Interconnections
Low power cells are usually connected together using nickel strips which are welded to the cell terminals or the case. Soldering is not recommended since the soldering process is apt to apply large, uncontrolled amounts of heat to the battery components which may damage the separators or the vents which are normally made of plastic. Modern computer controlled resistance welders allow much more precise control of the welding process, both limiting the amount of heat applied to the battery and localising the heat to a small desired area. Welding also provides a stronger, low resistance joint. The interconnecting strips often have complex shapes and profiles which may be stamped out of flat strip in a progressive die.
High power cells may use solid copper bus bars or braided straps.
The electronic components are usually mounted on a conventional printed circuit board (PCB).
Flexible PCBs may cost more than rigid PCBs but they can be used to reduce the overall product costs. Not only do they save weight and space but they also provide more packaging options and they simplify physical interconnections and assembly operations as well as eliminating the need for connectors. Connectors may in fact be specified to facilitate assembly and disassembly if the design requires that individual battery components need to be changed or serviced but there is usually a cost and reliability penalty associated with such designs.
External Connections
The type of terminals or connections to the external circuits depend on, the current to be carried, the frequency with which the battery may be connected and disconnected and the design of the design of the circuit to which the battery will be connected.
For low power circuits, gold plated contacts are the terminals of choice for connectors which are subject to frequent insertions. Gold is hard wearing, it has low contact resistance and doesn't oxidise. Flying leads with spade terminals or snap on studs are also used for low power applications. Metal tabs are also used on pouch cells.
Terminals for high power applications are usually threaded metal studs to ensure a reliable connection. Safety requirements on high voltage batteries may also dictate shrouded terminals to prevent accidental exposure of the operator to dangerous voltages or of the battery to short circuits. Keyed or terminals or connections are also advisable to prevent connection to incorrect chargers or loads.
Thermal Design
Thermal management is a major issue in high power designs, particularly for automotive applications. See details in the Thermal Management section. As part of the battery system, it may be necessary to provide air or water cooling ducts, pumps or fans and heat exchangers for high temperature working or heaters for operating in low temperature environments. The layout of the cells should be conducive to managing heat flows within the pack.
Battery Packaging
The battery casing has to provide the mechanical and electrical interfaces to the product it is designed to power as well as to contain all the components outlined above.
The simplest and least expensive packaging for small batteries is shrink wrap or vacuum formed plastic. These solutions are only possible if the battery is intended to be completely enclosed by the finished product.
Injection moulded plastics are used to provide more precision packs. For enclosed packs designs using a minimum of materials are based around which a plastic frame holds the components in place thus minimising the cost, the weight and the size of the pack. The overall product cost can be further reduced by using insert mouldings in which the interconnection strips and the terminals are moulded into the plastic parts to eliminate both materials and assembly costs. Overmoulding may also be used to encapsulate and protect small components or sub-assemblies.

Case for 3 AAA Cells

Case for a Single Prismatic Cell
Insert Mouldings Showing Cell Interconnecting Strips
In some designs the battery pack forms part of the outer case of the end product. The colours and textures of the plastic must match the plastics of the rest of the product even though they may come from a completely different supplier. These designs are usually required to incorporate a mechanical latch to hold the battery in place. Again this latch as well as the terminals must interface with plastic parts from a different supplier so high precision and tight tolerances are essential. ABS polymers are the materials typically used for this purpose.
Batteries for traction applications are usually very large and heavy and subject to large physical forces as well as vibrations so substantial fixings are required to hold the cells in place. This is particularly necessary for batteries made up from pouch cells which are vulnerable to physical damage. Automotive battery packs must also withstand abuse and possible accidental damage so metal casings will normally be specified. The metal pack casing also serves to confine any incendiary event resulting from the failure of a cell or cells within the battery and to provide a measure of protection for the user. At the same time the case must also protect the cells and the electronics from the harsh operating environments of temperature extremes, water ingress, humidity and vibration in which these batteries work.
Usually the complete pack is replaced when the battery has reached the end of its useful life. In certain circumstances however, for instance when the pack incorporates a lot of electronic circuits, it may be desirable to design the pack such that the cells within the pack can be replaced.

14.4V 12Ah Lithium battery pack in an off-the-shelf case.
If the design requires provision for replacement of the cells the casing of the battery pack must be designed to clip or screw together. Normally the parts of the plastic housing will be ultrasonically welded together both for security and for low cost as well as to prevent unauthorised tampering with the cells and the electronics.
Other Considerations
Thermal effects need to be taken into account and, tolerances must allow for potential swelling of the cells. Some Lithium pouch cells may swell as much as 10% or more over the lifetime of the cell. For this reason potting is not recommended. In low power designs groups of pouch cells may be shrink wrapped but for higher power applications plastic or metal frames may be used both to provide physical protection of the cells as well as to allow for swelling.
The battery pack should not normally be airtight or sealed since many batteries release hydrogen or oxygen during operation which could cause bursting of the pack or an explosion if the gases are allowed to accumulate. Lithium cells do not emit gases under normal circumstances, but in the case of failure and thermal breakdown, inflammable gases may be vented by the clells. Some form of ventilation or purging should be provided to avoid these problems.
Tolerances should also allow for the use of alternative cells from other manufacturers. While the cells may be "standard" sizes, there could still be differences between cells from different vendors.
High power batteries may need special ventilation or channels between the cells to permit forced air or liquid cooling.
The pack design must incorporate some form of identification to indicate the manufacturer's name, the cell chemistry, the voltage and the capacity as well as the country of manufacture. Most manufacturers will also include a date stamp and or serial number to assist traceability in case of problems. This information is usually provided on a label but it may also be printed directly on to the battery casing.
Pack Costs
The costs involved in designing custom battery packs are often underestimated.
As an indication of the order of magnitude, some very rough cost estimates are given below. They assume that the manufacturer possesses all the necessary standard production resources and they include the pack maker's profit margin and warranty provision. Costs could be lower if the packs are designed and made in house, but then some investment in capital equipment may be required and possibly some recruitment and training costs.
Design Engineering Costs
Low power batteries are usually designed for very high volume production and costs may be calculated to one thousandth of a cent. Most battery packs include some form of battery management electronics, even the smallest designs used in applications such as mobile phones incorporate integrated circuits mounted on a printed circuit board . The mechanical engineering effort however is the activity most often underestimated. It involves the design of precision thin section plastic parts and their associated complex moulding tools as well as metal stampings requiring precision stamping dies. Component sourcing as well as cell selection and qualification also add to the costs.
For low power packs these engineering costs could amount to $20,000 to $50,000.
High power batteries bring an additional set of challenges. Systems integration is much more complex due to the wider range of system functions and demands to be accommodated. For automotive applications the accuracy of the SOC estimation must be much higher and this may also require a major cell characterisation programme. The components are much larger and more expensive and the enormous energy content of the cells demands special safety considerations to prevent physical and electrical abuse and accidental damage. This requires robust steel frames and enclosures and fail safe electronics. Thermal management is also very important and designs may include both heating and forced cooling circuits. Expensive cable forms are needed to connect the cell voltage and temperature sensing signals to the BMS processing unit. All of these requirements add to the complexity, costs and timescales of the associated systems software.
Enginering costs for EV and HEV applications could be upwards of $200,000 and probably much more.
Tooling Costs
High volume products may require multi-cavity moulding tools and progessive stamping dies. In addition automated transfer mechanisms and assembly jigs and fixtures will be required for the manufacturing operations.All this could cost a minimum of $1,500. This assumes the manufacturing plant is already equipped with standard engineering, production and test facilities such as CAD and CAM, PCB assembly machines, conveyer belts, welders, presses, power supplies and electrical test equipment.
For manufacturing high power batteries, material handling and operator safety become major factors because of the heavy weight of the packs and the high voltages involved. Tooling costs may be double those needed for low power packs starting at $200,000 or more.
Prototypes could cost double the cost of volume production. Low volume purchases are more expensive and a considerable amount of manual work is involved. This is only significant for high power batteries.
Production Costs
The manufacturing costs for low power batteries used in mobile phones could be as low as $2.50 whereas a high capacity EV battery could cost upwards of $10,000. In both cases the major cost is the cells. In small batteries this may be 80% to 85% of the total costs. Large batteries use more electronics and higher power components. They are also more labour intensive. For large batteries the cost of the cells could be between 60% and 80% of the total costs depending on the battery specification. Since most cells are sourced from Asia, shipping costs also contribute significantly to the costs.
Two conclusions can be made from this .
Large production volumes are required to justify the development of custom battery packs.
For safety reasons, batteries for electric vehicles involve very high unavoidable engineering development costs, even for a single vehicle.

Material Safety Data Sheet-MSDS

A material safety data sheet (MSDS), safety data sheet (SDS), or product safety data sheet (PSDS) is an important component of product stewardshipand occupational safety and health. It is intended to provide workers and emergency personnel with procedures for handling or working with that substance in a safe manner, and includes information such as physical data (melting point, boiling point, flash point, etc.), toxicity, health effects, first aid, reactivity, storage, disposal, protective equipment, and spill-handling procedures. MSDS formats can vary from source to source within a country depending on national requirements. SDSs are a widely done system for cataloging information on chemicals, chemical compounds, and chemical mixtures. SDS information may include instructions for the safe use and potential hazards associated with a particular material or product. These data sheets can be found anywhere where chemicals are being used.

There is also a duty to properly label substances on the basis of physico-chemical, health and/or environmental risk. Labels can include hazard symbols such as the European Union standard black diagonal cross on an orange background, used to denote a harmful substance.
An SDS for a substance is not primarily intended for use by the general consumer, focusing instead on the hazards of working with the material in an occupational setting. In some jurisdictions, the SDS is required to state the chemical's risks, safety, and effect on the environment. It is important to use an SDS specific to both country and supplier, as the same product (e.g. paints sold under identical brand names by the same company) can have different formulations in different countries. The formulation and hazard of a product using a generic name (e.g. sugar soap) may vary between manufacturers in the same country.

Battery Manufacturing

Electrode Coating

The anodes and cathodes in Lithium cells are of similar form and are made by similar processes on similar or identical equipments. The active electrode materials are coated on both sides of metallic foils which act as the current collectors conducting the current in and out of the cell. The anode material is a form of Carbon and the cathode is a Lithium metal oxide. Both of these materials are delivered to the factory in the form of black powder and to the untrained eye they are almost indistinguishable from eachother. Since contamination between the anode and cathode materials will ruin the battery, great care must be taken to prevent these materials from coming into contact with eachother. For this reason the anodes and cathodes are usually processed in different rooms.

Particle size must be kept to a minimum in order to achieve the maximum effective surface area of the electrodes needed for high current cells. Particle shape is also important. Smooth spherical shapes with rounded edges are desirable since sharp edges or flaky surfaces are susceptible to higher electrical stress and decomposition of the anode passivating SEI layer, which can lead to very large heat generation and possible thermal runaway when the cells are in use.

The metal electrode foils are delivered on large reels, typically about 500 mm wide, with copper for the anode and aluminium for the cathode, and these reels are mounted directly on the coating machines where the foil is unreeled as it is fed into the machine through precision rollers.

The first stage is to mix the electrode materials with a conductive binder to form a slurry which is spread on the surface of the foil as it passes into the machine. A knife edge is located just above the foil and the thickness of the electrode coating is controlled by adjusting the gap between the knife edge and the foil. Since it is not unusual for the gravimetric or volumetric energy storage capacity of the anode material to be different from that of the cathode material, the thickess of the coating layers must be set to allow the energy storage per unit area of the anode and cathode electrodes to be matched.

From the coater, the coated foil is fed directly into a long drying oven to bake the electrode material onto the foil. As the coated foil exits the oven it is re-reeled. The coated foils are subsequently fed into slitting machines to cut the foil into narrower strips suitable for different sizes of electrodes. Later they are cut to length. Any burrs on the edges of the foil strips could give rise to internal short circuits in the cells so the slitting machine must be very precisely manufactured and maintained.

Cell Assembly

In the best factories cell assembly is usually carried out on highly automated equipment, however there are still many smaller manufacturers who use manual assembly methods.

The first stage in the assembly process is to build the electrode sub-assembly in which the separator is sandwiched between the anode and the cathode. Two basic electrode structures are used depending on the type of cell casing to be used, a stacked structure for use in prismatic cells and a spiral wound structure for use in cylindrical cells.

Prismatic Cells

Prismatic cells are often used for high capacity battery applications to optimise the use of space. These designs use a stacked electrode structure in which the anode and cathode foils are cut into individual electrode plates which are stacked alternately and kept apart by the separator. The separator may be cut to the same size as the electrodes but more likely it is applied in a long strip wound in a zig zag fashion between alternate electrodes in the stack. While this case design makes optimum use of space when used in a battery pack, it has the disadvantage that it uses multiple electrode plates which need a clamping mechanism to connect all the anodes together and to the main terminal post and a similar mechanism for the cathodes. This all adds to the complexity and labour content of the cell and consequently to the costs.Some prismatic cells are also made by the simpler method of winding the electrodes on a flat mandrel. Stacked electrodes are also used for the production of pouch cells.

Cylindrical Cells

For cylindrical cells the anode and cathode foils are cut into two long strips which are wound on a cylindrical mandrel, together with the separator which keeps them apart, to form a jelly roll (Swiss roll in the UK). Cylindrical cells thus have only two electrode strips which simplifies the construction considerably. A single tab connects each electrode to its corresponding terminal, although high power cells may have multiple tabs welded along the edges of the electrode strip to carry the higher currents.

The next stage is to connect the electrode structure to the terminals together with any safety devices and to insert this sub-assembly into the can. The can is then sealed in a laser welding or heating process, depending on the case material, leaving an opening for injecting the electrolyte into the can. The following stage is to fill the cell with the electrolyte and seal it. This must be carried out in a "dry room" since the electrolyte reacts with water. Moisture will cause the electrolyte to decompose with the emission of toxic gases. Lithium Hexafluoride (LiPF6) for instance, one of the most commonly used electrolyte materials, reacts with water forming toxic hydrofluoric acid (HF).

Afterwards the cell is given an identification with a label or by printing a batch or serial number on the case.


Once the cell assembly is complete the cell must be put through at least one precisely controlled charge / discharge cycle to activate the working materials, transforming them into their useable form. Instead of the normal constant current - constant voltage charging curve, the charging process begins with a low voltage which builds up gradually. This is called the Formation Process. For most Lithium chemistries this involves creating the SEI (solid electrolyte interface) on the anode. This is a passivating layer which is essential for moderating the charging process under normal use.

During formation, data on the cell performance such as capacity and impedance, are gathered and recorded for quality analysis and traceability. The spread of the performance measurements also gives an indication of whether the process is under control. (Beware of manufacturers who use this process for sorting their cells into different performance groups for sale with alternative specifications).

Although not the prime purpose of formation, the process allows a significant percentage of early life cell failures due to manufacturing defects, the so called "infant mortalities", to occur in the manufacturer's plant rather than at the customers' premises.

Process Control

Tight tolerances and strict process controls are essential throughout the manufacturing process. Contamination, physical damage and burrs on the electrodes are particularly dangerous since they can cause penetration of the separator giving rise to internal short circuits in the cell and there are no protection methods which can prevent or control this.

Support Services

Cleanliness is essential to prevent contamination and cells are normally manufactured in clean room conditions with controlled access to the assembly facilities often via air showers.

Apart from the production test equipment, a battery manufacturer should be expected to have a materials laboratory equipped to carry out a full analysis of the materials used in the production of the cells as well as to carry out failure analysis. The following list shows some of the major equipment used.

-Scanning electron microscope (SEM) for investigating the physical structure of the materials

-Mass spectrometer for analysing the chemical content of the materials

-Calorimeters for checking the thermal properties of the materials and the cells

-Programmable charge/discharge cycle test equipment to exercise the cells and verify their lifetime

-Environmental chambers and vibration tables for investigating the performance of the cells under their expected operating conditions

-Mechanical stress tesing equipment

Lithium Based Batteries

Other Lithium Cathode Chemistry Variants

Numerous variants of the basic Lithium-ion cell chemistry have been developed. Lithium Cobalt and Lithium Manganese were the first to be produced in commercial quantities but Lithium Iron Phoshate is taking over for high power applications because of its improved safety performance. The rest are either at various stages of development or they are awaiting investment decisions to launch volume production.


Doping with transition metals changes the nature of the active materials and enables the internal impedance of the cell to be reduced.

The operating performance of the cell can also be be "tuned" by changing the identity of the transition metal. This allows the voltage as well as the specific capacity of these active materials to be regulated. Cell voltages in the range 2.1 to 5 Volts are possible.


While the basic technology is well known, there is a lack of operating experience and hence system design data with some of the newer developments which also hampers their adoption. At the same time patents for these different chemistries tend to be held by rival companies undertaking competitive developments with no signs of industry standardisation or adoption of a common product. (The original patent on Lithium Cobalt technology has now expired which is perhaps one explanation for its popularity).


Lithium Cobalt LiCoO2

Lithium Cobalt is a mature, proven, industry-standard battery technology that provides long cycle life and very high energy density. The polymer design makes the cells inherently safer than "canned" construction cells that can leak acidic electrolyte fluid under abusive conditions. The cell voltage is typically 3.7 Volts. Cells using this chemistry are available from a wide range of manufacturers.

The use of Cobalt is unfortunately associated with environmental and toxic hazards.


Lithium Manganese LiMn2O4

Lithium Manganese provides a higher cell voltage than Cobalt based chemistries at 3.8 to 4 Volts but the energy density is about 20% less. It also provides additional benefits to Lithium-ion chemistry, including lower cost and higher temperature performance. This chemistry is more stable than Lithium Cobalt technology and thus inherently safer but the trade off is lower potential energy densities. Lithium Manganese cells are also widely available but they are not yet as common as Lithium Cobalt cells.

Manganese, unlike Cobalt, is a safe and more environmentally benign cathode material.

Manganese is also much cheaper than Cobalt, and is more abundant.


Lithium Nickel LiNiO2

Lithium Nickel based cells provide up to 30% higher energy density than Cobalt but the cell voltage is lower at 3.6 Volts. They also have the highest exothermic reaction which could give rise to cooling problems in high power applications. Cells using this chemistry are therefore not generally available.


Lithium (NCM) Nickel Cobalt Manganese - Li(NiCoMn)O2

Tri-element cells which combine slighlty improved safety (better than Cobalt oxide) with lower cost without compromising the energy density but with slightly lower voltage. Different manufacturers may use different proportions of the three constituent elements, in this case Ni, Co and Mn.


Lithium (NCA) Nickel Cobalt Aluminium - Li(NiCoAl)O2

As above, another tri-element chemistry which combines slighlty improved safety (better than Cobalt oxide) with lower cost without compromising the energy density but with slightly lower voltage.


Lithium Iron Phosphate LiFePO4

Phosphate based technology possesses superior thermal and chemical stability which provides better safety characteristics than those of Lithium-ion technology made with other cathode materials. Lithium phosphate cells are incombustible in the event of mishandling during charge or discharge, they are more stable under overcharge or short circuit conditions and they can withstand high temperatures without decomposing. When abuse does occur, the phosphate based cathode material will not burn and is not prone to thermal runaway. Phosphate chemistry also offers a longer cycle life.

Recent developments have produced a range of new environmentally friendly cathode active materials based on Lithiated transition metal phosphates for Lithium-ion applications.


Phosphates significantly reduce the drawbacks of the Cobalt chemistry, particularly the cost, safety and environmental characteristics. Once more the trade off is a reduction of 14% in energy density, but higher energy variants are being explored.

Due to the superior safety characteristics of phosphates over current Lithium-ion Cobalt cells, batteries may be designed using larger cells and potentially with a reduced reliance upon additional safety devices.

The use of Lithium Iron Phosphate chemistry is the subject of patent disputes and some manufacturers are investigating other chemistry variants mainly to circumvent the patent on the LiFePO4 chemistry.


Lithium Metal Polymer

Developed specifically for automotive applications employing 3M polymer technology and independently in Europe with technology from the Fraunhofer Institute, they have been trialled successfully in PNGV project demonstrators in the USA. They use metallic Lithium anodes rather than the more common Lithium Carbon based anodes and metal oxide (Cobalt) cathodes.


Some versions need to work at temperatures between 80 and 120ºC for optimum results although it is possible to operate at reduced power at ambient temperature.

The Fraunhofer technology uses an organic electrolyte and the cell voltage is 4 Volts. It is claimed that their the cell chemistry is more tolerant to abuse.

These products are not yet in volume production.


Lithium Sulphur Li2S8

Lithium Sulphur is a high energy density chemistry, significantly higher than Lithium-ion metal oxide chemistries. This chemistry is under joint development by several companies but it is not yet commercially available. A major issue is finding suitable electrolytes which are not subject to the numerous unwanted side reactions which plague the current designs.

Lithium Sulphur cells are tolerant of over-voltages but current versions have limited cycle life. The cell voltage is 2.1 Volts

See also Dissolution of the Electrodes on the New Cell Designs and Chemistries page.


Alternative Anode Chemistry (LTO)

The anodes of most Lithium based secondary cells are based on some form of carbon (graphite or coke). Recently Lithium Titanate Spinel (Li4Ti5O12) has been introduced for use as an anode material providing high power thermally stable cells with improved cycle life.

This has the following advantages

Does not depend on SEI Layer for stability

No restriction on ion flow hence significantly higher charge and discharge rates possible as well as better low temperature performance.

Lower internal impedance of the cell

Higher temperatures can be tolerated.

No SEI build up over time means very long cycle life possible (10,000 deep cycles)

Public domain technology (No patent disputes)

Disadvantages are

Lower anode reactivity means cell voltage reduced to 2.25 Volts when used with Spinel cathode. (Other cathode chemistries possible)

25% to 30% Lower energy density hence bulkier cells


Lithium Air Cells

Originally conceived as primary cells (see Lithium Pimary Cells), Lithium air cells offer a very high energy density. Rechargeable versions are now under development which promise energy densities of 10 times more than the current generation of Lithium cells, approaching that of Gasoline/Petrol.

The anode is Lithium and the cathode is not air but in fact gaseous Oxygen from the air. Because the cell does not have a solid cathode in the conventional sense it eliminates the weight and volume of the cathode as well as its mechanical supporting structure.

This would enable very small batteries to be made with the same range as current technology, or alternatively, electric drive ranges of several hundred miles could be obtained from batteries the same physical size as those available today.


The Lithium is oxidised by the Oxygen during discharging and charging drives the Oxygen off again, a relatively simple chemistry. There are however problems in preventing the other constituents of the air from poisoning the Lithium electrode. There are also potential safety concerns with the metallic Lithium anodes. The cells demonstrate very high hysteresis with the charging voltage considerably higher than the discharge voltage This corresponds to a low Coulombic efficiency, currently only about 60% to 70%.

The cell voltage is 2.5 Volts.

.See also Energy Density on the New Cell Designs and Chemistries page.


See note on the Toxicity of Lithium

Characteristics of some common Lithium chemistries used in high power batteries


International Battery Standards

National and international standards organisations were set up to facilitate trade by encouraging greater product interoperability and compatibility as well as setting standards for acceptable product safety, quality and reliability.

Below are listed some of the most common standards applicable to battery applications and some of the organisations who issue them and or carry out quality assurance and conformance testing. InEurope, European standards are gradually being adopted in replacement of the previous national standards.


Copies of the relevant standards can be obtained directly from the issuing organisations or from public libraries.

Standards Setting and Safety Testing Organisations




Asociación Española de Normalización y Certificación (Spain)


American National Standards Institute sponsored by NEMA


Australian Standard


Association Suisse des Electriciens (Swiss)


American Society for Quality Control


American Society for Testing and Materials


Explosive Atmospheres (Safety directive)


Battery Council International (Publishes AutomotiveBatteryStandards)


British Standards


California Air Resources Board (Automotive Emission Standards)


Conformance with EU directives


European Committee for Normalisation (Standards Committee)


European Committee for Electrotechnical Standardisation


International Special Committee on Radio Interference


Committee on Data for Science and Technology (Committee of ICSU)


Canadian Standards Association


Defence Standards (UK)


Danmarks Electriske Materielkontrol (Denmark)


Deutsches Institut für Normung (German Institute for Standardisation)


Economic Commission forEuroperegulations.


Electronics Industry Association (USA)


European Norms (Standards)


Federal Communications Commission (USA)


Finnish Electrical Inspectorate


Foundation for Intelligent Physical Agents (Interoperability standards)


Guo Biao = National Standard (People's Republic ofChina)


Health & Safety Executive (UK)


International Council for Science


International Electrotechnical Commission


Institution of Electrical Engineers (UK)


Institute of Electrical and Electronics Engineers (USA)


Instituto ItalianodelMarchio de Qualitá


Ingress Protection


International Standards Organisation


International TelecommunicationsUnion


Japanese Industrial Standard


Keuring van Elektrotechnishe Materialen (Netherlands)


KoreanInstituteofStandardsand Technology


Military Standards (USA)


Motor Industry Software Reliability Association (UK)


Motor Vehicle Emission Group (EU Emission standards)


National Measurement Accreditation Service (UKCalibration)


National Electric Manufacturers Association (USA)


Norges Electriske Materiellkontroll (Norway)


Norme Française (France)


National Fire Protection Association (USA)


National Institute of Justice (USA)


US Department of Labor - Occupational Safety & Health Administration


Osterreichischer Verband für Elektrotechnik (Austria)


Automotive 42 VoltBatteryStandard


Rehabilitation Engineering & Assistive Technology Society ofNorth America


Society of Automotive Engineers (USA)


Svenska Elektriska Materielcontrollanstalten (Sweden)


Schweitzerischer Elektrotechnische Verein (Swiss)


NATO Standards Agreements


DTI Standards and Technical Regulations Directorate (UK)


Telecommunications Industry Association (USA)


Technical Report (Used by IEC)


TÜV Rheinland Group (TUV - Technical Inspection Asssociation)


UK Accreditation Service (Assessment of test services)/(Calibration)


Underwriters Laboratories Requirements (USA)


United States AdvancedBatteryConsortium


United States National Electrical Code


Union Technique de l'Electriciteé (France)


Verband Deutscher Elektrotechniker (Germany)




Standard Number


IEC 60050

International electrotechnical vocabulary. Chapter 486: Secondary cells and batteries.

IEC 60086-1, BS 387 

Primary Batteries - General

IEC 60086-2, BS

Batteries - General


Portable Primary Cells and Batteries with Aqueous Electrolyte - General and Specifications


Portable Rechargeable Cells and Batteries - General and Specifications


Portable Lithium Primary Cells and Batteries - General and Specifications

UL 2054

Safety of Commercial and HouseholdBatteryPacks - Testing

IEEE 1625

Standard for Rechargeable Batteries forMobileComputers

USNEC Article 480

Storage Batteries

ISO 9000

A series of quality management systems standards created by the ISO. They are not specific to products or services, but apply to the processes that create them.

ISO 9001: 2000

Model for quality assurance in design, development, production, installation and servicing.

ISO 14000

A series of environmental management systems standards created by the ISO.

ISO/IEC/EN 17025

General Requirements for the Competence of Calibration and Testing Laboratories


Standard Number


BS 2G 239:1992

Specification for primary active lithium batteries for use in aircraft

BS EN 60086-4:2000, IEC 60086-4:2000

Primary batteries. Safety standard for lithium batteries

BS EN 61960-1:2001, IEC 61960-1:2000

Secondary lithium cells and batteries for portable applications. Secondary lithium cells

BS EN 61960-2:2002, IEC 61960-2:2001

Secondary lithium cells and batteries for portable applications. Secondary lithium batteries

02/208497 DC

IEC 61960. Ed.1. Secondary cells and batteries containing alkaline or other non-acid electrolytes. Secondary lithium cells and batteries for portable applications

02/209100 DC

IEC 62281. Ed.1. Safety of primary and secondary lithium cells and batteries during transport

BS G 239:1987

Specification for primary active lithium batteries for use in aircraft

BS EN 60086-4:1996, IEC 60086-4:1996

Primary batteries. Safety standard for lithium batteries

UL 1642

Safety of Lithium-Ion Batteries - Testing

GB /T18287-2000

Chinese National Standard for Lithium Ion batteries for mobile phones



United Nations recommendations on the transport of dangerous goods

Battery Related Glossary
AC Inverter - An electrical circuit which generates a sine-wave output (regulated and without breaks) using the DC current supplied by the rectifier-charger or the battery. The primary elements of the inverter are the DC/AC converter, a regulation system and an output filter.
A/D Converter (ADC) Analogue/Digital Converter. A device which converts continuously varying analogue signals into a binary coded digital form.
Acid - A proton donor. A compound containing hydrogen which dissociates in aqueous solution producing positively charged hydrogen ions (H+). An acidic solution has a pH less than 7.0
Active material - The chemically reactive materials in an energy cell which react with each other converting from one chemical composition to another while generating electrical energy or accepting electric current from an external circuit.
Ageing - Permanent loss of capacity with frequent use or the passage of time due to unwanted irreversible chemical reactions in the cell.
AGM (Absorbtive Glass Mat) battery - A lead acid battery using a glass mat to promote recombination of the gases produced by the charging process.
Alkali - A compound which dissolves in water producing negatively charged hydroxide ions. Alkaline solutions are strongly basic and neutralise acids forming a salt and water.
Alkaline battery - A battery which uses an aqueous alkaline solution for its electrolyte.
Allotrope - Two or more forms of the same element in the same physical state (solid, liquid, or gas) that differ from each other in their physical, and sometimes chemical properties. The term allotropy applies to elements only, not compounds. The more general term, used for any crystalline material, is polymorphism. See also isotope.
Ambient temperature - The average temperature surrounding the battery, typically air.
Amorphous - Without definite shape or structure, without crystalline structure.
Ampere (Amp) - The unit of current flow equal to one coulomb per second.
Ampere hours (Ah) or Amphours - The unit of measure used for comparing the capacity or energy content of a batteries with the same output voltage. For most batteries it defines the battery's C rate. For automotive (Lead Acid) batteries the SAE defines the Amphour capacity as 20 times the current delivered for a period of 20 hours when the battery is discharged at 1 twentieth of the C rate until the cell voltage drops to 1.75 Volts.
Strictly - One Ampere hour is the charge transferred by one amp flowing for one hour. 1Ah = 3600 Coulombs.
The true capacity of any battery is its energy content and this is measured in WattHours (Wh). It is the battery's Amphour capacity multiplied by the battery voltage.
Ampoule battery - A battery in which the electrolyte is stored in a separate chamber from the cell electrodes until the battery is needed.
Analogue (Analog) circuit - An electronic circuit in which an electrical value (usually voltage or current, but sometimes frequency, phase) represents something in the physical world.The magnitude of the electrical value varies with with the intensity of an external physical quantity.
Also - An electrical circuit which provides a continuous quantitative output ( as opposed to a digital output which may be a series of pulses or numbers) in response to its input.
Anechoic chamber - A room whose walls do not reflect either electromagnetic or acoustic waves.
Anion - Particles in the electrolyte of a galvanic cell carrying a negative charge and moving toward the anode during operation of the cell. See also cation
Anisotropic - Showing differences of property or of effect in different directions.
Anode - The electrode in an electrochemical cell where oxidation takes place, releasing electrons. During discharge the negative electrode of the cell is the anode. During charge the situation reverses and the positive electrode of the cell is the anode.
ANSI - The American National Standards Institute publish standards for batteries jointly with NEMA. (See below)
Aqueous solution - Chemical components in liquid or gel form.
Arrhenius Equation - The relationship between the rate at which a chemical reaction proceeds and its temperature. In general terms, heat speeds up the chemical action.
Assembled battery - A battery composed of two or more cells.
Atomic Number - Specific to individual elements - represents the number of protons in the atomic nucleus. The same as the number of electrons.
Atomic Mass - The number of nucleons (protons and neutrons) in the atomic nucleus.
Auger analysis - Similar to ESCA but does not provide information on the chemical state (oxidation etc.) of the elements.
Authentication - Verification that an item is from an approved source and/or that it is able to meet its declared specification.
Avogadro's Number (NA) - The number of atoms in 12grams of Carbon-12 (definition) = 6.022 x 1023. By extension, the number of particles in 1 mole of a substance.
Base - A proton acceptor. A compound containing hydrogen which dissociates in aqueous solution producing negatively charged hydroxide (OH-) or other ions. Alkalis are bases and a basic solution has a pH greater than 7.0
Battery - Two or more electrochemical energy cells connected together to provide electrical energy.
Battery Management System (BMS) - Electronic circuits designed to monitor the battery and keep it within its specified operating conditions and to protect it from abuse during both charging and discharging.
Battery Monitoring - Sometimes confused with BMS (above) of which it is an essential part, these circuits monitor the key operating parameters (current, voltage, temperature, SOC, etc.) of the battery and provide information to the user.
Bobbin - A cylindrical cell design utilizing an internal cylindrical electrode and an external electrode arranged as a sleeve inside the cell container.
Bootstrap - To do something seemingly impossible using only the available resources. In the context of DC battery power circuits it means generating a DC voltage higher than the battery voltage.
British Thermal Units (BTU) - A unit of heat energy defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. One Btu is equal to about 252 calories, or 778 foot pounds, or 1.055 kilojoules or 0.293 watt hours.
Buck regulator - A switching regulator which incorporates a step down DC-DC converter. A transformerless design in which the lower output voltage is achieved by chopping the input voltage with a series connected switch (transistor) which applies pulses to an averaging inductor and capacitor.
Butler Volmer equation - Used by cell designers to predict the current which will flow in a battery. It is the sum of the anodic and cathodic contributions and is directly proportional to the surface area of the electrodes, increasing exponentially with temperature.
Button cell - Miniature cylindrical cell with a characteristic disc shape.

C Programming Language - The preferred programming language for embedded software used in many battery management applications. Robust, fast and powerful, it allows low level access to information and commands while still retaining the portability and syntax of a high level language.
C Rate - C is a value which expresses the rated current capacity of a cell or battery. A cell discharging at the C rate will deliver its nominal rated capacity for 1 hour. Charging and discharging currents are generally expressed as multiples of C. The time to discharge a battery is inversely proportional to the discharge rate.
• NC is a charge or discharge rate which is N times the rated current capacity of the battery where N is a number (fraction or multiple)
• CN is the battery capacity in AmpHours which corresponds to complete discharge of the battery in N hours (N is usually a subscript). Also written as the N-Hour rate.
Calendar life - The expected life time duration of a cell whether it is active use or in storage
CAN Bus - Controller Area Network The automotive industry standard for on-board vehicle communications. It is a two wire, serial communications bus which is used for networking intelligent sensors and actuators
Calorimeter - A device or chamber for measuring the heat generated by objects placed inside it.
Capacitance (C) - A measure of the ability of a device to store charge per unit of voltage applied across the device. C=Q/V Farads.
The capacitance of a parallel plate capacitor is given by C = ε A/d where ε is the permittivity of the dielectric, A is the area of the plates (electrodes) and d the distance between them.
1 Farad = 1 Coulomb per Volt. (Q / V)
The current through the capacitor is given by the relationship i =C d/dtV(t)
Capacitor - A passive electrical device that stores energy in an electric field.
Capacity - The electric energy content of a battery expressed in "Watt hours". Batteries with the same output voltage also use "Ampere hours" for comparing capacities.
Capacity offset - A correction factor applied to the rating of a battery if discharged under different C-rates from the one rated.
Catalyst - A chemical agent which promotes or influences a chemical reaction without itself being permanently changed by the reaction. Used in recombinant cells and fuel cells
Cathode - The electrode in an electrochemical cell where reduction takes place, gaining electrons. During discharge the positive electrode of the cell is the cathode. During charge the situation reverses and the negative electrode of the cell is the cathode.
Cation - Particles in the electrolyte of a galvanic cell carrying a positive charge and moving towards the cathode during operation of the cell. See also anion
CCA - Cold Cranking Amperes - A measure used to specify the cold cranking capability of automotive SLI batteries. For Lead Acid batteries it is the constant current a battery can deliver during a continuous discharge over a period of 30 seconds at -18°C without the terminal voltage dropping below a minimum of 1.2 Volts/cell.
CE - The CE marking indicates that the product has been designed and manufactured in conformity with the essential requirements of all relevant EU directives, and submitted to the relevant conformity assessment procedure.
Cell - A closed electrochemical power source. The minimum unit of a battery.
Cell balancing - The process used during charging to ensure that every cell is charged to the same state of charge. Also called "Equalisation".
Cell chemistry - The active materials used in the energy cell.
Cell reversal - A condition which may occur multi cell series chains in which an over discharge of the battery can cause one or more cells to become completely discharged. The subsequent volt drop across the discharged cell effectively reverses its normal polarity.
Charge - The process of replenishing or replacing the electrical charge in a rechargeable cell or battery. see also Electric charge
Charge acceptance - The ability of a secondary cell to convert the active material to a dischargeable form. A charge acceptance of 90% means that only 90% of the energy can become available for useful output. Also called Coulombic Efficiency or Charge Efficiency. See alternative definition below.
Charge carriers - The particle carrying the electrical charge during the flow of electrical current . In metallic conductors the charge carriers are electrons , while ions carry the charges in electrolyte solutions .
Charge efficiency - The ratio (expressed as a percentage) between the energy removed from a battery during discharge compared with the energy used during charging to restore the original capacity. Also called the Coulombic Efficiency or Charge Acceptance. See alternative definition above.
Charge pump - A power supply which uses capacitors instead of inductors to store and transfer energy to the output. A voltage doubler or tripler.
Charge rate - The current at which a cell or battery is charged. Generally expressed as a function of rated capacity C.
Charge retention - The ability of a battery to retain its charge in zero current conditions. Charge retention is much poorer at high temperatures. See also Self Discharge
Charge, state of - The available or remaining capacity of a battery expressed as a percentage of the rated capacity.
Charge transport - The movement of electrical charge from one part of the system to another, occurring through the drift of ions under the influence of electrical potential difference. Also called Electromigration.
Chemical species - Atoms, molecules, molecular fragments, ions, etc., as entities being subjected to a chemical process or to a measurement.
CID Circuit Interrupt Device - A small mechanical switch which interrupts the current through an energy cell if the internal pressure exceeds a predetermined limit. Usually applied in small cells only.
Coercivity - The resistance of a ferromagnetic material to becoming demagnetised. Measured in Oersteds.
Coin cell - Small cylindrical cell with a disc shape.
Conditioning - Cycle charging and discharging to ensure that formation (see below) is complete when a cell enters service or returns to service after a period of inactivity
Conductance - Strictly speaking the Conductance applies to resistive circuits and is the reciprocal of the Resistance. Battery manufacturers have their own definition which applies to the frequency dependent elements of the circuit, that is - C= I/E where C is the conductance, I is the test current applied to a component (the cell) and E is the in phase component of the ac voltage E producing it.(Compare with Ohm's Law R=E/I) Measuring the conductance of a battery gives a good indication of its state of health.
Conducting polymer - Plastic materials which have some of the properties of metals. Used as solid electrolytes in batteries. Also used in the construction of fuel cell membranes, capacitor electrodes and in applications requiring anti-static plastics. (See also Polymerbelow)
Constant current charge CC - A charging scheme which maintains the current through the cell at a constant value.
Constant voltage charge CV - A charging scheme which maintains the voltage across the battery terminals at a constant value.
Contacts - The battery output terminals.
Conversion Efficiency - The percentage of the input energy of a process that is converted to energy of the desired type.
Coulomb - A unit of electric charge. One coulomb (1C) is equal to the charge transferred by a current of one ampere in one second.
Coulomb Counting - A method of determining the state of charge of a battery by integrating the ingoing and outgoing discharge currents of a battery over time.
Coulombic Efficiency - The ratio (expressed as a percentage) between the energy removed from a battery during discharge compared with the energy used during charging to restore the original capacity. Also called Charge Efficiency or Charge Acceptance.
Coup de fouet (Whiplash) - A dramatic initial voltage drop when a battery is suddenly called upon to supply a heavy load. The voltage recovers after a short time once the electro-chemical discharge process stabilises.
Critical Temperature (Superconductor) - The temperature below which a superconducting material must be cooled in order to exhibit the property of superconductivity.(See below)
CSA - The Canadian Standards Association is a not-for-profit membership-based association serving business, industry, government and consumers in Canada and the global marketplace.
Curie point or Curie temperature - The temperature above which a ferromagnets and some other materials undergo a sharp change in their magnetic properties losing their ability to possess a net spontaneous or remanent magnetization in the absence of an external magnetic field.
Current limit - The maximum current drain under which the particular battery will perform adequately under a continuous drain.
Current shunt - A current shunt is an low value resistance, whose value is accurately known, placed in series between the battery and the load. The voltage drop across the shunt is used to determine the value of the current using Ohm's Law. Used in series, it is not ashunt in the literal sense of the word. Its name derives from the fact that early ammeters could not handle high currents and the shunt was used to bypass most of the current around the meter.
Cut-off voltage - The specified voltage at which the discharge of a cell is considered complete. See also End voltage and Termination voltage
CVT - Constant Voltage Transformer
Cycle - A single charge and discharge of a battery.
Cycle life - The number of cycles a battery can perform before its nominal capacity falls below 80% of its initial rated capacity. See also Float life below.
Cylindrical cell - A cell in which the electrodes are rolled up in a spiral and placed into a cylindrical container.

D/A Converter (DAC) Digital/Analogue Converter - A device which converts a digitally coded signal into an equivalent analogue signal.
DC-DC Converter - An electronic circuit which takes a DC input voltage and converts it to a different, desired DC output voltage.
Deep cycle battery - A battery designed to be discharged to below 80% Depth of Discharge. Used in marine, traction and EV applications.
Deep discharge - Discharge of at least 80% of the rated capacity of a battery.
Delta V - The voltage drop which occurs in some cells, notably NiCads, which indicates that the cell is fully charged.
Dendritic growth - The formation from small crystals in the electrolyte of tree like structures which degrade the performance of the cell.
Depth of discharge DOD - The ratio of the quantity of electricity or charge removed from a cell on discharge to its rated capacity.
Diamagnetism - The property of a substance which is repelled instead of attracted by a magnet. A diamagnetic material will be repelled from a magnet no matter what pole it is near. It is exhibited by all common materials, but is very weak and often swamped by stronger paramagnetic or ferromagnetic effects. Metals such as bismuth, copper, gold, silver and lead, as well as many nonmetals such as graphite, water and most organic compounds are diamagnetic. See also Ferromagnetism and Paramagnetism.
Dielectric - A nonconductor of electricity, such as an insulator, or a substance in which an electric field can be maintained with a minimum loss of power. The material used between two conducting plates to form a capacitor. When a dielectric or insulator is placed in an electric field, electric charges do not flow through the material but shift only slightly from their average equilibrium positions causing the dielectric to become polarised with a positive charge on one side and a negative charge on the other.
Dielectric Constant - Used to determine the ability of an insulator to store electrical energy. The dielectric constant is the ratio of the capacitance induced by two metallic plates with an insulator between them to the capacitance of the same plates with air or a vacuum between them.
Discharge - The change from chemical energy within the cell into electrical energy to operate a external circuit.
Discharge capacity - The amount of energy taken from the battery when discharged at the rated current and ambient temperature until the discharge end voltage is reached. Generally expressed in units of Watt hours (or Ampere hours for batteries with the same voltage).
Discharge rate - The current delivered by the cell during discharging. Expressed in Amperes or multiples of the C rate.
Discharge voltage - The voltage between the terminals of a cell or battery under load, during discharge.
DOD - Depth of Discharge (see above)
Dropout - In a voltage regulator, the lower limit of the AC input voltage where the power supply just begins to experience insufficient input to maintain regulation. The dropout voltage for linears is quite load dependent. For most switchers it is largely design dependent, and to a smaller degree, load dependent.
Dry Cell - A Leclanché cell with a gel electrolyte.
DST - Dynamic Stress Test. Accelerated battery life tests specified by the USABC. Cycling down to 80% DOD twice per day at different temperatures.
Duty Cycle - The load current or power a battery is expected to supply for specified time periods.
dT/dt - The rate of change of temperature with time. The rapid rate of temperature rise is used to detect the end of the charging cycle in NiMH batteries.

Earth Leakage Trip - See Ground Fault Interruptor
ECE-15 - The United Nations Economic Commission for Europe specification for urban driving cycle simulation.
E Rate - Discharge or charge power, in watts, expressed as a multiple of the rated capacity of a cell or battery which is expressed in watt-hours. For example, the E/10 rate for a cell or battery rated at 23.4 watt-hours is 2.34 watts. (This is similar to the method for calculating C-Rate.)
Elastomer - elastic or plastic materials that resemble rubber which resume their original shape when a deforming force is removed.
Electret - The electrostatic equivalent of the permanent magnet. Dielectric materials that have been permanently electrically charged or polarised.
Electric charge is a physical property of matter which causes it to experience a force when near other electrically charged matter. The charge may be positive or negative. Similar charges repel each other while opposite charges attract each other. The unit of electric charge is the Coulomb (C).
Electrochemical equivalent - The weight of a substance which is deposited by the passage of one coulomb of current.
Electrode - Conducting element within a cell in which an electrochemical reaction occurs.
Electrode potential - The voltage developed by a single electrode, determined by its propensity to gain or lose electrons.
Electrolysis - Chemical modifications, oxidation and reduction produced by passing an electric current through an electrolyte. See also Faraday's Law of Electrolysis
Electrolyte - A substance which dissociates into ions (charged particles) when in aqueous solution or molten form and is thus able to conduct electricity. It is the medium which transports the ions carrying the charge between the electrodes during the electrochemical reaction in a battery.
Electromotive Force EMF - The ability of an electrical source to deliver energy. It is the difference of potentials which exists between the two electrodes of opposite polarity in an electrochemical cell. Also known as the Cell voltage. The unit of EMF is the Volt.
Embedded System - A special-purpose computer system, which is completely encapsulated within the device it controls, usually performing a limited range of specific pre-determined tasks. This allows the use of simpler or cheaper dedicated microprocessors providing only the minimum functionality required by the application, or alternatively the entire processing power of the microprocessor can be focused on a single task. Battery Management Systems will normally be implemented with an embedded system.
EMC - Electromagnetic compatibility (EMC) is the ability of electronic and electrical equipment and systems to operate without adversely affecting other electrical or electronic equipment or being affected by other sources of electromagnetic interference. (RFI)
End voltage - The prescribed voltage that indicates that the discharge is complete. (see also Cut-off voltage)
Endothermic - Describes a chemical action in which heat is absorbed.
Energy Content - The absolute amount of energy stored in a battery expressed in Wh or Joules
Energy density - The amount of energy stored in a battery. It is expressed as the amount of energy stored per unit volume or per unit weight (Wh/L or Wh/kg).
Enthalpy - The amount of energy released or absorbed by a chemical reaction. The "Free Enthalpy" (also called the " Change in Gibbs Free Energy") in a reaction is the maximum amount of chemical energy available from a system that can be converted into electrical or mechanical energy and vice versa. (discharge and charge respectively)
Entropy - A measure of the disorder of a system. Used as a measure of heat content.
EPROM - Electronically Erasable Programmable Read-Only Memory. Re-writable memory that does not lose data if power is lost to the system (non-volatile). Available in three types:
• OTP One Time Programmable non-erasable.
• Windowed (ultraviolet light erasable) used for prototyping and development work.
• EEPROM Electronically Erasable Programmable Read-Only memory.
Equalisation - The process of bringing every cell in a battery chain to the same state of charge (SOC)
ESCA - Electron Spectroscopy for Chemical Analysis. Equipment using x-ray irradiation to identify the presence of individual chemical elements particularly for surface coatings and thin films where it can be used for selected element depth profiling. A machine typically costs about $750,000
ESD - Electrostatic Discharge
EUDC - Extra Urban Driving Cycle. European additional specification for urban driving cycle simulation.
EUROBAT - The Association of European Storage Battery Manufacturers. (Mainly Lead acid)
Eutectic - A mixture in such proportions that the melting-point is as low as possible, the constituents melting simultaneously.
EV - Electric Vehicle
Exercise - Commonly describes the discharging to one volt per cell and subsequent charging. Used to maintain or condition NiCad and NiMH cells.
Exothermic - Describes a chemical action in which heat is produced.

Farad - The charge in Coulombs necessary to change the potential between the plates of a capacitor by 1 volt.
1 Farad = 1 Coulomb per Volt. (Q / V)
Faraday cage - An enclosure with no apertures (holes, slits, windows or doors) made of a perfectly conducting material. No electric fields are produced within the Faraday cage by the incidence of external fields upon it or by currents flowing on the perfect conductor such that the perfectly conducting enclosure is a perfect electromagnetic shield.
Faraday constant- The magnitude of electric charge per mole of electrons or protons. It is equal to Avogadro's Number times the charge on the electron. F= NA.e
Faraday's Law of Electrolysis - The mass of a substance altered at an electrode during electrolysis is directly proportional to the quantity of electrical charge (measured in Coulombs) transferred at that electrode.
Faraday's Law of Induction - The induced EMF in a closed circuit is proportional to the rate of change of the magnetic flux through the circuit. see also Inductance
Fast charge - Charging in less than one hour at about 1.0C rate. Needs special charger.
FCC - The Federal Communications Commission is an independent United States government agency charged with regulating interstate and international communications by radio, television, wire, satellite and cable.
Ferromagnetism - The property of a substance which is attracted to a magnet. Iron, cobalt, nickel, gadolinium, dysprosium and alloys containing these elements are ferromagnetic. See also Diamagnetism and Paramagnetism.
FET - Field Effect Transistor - A semiconductor device designed for fast, current switching applications.
Firmware - Instructions programmed into a micro-controller that controls its operation. A combination of hardware and software.
FlexRay Bus - A fault tolerant, high speed data communications bus designed for complex automotive control applications.
Float charge - An arrangement in which the battery and the load are permanently connected in parallel across the DC charging source, so that the battery will supply power to the load if the charger fails. Compensates for the self-discharge of the battery.
Float life - The expected lifetime in hours of a battery when used in a float charge application. See also Cycle life above.
Flooded Lead Acid cell -In "flooded" batteries, the oxygen created at the positive electrode is released from the cell and vented into the atmosphere. Similarly, the hydrogen created at the negative electrode is also vented into the atmosphere. This can cause an explosive atmosphere in an unventilated battery room. Furthermore the venting of the gasses causes a net loss of water from the cell. This lost water needs to be periodically replaced. Flooded batteries must be vented to prevent excess pressure from the build up of these gasses. See also Sealed Lead Acid (SLA) Cells which overcome these problems.
Flow battery - A battery in which the electrolyte flows or is pumped through the electrodes
Flywheel battery - A flywheel stores kinetic energy in a high speed (up to 100,000 rpm) rotating cylinder and is "charged" and "discharged" via an integral motor/generator. High power availability but low capacity.
Formation - Electrochemical processing of a cell electrode(or plate) between manufacturing and first discharge, which transforms the active material into its useable form.
FPGA - Field Programmable Gate Array. A microchip which can be made with thousands of programmable logic gates. Often used for prototype or custom designs, they permit short development times and low production costs.
FUDS - Federal Urban Driving Schedule specification for urban driving cycle simulation.
Fuel Cell - An electrochemical generator in which the reactants are stored externally and may be supplied continuously to a cell.
Fuel Gauge - An indication of the State of Charge (SOC) or how much charge is remaining in a battery. Also called a Gas Gauge.
Fuzzy Logic - A method of deriving precise answers from vague data.

Galvanic cell - An electrolytic cell in which chemical energy is converted to electrical energy on demand
Gas chromatography - The separation and identification of individual chemical components from a sample. A typical machine costs over $250,000..
Gas gauge - An electrical circuit which indicates the amount of charge remaining in a battery.
Gassing - The generation of a gaseous product at one or both electrodes as a result of the electrochemical action. In Lead Acid batteries gassing produces hydrogen and oxygen.
Gel cell - A battery which uses gelled electrolyte, an aqueous electrolyte that has been fixed by the addition of a gelling agent.
Gibbs Free Energy - See Enthalpy
GMR (Giant MagnetoResistance) A spintronic effect that produces a large change in resistance of the conducting layers that occurs when thin stacked layers of ferromagnetic and nonmagnetic materials are exposed to a magnetic field. "Giant" refers to its very large electrical signal. The technology is used to manufacture current and magnetic sensors.
Gravimetric Energy Density (WhKg) - The energy output per unit weight of a battery.
Gravimetric Power Density (W/Kg) - The power output per unit weight of a battery.
Ground Fault Interruptor - Also called an Earth Leakage Trip - A safety device which disconnects the mains power if an earth leakage (unsafe) condition is detected. A sensing coil detects fault currents from the live wire to the earth (ground) wire and switches off the power when a predetermined threshold is reached. The device is designed to protect the electrical installation from faults and does not sense fault currents from the live wire to any other earthed body. See also Residual Current Device (RCD) which also protects the user.
Ground Loop - An unintentionally induced feedback loop or crosstalk caused by two or more circuits sharing a common electrical ground.

Half Cell Reaction - The electrochemical reaction between the electrode and the electrolyte.
Hall effect - When a fixed conductor carrying an electric current is placed in an external magnetic field perpendicular to the current there is voltage drop across the conductor at right angles to the current which is proportional to the magnetic field. Used to measure magnetic field strength.
Heavy Duty battery - An ill defined battery characteristic. See Battery Performance.
Henry (H) - The unit of inductance. The inductance L in a circuit =1 Henry if the rate of change of the current of 1 Ampre per second in the circuit produces an EMF of 1 Volt.
1 Henry = 1 Weber per Amp (Wb / A)
Hertz (Hz) - The standard unit of frequency of one cycle per second.
HEV - Hybrid Electric Vehicle (See below)
Hibernation state - A state in which the the status of the various functions of a circuit has been saved in memory and the circuit has been switched off save energy. When power is reapplied, data taken from the memory is used to restore the circuit to the status it had before switch off. (See also "Standby state" below)
High Energy battery - An ill defined battery characteristic. See Battery Performance.
High rate discharge - Discharge at a current of 2C or more.
Horse Power (Hp) - The rate of doing work. 1 Hp = 746 Watts or 550 foot pounds per second.
Hybrid Electric Vehicle (HEV) - A vehicle which has two forms of motive power one of which is electric.
Hydrometer - A device used for measuring the specific gravity of a fluid. In the case of lead acid batteries the specific gravity provides a measure of the state of charge of the cell.
Hygrometer - An instrument for measuring humidity. Often confused with a hydrometer.
Hysteresis - A property of physical and chemical systems that do not instantly follow the forces applied to them, but react slowly, or do not return completely to their original state. In the case of magnetic systems, when an external magnetic field is applied to a magnetic material, the material becomes magnetised absorbing some of the external field. When the external field is removed the material remains magnetised to some extent, retaining some magnetic field. See also hysteresis in batteries.

IEC - The International Electrotechnical Commission (IEC), founded in London in 1906, is the leading global organization that prepares and publishes international standards for all electrical, electronic and related technologies. See also Standards
IGBT - Insulated Gate Bipolar Transistor. It has the output switching and conduction characteristics of a bipolar power transistor but is voltage controlled like the MOSFET giving the high current carrying capability of the bipolar transistor but the ease of control of the MOSFET.
Immobilised electrolyte - A construction technique used in lead-acid batteries. The electrolyte (the acid) is held in place against the plates instead of being a free-flowing liquid. The two most common techniques are Gel Cell and Absorbed Glass Mat.
Impedance - A measure of the response of an electric circuit to an electric current. The actual value is frequency dependent. The current is opposed by the capacitance, inductance and resistance of the circuit.
Impedance testing - Determination of the battery's internal impedance by measuring the voltage drop across a cell when it carries a sample alternating current.
Inductance (L) - A measure of the ability of a device to store magnetic flux per unit of rate of change of current passing through the device. Measured in Henries. 1 Henry = 1 Weber per Amp (Wb / A)
See also Faraday's Law of Induction
The voltage across the inductor is given by the relationship v = - L d/dtI(t)
Inductive charging - A charger in which the charging current is induced by an external induction coil into a secondary transformer winding housed within the battery together with rectifying and charge control circuits.
Inductor - A passive electrical device that stores energy in a magnetic field
Infra red radiation - The spectrum of the heat radiated by a warm body.
Inhibitor - A substance added to the electrolyte to prevent or slow down an unwanted electrochemical process. Used to prevent corrosion of the electrodes or the formation of dendrites.
Insert mouldings - Plastic parts containing metal inserts used to simplify product assembly and reduce costs. Inserts made from metal or other materials are placed in the mould prior to the injection of plastic. The plastic flows around the inserts and fixes their position.
Intelligent battery - Battery containing circuitry enabling some communication between the battery and the application or with the charger.
Intelligent charger - Charger which is able to react to inputs from an intelligent battery to control or optimise the charging process.
Intelligent Energy Manager IEM - A system for reducing the demands that power hungry applications place on the battery.
Intercalation - This insertion of ions into the crystalline lattice of a host electrode without changing its crystal structure.
Internal impedance - Resistance to the flow of AC current within a cell. It takes into account the capacitive effect of the plates forming the electrodes.
Internal resistance - Resistance to the flow of DC electric current within a cell, causing a voltage drop across the cell in closed circuit proportional to the current drain from the cell. A very low internal impedance is usually required for a high rate (high power) cell.
Inverter - An electrical circuit which generates a sine-wave output (regulated and without breaks) using the DC current supplied by the rectifier-charger or the battery. The primary elements of the inverter are the DC/AC converter, a regulation system and an output filter.
Ion - An atom or group of atoms which is electrically charged. Depending on how they were created - through release or absorption of electrons - ions can be either positively charged (Cations) or negatively charged (Anions). See also Ionisation
IP Code - Ingress Protection Rating. It consists of the letters IP followed by two digits. The first digit represents the degree of protection against dust and solids. The second digit represents the degree of protection against moisture and water.
IR drop - The voltage drop across a battery due to its internal impedance. See also Ohmic loss below.
I2R loss - The energy generated or lost as heat due to the internal resistance of the battery. Also known as the Joule heating effect.
ISO - A network of national standards institutes from 148 countries, founded in 1946, working in partnership with international organizations, governments, industry, business and consumer representatives.The name, "ISO" was not intended as an acronym for anInternational Standards Organisation but was derived from the Greek word "isos" meaning "equal". See also Standards
Isotope - Atoms of the same element with the same atomic number ( the same number of protons) but with different numbers of neutrons an hence different weights. See also allotrope.

Josephson effect - The flow of electric current through nonconductive material when placed between two superconductors. Used to detect very weak magnetic fields.
Joule - "J" A measure of work, energy or cell capacity. For electrical energy, one Joule is one Amp at one Volt for one Second, or one WattSecond. 1 Wh = 3.6kJ. For mechanical energy one Joule is a force of one Newton acting over one metre i.e. One newton metre.
Joule heating - The I2R loss or heating effect of a current I flowing through a resistance R.

Kalman Filter - A mathematical technique for deriving accurate information from inaccurate data.
Kelvin Bridge - An electrical circuit for measuring very low impedances such as battery internal impedance, contact resistance and resistance of circuit elements such as wires and cables. Also known as the Kelvin Connection for voltage sensing.
Keyed connectors - Plug and socket pairs with a unique mechanical profile which can only be mated with eachother in a particular orientation and which do not allow mating with connectors of a different design.

LDO (Low Drop Out) Regulator - An LDO is a type of linear regulator. Dropout voltage is the minimum input to output voltage differential required for the regulator to sustain an output voltage within 100mV of its nominal value.
Leakage - The escape of electrolyte to the outer surface of the battery or cell.
Leclanché Cell - A zinc carbon or zinc chloride cell.
Lifetime Energy Throughput - The total amount of energy in Watthours which can be taken out of a rechargeable battery over all the cycles in its lifetime before its capacity reduces to 80% of its initial capacity when new.
LIN Bus - Local Interconnect Network An automotive industry standard for on-board vehicle communications. It is a single wire, serial communications bus which is used for networking intelligent sensors and actuators
Linear charger - Charger which uses a series regulator. The simplest and cheapest type but less efficient than a Switch mode charger.
Linear Regulator - A linear, or Series, regulator is a voltage regulator which uses a transistor or FET in series with the load, operating in its linear region, to subtract excess voltage from the applied input voltage, producing a regulated output voltage.
Lithium Ion Cell - A secondary lithium cell in which both the negative and positive electrodes are lithium insertion (intercalation) compounds. Also known as rocking chair, shuttlecock or swing cell.
Lithium Polymer Cell - A lithium ion cell with a solid polymer electrolyte.
Load current - The discharge current provided by a battery, or drawn by a battery powered device.
Long Life battery - An ill defined battery characteristic. See Battery Performance.

Magnetic flux ( Φ ) - is a measure of the magnetic field strength. Measured in Webers
Magnetic flux density (B) - is the magnetic flux per unit area. B = (Φ / A) Teslas. The flux density resulting from a magnetic field is given by B = μH where μ is the permeability of the medium.
Magnetic field strength (H) - is a measure of the magnetic field surrounding a wire (or moving charge). H = I / (2 π r) Amps per metre, where I is the current in the wire and r the distance from the wire.
Magnetic Resonance Imaging (MRI) - A method of looking inside the human body without using surgery, harmful dyes or x-rays based on Nuclear Magnetic Resonance (NMR).
Magnetohydrodynamic Generator MHD - The production of electricity by passing a conducting fluid or plasma through a magnetic field.
Magnetomotive Force (MMF) - is the strength of a magnetic field, or magnetic potential, in a current carrying coil of wire. It is the work that would be required to carry a hypothetical isolated magnetic pole of unit strength completely around a magnetic circuit and is equivalent to the current I multipled by the number of turns N in the coil producing the field. It is expressed in units called ampere-turns (At). The MMF = ampere-turns = NI = Number of turns (N) X Current in the wire (I)
Magnetostriction - A property which causes the shape or dimensions of ferromagnetic materials to change during the process of magnetisation.
Mass spectrometer - A device which produces a mass spectrum of a sample to find out its composition by ionising the sample and separating ions of differing masses and recording their relative abundance by measuring intensities of ion flux. Mass spectroscopy allows detection of compounds by separating ions by their unique mass. A typical machine costs around $250,000
Memory effect - Reversible, progressive capacity loss in nickel based batteries found in NiCad and to a lesser extent in NiMH batteries. It is caused by a change in crystalline formation from the desirable small size to a large size which occurs when the cell is recharged before it is fully discharged.
Mechanical charging - Charging by replacing one or more of the active chemicals in the cell.
Meissner effect - When a superconducting material is cooled below its critical temperature it will exclude or repel a magnetic field. A magnet moving by a conductor induces currents in the conductor. This is the principle upon which the electric generator operates. But, in a superconductor the induced currents exactly mirror the field that would have otherwise penetrated the superconducting material causing the magnetic field to be excluded and magnet to be repulsed. This phenomenon is known as diamagnetism (see above) and is so strong that a magnet can actually be levitated over a superconductive material.
Metal hydride - A metallic compound which is able to absorb hydrogen. Used as the negative electrode (anode) of a Nickel Metal Hydride battery.
Microcycles - Rapid, shallow charge and discharge cycles which occur in automotive battery applications, particularly those which involve regenerative braking.
MISRA - UK Motor Industry Software Reliability Association.
Mole (n) - The amount of substance of a system that contains as many "elemental entities" (e.g., atoms, ions, electrons, molecules) as there are atoms in 12 grams of carbon-12 (Avogadro's number of particles). It is an amount, not a physical quality. 1 mole of a pure substance has a mass in grams equal to its molecular mass (M).
Molar mass - The mass in grams of one mole (or 6.02 x 1023 molecules) of any chemical compound.
Monomer - A small molecule that may become chemically bonded to other monomers to form a polymer. From Greek mono "one" and meros "part".
Morphology - The microstructure of the solid phases of materials. The grain shapes and structure of crystals of the chemical components of a battery.
MOSFET - A Field Effect Transistor made using Metal Oxide Semiconductor technology. Controlled by voltage rather than current like a bipolar transistor. MOSFET's have a significantly higher switching speed than bipolar power transistors. Suitable for high power circuits, they generate almost no loss (little heat generation), enabling fast response, excellent linearity, and high efficiency. The positive temperature coefficient inhibits thermal runaway. (Degrades to an SFET - Smoke and Fire Emitting Transistor if subject to excessive voltages). See also IGBT.
MSDS - Material Safety Data Sheet. Information provided by battery or cell manufacturers about any hazardous materials used in their products.
Multiplexer - A multiplexer is a device which enables several communications links or signals to share a single communications channel. At the receiving end of the link a demultiplexer separates the signals again. Various coding schemes are possible which enable the signals to be transmitted simultaneously or sequentially.

Nano - From the latin word meaning "dwarf". One billionth or 10 -9. One micron = 1000 nanometers. One nanometer is about the diameter of 3 to 6 atoms (depending on the element).
Nanotechnology - Nanomaterials (nanocrystalline materials) are materials possessing grain sizes on the order of a billionth of a meter. Used for electrodes and separator plates in NiMH and Lithium ion batteries and also in supercapacitors. Their foam-like (aerogel) structure provides a very large effective surface area which can hold considerably more energy than their conventional counterparts.
Nanobattery - Very small battery built using nano technology. Of microscopic size 1 micron diameter they deliver 3.5 volts. The electrodes are ceramic or carbon particles and the electrolyte is a solid polymer impregnated in an aluminium oxide membrane.
Negative Delta V (NDV) - The NDV is the drop in the battery voltage which occurs when NiCad or NiMH cells reaches their fully charge state. Used to detect the end of the charging cycle in Nicads.
Negative electrode - The electrode which has a negative potential. The anode.
NEMA - The National Electric Manufacturers Association in the USA publish standards for batteries jointly with ANSI. (See above)
Nernst equation - Used by cell designers to calculate the voltage of a chemical cell from the standard electrode potentials, the temperature and to the concentrations of the reactants and products.
Neural Network - A powerful data modeling tool that is able to capture and represent complex input/output relationships. It is used as a basis for self learning systems.
NIH - Not Invented Here. Used to describe engineers and managers who are reluctant to accept ideas from another organisation.
Nominal capacity - Used to indicate the average capacity of a battery. It is the average capacity when batteries are discharged at 0.2C within one hour of being charged for 16 hours at 0.1C and 20± 5°C. (or discharge at 0.05C for automotive batteries - SAE) Definition depends on the conditions. See Ampere Hours Ah above
Nominal voltage - Used to indicate the voltage of a battery. Since most discharge curves are neither linear nor flat, a typical value is generally taken which is close to the voltage during actual use.
NRE - Non-Recurring Engineering costs. A one time charge for design and implementation of custom battery packs or other products.
NTC - A thermistor with a negative temperature coefficient, whose resistance decreases with increasing temperature.
Nuclear fission - Occurs when the atomic nucleus splits into two or more smaller nuclei plus some by-products. These by-products include free neutrons and photons (usually gamma rays). Fission releases substantial amounts of energy (the nuclear binding energy ). The neutrons released by the fission process may collide with other nuclei causing them in turn to undergo fission initiating to a chain reaction.
Nuclear fusion - A process in which two nuclei join together to form a larger nucleus and releasing energy. It takes considerable energy to overcome the repulsion between the two positively charged nuclei to force them to fuse. The fusion of lighter nuclei, which creates a heavier nucleus and a free neutron, will generally release even more energy than it took to force them together. It is an exothermic process which could produce self-sustaining reactions.
Nuclear Magnetic Resonance (NMR) - The interaction of atomic nuclei placed in an external magnetic field with an applied electromagnetic field oscillating at a particular frequency . Magnetic conditions within the material are measured by monitoring the radiation absorbed and emitted by the atomic nuclei. Used in MRI scanners and as a spectroscopy technique to obtain physical, chemical, and electronic properties of molecules.

OEM Original Equipment Manufacturer - A company with the prime responsibility for conceiving, designing, manufacturing and distributing a particular product line.
Ohmic loss - The voltage drop across the cell during passage of current due to the internal resistance of the cell. Also known as IR loss or IR drop.
Open circuit voltage OCV - The voltage of a cell or battery with no load attached measured with a voltmeter at room temperature.
Operating voltage - Voltage between the two terminals of the battery with a load connected.
Operational amplifier (Op amp) - A high gain DC amplifier with a voltage gain of 100 to 100,000 or more and a very high (ideally infinite) input impedance and very low (ideally zero) output impedance. Op-amps are the basic building block of linear integrated circuits used for analogue circuit applications. They have positive and negative inputs which allow circuits which use feedback to achieve a wide range of functions.
Opportunity charging - Intermittent charging from sources whenever or wherever power is available.
Opto-isolator - Also called opto-coupler. An isolation device using optical techniques (an LED transmitting across a small gap to a photocell) to isolate the electrical connections between a transmitter and a receiver. Used to pass signals between high voltage and low voltage circuits and to replace switches and relays. Having no electrical connection they also help to cut down on ground loops.
Osmosis - The diffusion of a solvent through a semi permeable membrane from a region of low solute concentration to a region of high solute concentration. The semi permeable membrane is permeable to the solvent, but not to the solute, resulting in a chemical potential difference across the membrane which drives the diffusion. The solvent flows from the side of the membrane where the solution is weakest to the side where it is strongest to equalise the concentration on both sides.
Over-charge - Continuous charging of the battery after it reaches full charge. Generally overcharging will have a harmful influence on the performance of the battery which could lead to unsafe conditions. It should therefore be avoided.
Over-current - Exceeding the manufacturer's recommended maximum discharge current for a cell or battery.
Over-discharge - Discharging a battery below the end voltage or cut-off voltage specified for the battery.
Overmoulding - An injection moulding technique used to encapsulate and protect components or small sub-assemblies, usually by moulding a soft, flexible, cosmetically attractive plastic over the components which must be able to withstand the temperatures and pressures of the moulding process. Used for cable connectors, gaskets, and for incorporating small components into cables. Two shot moulds may be used to provide soft plastic grips over a hard plastic shell. It provides rugged, almost unbreakable protection with built in strain relief.
Over-voltage - The difference between the actual potential at which an electrochemical reaction occurs, and its theoretical equilibrium potential.
Oxidation - The loss of electrons by a chemical species

Packaging - In a battery, the mechanical structure used to contain and protect its components (cells, electronic circuits, contacts etc.).
Parallel connection - The connection together of, two or more, similar cells to form a battery of higher capacity by connecting together all the cell terminals of the same polarity.
Paramagnetism - The property of a substance which is attracted to a magnet. It is similar to ferromagnetism except that the attraction is weaker. When a paramagnetic material is placed in a strong magnetic field, it becomes a magnet as long as the strong magnetic field is present. But when the strong magnetic field is removed the magnetic effect is lost. Below the substance's Curie temperature a paramagnetic material becomes ferromagnetic. Paramagnetism is exhibited by materials containing transition elements, rare earth elements and actinide elements. Liquid oxygen and aluminium are also examples of paramagnetic materials. See also Diamagnetism and Ferromagnetism.
Passivation layer - A resistive layer that forms on the electrodes in some cells after prolonged storage impeding the chemical reaction. This barrier must be removed to enable proper operation of the cell. Applying charge/discharge cycles often helps in preparing the battery for use. In other applications, passivation is used as a method of shielding a metal surface from attack.
Periodic Table of the Elements - A tabular display of the known chemical elements. The elements are arranged by electron structure so that many chemical properties vary regularly appearing in groups with common properties across the table. Each element is listed by its atomic number and chemical symbol .
Permanent charge - The charging current which can safely be continuously supported by the battery, regardless of the state of the charge.
Permeability (μ) - The measure of the characteristic of a medium to support the formation of a magnetic field. It indicates degree of magnetisation that a material obtains in response to an applied magnetic field. It is measured in units of Henries per metre (H / m)
Permittivity (ε) - The measure of the characteristic of a medium to resist the formation of an electric field. It gives an indication of how much electrical charge a material can store in a given volume. It is measured in units of Farads per metre (F/ m)
Peukert's equation An empirical formula that approximates how the available capacity of a battery changes according to the rate of discharge. The equation shows that at higher currents, there is less available energy in the battery.
Peukert number A value that indicates how well a battery performs under heavy currents. A value close to 1 indicates that the battery performs well; the higher the number, the more capacity is lost when the battery is discharged at high currents. The Peukert number of a battery is determined empirically.
pH - (potential (of) hydrogen) is a logarithmic measure of the concentration of hydrogen ions (H + ) in a solution and, therefore, its acidity or alkalinity (basicity). pH = -log[H + ]
The "pH" scale extends from 0 to 14 (in aqueous solutions at room temperature). A pH value of 7 indicates a neutral (neither acidic nor basic) solution. A pH value of less than 7 indicates an acidic solution, the acidity increases with decreasing pH value. A pH value of more than 7 indicates an alkaline or basic solution, the alkalinity or basicity increases with increasing pH value.
Photovoltaic cell - A device that directly converts the energy in light into electrical energy. Also called a photocell, a solar cell or a PV cell.
Photovoltaic effect - The generation of an electromotive force as a consequence of the absorption of radiation. In practice a current which flows across the junction of two dissimilar materials when light falls upon it.
Pilot Cell - A selected cell whose condition is assumed to indicate the condition of the entire battery.
Plates - The electrodes used in a flat plate cell.
PNGV - Partnership for a New Generation of Vehicles. A partnership between government, industry and academia in the USA to improve all aspects of automotive design in which batteries figure highly.
Polarisation - The change in the potential of a cell or electrode from its equilibrium value caused by the passage of an electric current through it. There are two irreversible electrochemical components, the "electrode polarisation" at the electrodes and the "concentration polarisation" in the electrolytic phase plus an ohmic loss component due to the electrical resistance of the cell. Also due to the build up of gas bubbles on the electrodes.
Polarity reversal - Reversal of the polarity of a battery or cell due to over discharge.
Polymer - Strictly it is a substance made of long repeating chains of molecules called monomers which may be identical or different. The term polymer is often used in place of plastic, rubber or elastomer. In battery technology "polymer" usually refers to a solid (plastic) ionic conductor that is an electrical insulator but passes ions. (See also Conducting Polymer above)
Polymorphism - The ability of solid materials or compounds with the same chemical composition to exist in more than one form or crystal structure giving rise to materials with different physical or chemical properties. When the material consists of a single element, the property is known as allotropy.
Polyswitch - A resettable fuse. (See below)
Positive electrode - The electrode which has a positive potential. The cathode. Electric current from this electrode flows into the external circuit.
Pouch cell - A battery or cell contained in a flexible metal foil pouch.
Power density - The amount of power available from a battery. It is expressed as the power available per unit volume or per unit weight (W/L or W/kg).
PowerNet - The standard proposed for next generation of automotive batteries. Nominally 42 Volt systems.
Power transistor - A high current, bipolar transistor controlled by the current through the gate. Used in linear (series) regulators as the voltage dropper between the unregulated voltage input and the regulated output. Also used as a high current switching device in control and protection circuits. Needs a high current to turn it on and is slow to turn off and its negative temperature coefficient makes it prone to thermal runaway. For these reasons it was mostly superceded by MOSFETs in high power battery switching applications. See also Thyristor and IGBT.
ppm - Parts Per Million
Precursor - A chemical compound that participates in a chemical reaction which produces another compound.
Primary battery - A battery that is non-rechargeable.
Prismatic cell - A slim rectangular sealed cell in a metal case. The positive and negative plates are stacked usually in a rectangular shape rather than rolled in a spiral as done in a cylindrical cell.
Progressive dies - Multi-stage stamping tools for producing complex metal components from flat metal strip in a hydraulic or eccentric press. The die consists of two or more stages each of which carries out punching, drawing or folding operations with each down stroke of the press. Between each stroke, the strip moves from stage to stage through the die. Complex profiles and three dimensional shapes can be built up from a series of simpler operations which take place progressively at each stage as the strip passes through the die.
Protection - A facility incorporated into battery packs to protect the cells from out of tolerance working conditions or misuse.
PTC - A thermistor with a positive temperature coefficient, whose resistance increases with temperature.
PPTC - A Polymeric Positive Temperature Coefficient device. It is a non-linear thermistor, more commonly called a resettable fuse.
Pulse charger - Versatile, hybrid charger having some of the advantages of both switch-mode and linear chargers. More costly than both.
Pulse discharge - A high rate discharge, usually of 1 second or less.

Quick charge - Charging in three to six hours at about 0.3C rate. Needs special charger.
Quiescent current - The current which continues to be drawn from the battery when the application it powers is in standby or hibernation mode.

Ragone Plot - The graphical illustration of the specific energy of a cell as a function of its specific power.
RAM cells - Rechargeable Alkaline Manganese cells.
RAPS - Remote-Area Power Supplies - Power systems deriving their energy from local solar or wind sources using a battery for energy storage and supplying the load through DC-DC converters or AC inverters.
Rare earth elements - The rare earth metals belong to group 3 of the periodic table in two blocks, the Lanthanide series and the Actinide series. Originally found in small quantities they are not particularly rare. They are silver, silvery-white, or grey metals with a high electrical conductivity and a bright lustre which tarnishes readily in air.
Rate - When applied to cells it usually means the cells current carrying capacity.
Rated capacity - The specified capacity of a battery.
Reconditioning - One or more deep discharges below 1.0 V/cell with a very low controlled current, causing a change to the molecular structure of the cell and a rebuilding of its chemical composition. Reconditioning helps break down large crystals to a more desirable small size, often restoring the battery to its full capacity. Applies to nickel-based batteries. See also refurbishment (below)
Recombinant system - Sealed secondary cells in which gaseous products of the electrochemical charging cycle are made to recombine to recover the active chemicals. A closed cycle system preventing loss of active chemicals. Used in Nicads and SLA batteries.
Recovery - The lowering of the polarization of a cell during rest periods.
Recycling - Reclamation of materials without endangering human health and the environment.
Redox - A contraction of the words "reduction" and "oxidation". The two chemical reactions on which cell chemistries depend.
Redox Battery - A battery in which the chemical energy is stored in two dissolved ionic reactants separated by a membrane.
Reduction - The gain of electrons by a chemical species.
Refurbishing - The repair of worn out or damaged batteries. This is not the same as reconditioning (see above).
Regenerative braking - This uses the electrical drive motor in an electric vehicle to act as a generator returning energy to the battery when overdriven mechanically by the vehicle wheels. This provides a powerful braking effect and at the same time captures energy which would otherwise be wasted or dissipated in the brakes.
Regenesys - A high power Sodium Polysulfide Bromine "Flow Battery".
Regulator - See Voltage regulator.
Relay - A mechanical switch operated by a solenoid.
Resealable safety vent - The resealable vent internal to a cell to release excessive internal pressure.
Reserve battery - Batteries which are stored in an inactive state without their electrolyte. They are only activated when needed by the introduction of the electrolyte. See also Water-activated batteries and Ampoule batteries.
Reserve capacity - The number of minutes at which the battery can be discharged at 25 Amps and maintain a terminal voltage higher than 1.75 volts per cell, on a new, fully charged battery at 80degrees Fahrenheit (27 °C). Defines a battery's ability to power a vehicle with an inoperative alternator or fan belt. Used for comparing automotive SLI batteries.
Resettable fuse - A fuse which protects against excessive current and temperature by interrupting the flow of current. After opening it will reset after the fault conditions have been removed but only after it has cooled. It requires no manual resetting or replacement. The "Polyswitch" is an example of this.
Residual Circuit Breaker (RCCB), or Residual Current Device (RCD) - an electrical safety device which interrupts a circuit whenever it detects that the current is not balanced between the live (high voltage) conductor and the return neutral conductor. It can be used as a safety device by cutting off the supply voltage when it detects current leakage through the body of a person who is earthed (grounded) accidentally touching a live part of the circuit. See also Ground Fault Interruptor / Earth Leakage Trip.
Resistance welding - Resistance welding is a process used to join metallic parts with electric current. There are several forms of resistance welding, including spot welding, seam welding, projection welding, and butt welding.
Rest periods - Interruptions to the charging process to allow the chemical reactions in the battery to stabilise.
Reversible reaction - A chemical reaction which can be reversed to reconstitute the original components.
RFI - Radio Frequency Interference. Transmitted/emitted RFI affects other external equipment. Susceptibility measures the immunity of equipment from received RFI. See also EMC and Electromagnetic Radiation
RFID - Radio Frequency Identification. Small tags incorporating a radio transmitter which can be used to identify or track items of value.
Rocking Chair Cell- A lithium ion cell
RS232 connection - A standard for serial transmission of data between two devices.
RS485 connection - A standard for serial transmission of data between multiple devices.

SAE - Society of Automotive Engineers. The SAE Technical Standards Board issues and recommends industry standards. See also Standards
Safety vent - A safety mechanism that is activated when the internal gas pressure rises above a normal level.
Sampling Rate - The repetition frequency at which digital samples are taken of an analogue quantity.
Sealed cells - A cell which remains closed and does not release gas or liquid when operated within the limits of charge and temperature specified by the manufacturer. An essential component in recombinant cells.
Secondary battery - A battery which can be recharged and used repeatedly.
Self-discharge - Capacity loss during storage due to the internal current leakage between the positive and negative plates.
SEM (Scanning Electron Microscope) - Apparatus used to investigate the physical structure of cell components and surfaces. They typically cost about $500,000 or more.
Semiconductor - An insulator whose conductivity can be manipulated by the addition of impurities ( doping ), by introduction of an electric field, by exposure to light , or by other means.
Separator - A non-conductive semi-permeable film or grid to separate 2 electrodes to prevent them from contacting each other and short-circuiting but which allows the passage of ions through it.
Series connection - The connection of, two or more, similar cells in a chain to form a battery of higher voltage by connecting the positive terminal of each cell to the to negative terminal of the next cell.
Series regulator - Another name for a Linear regulator
Service life - The period of useful life of a battery before a predetermined end point is reached.
Shaft encoder - An electro-mechanical or optical device which converts the angular position or motion of a shaft or axle to an analogue or digital electrical signal. Also called a rotary encoder.
Shedding - The loss of material from the plates of Lead Acid batteries.
Shelf life - The duration a cell can be kept in storage and still retain its ability to give a specified performance. See also Battery Storage
Shrouded terminals - Terminals surrounded by an insulating shroud which prevents accidental contact with the terminal.
Shunt - A device which allows electric current to pass around another point in the circuit.
Shunt regulator - A voltage regulator which uses a transistor or FET, in parallel with the load, which shorts out the excess voltage when the applied input voltage exceeds a specified limit producing a regulated output voltage. It is a simple but lossy design.
Shuttlecock cell - A lithium ion cell.
Sintering - Heating a mixture of powdered metals, sometimes under pressure, to the melting-point of the metal in the mixture which has the lowest melting-point, the melted metal binding together the harder particles.
SLA Battery - Sealed Lead Acid battery. In sealed batteries the generated oxygen combines chemically with the lead and then the hydrogen at the negative electrode, and then again with reactive agents in the electrolyte, to recreate water. A recombinant system. The net result is no significant loss of water from the cell. See also Flooded Lead Acid cell.
SLA - Equipment used for rapid prototyping. See StereoLithography Apparatus below.
SLI Battery - Common automotive battery used for Starting Lighting and Ignition
Slow charge - Charging overnight in 14 to 16 hours at about 0.1C rate. Safe and simple.
Smart Battery - An intelligent battery which contains information about its specification, its status and its usage profile which can be read by its charger or the application in which it is used.
SMBus - System Management Bus. A two wire, 100 KHz, serial bus for interconnecting Smart Batteries which have built in intelligence, with their associated chargers or applications.
Solar cell - A photovoltaic cell. Solar cells convert sunlight energy into electric current. They do not store energy.
Solar panel - An array of photocells providing an increased output.
Solenoid - A coil containing an iron plunger which moves when a current is passed through the coil.
Solid State Battery - Cells with solid electrolytes. Lithium polymer cells are examples of this technology
SOC - State of Charge. See below.
SOH - State of Health. See below.
Specific Energy - Same as Gravimetric Energy Density (Wh/Kg)
Specific Gravity SG - The ratio of the weight of a solution compared with the weight of an equal volume of water at a specified temperature. It is used to determine the charge condition in lead acid batteries.
Specific Power - Same as Gravimetric Power Density (W/Kg)
Spintronics - A technology used in solid state devices which exploits the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge. Also known as magnetoelectronics.
Spiral Wound - Battery construction in which the electrodes with the electrolyte and separator between them are rolled into a spiral like a jelly roll (Swiss roll).
Stacked Electrodes -
Standard charge - The normal C/10 charge used to recharge a cell or battery in 10 hours. Other definitions (charging periods) also apply.
Standby power - A fully charged battery ready to take over supplying a load in case of emergency.
Standby state - A state in which the main functions of a circuit have been powered down to save energy, but power remains applied to the circuit ready to make a rapid restart. (See also "Hibernation state" above)
State of Charge- SOC - The available capacity of a battery expressed as a percentage of its rated capacity.
State of Health- SOH - A measurement that reflects the general condition of a battery and its ability to deliver the specified performance compared with a fresh battery. It takes into account such factors as charge acceptance, internal resistance, voltage and self-discharge. It is not as precise as the SOC determination.
Stereolithography (SLA) - A Rapid Prototyping (RP) system for creating plastic parts directly from 3D CAD files. The RP model speeds design validation and is also finds use as a master pattern.
Stoichiometry - The branch of chemistry that deals with the numerical proportions in which substances react.
Storage life - The length of time a cell or battery can be stored on open circuit without permanent deterioration of its performance. See also Battery Storage
Studs - Threaded bolt connectors used on high power cells
Sulphation - Growth of lead sulphate crystals in Lead-Acid batteries which inhibits current flow. Sulphation is caused by storage at low state of charge.
Supercapacitor - A capacitor that can store a large amount of energy. Also called Ultracapacitor or Booster capacitor.
Superconductivity - A phenomenon occurring below a very low, characteristic critical temperature in certain materials (superconductors), characterised by the complete absence of electrical resistance and the damping of the interior magnetic field (the Meissner effect). Superconductors can carry currents that will not decay.
Swelling - Distortion of cells caused by expansion of the active chemicals due to temperature and pressure effects.
Swing cell - A lithium ion cell
Switcher - A switch mode regulator.
Switch mode charger - Charger which uses a switch mode regulator. More efficient but more costly than a Linear charger.
Switch mode regulator - A switching regulator is a voltage regulator which uses an output stage, switched repetitively on and off, together with energy storage components (capacitors and inductors) to generate a DC output voltage. Regulation is achieved through Pulse Width Modulation (PWM). Output voltages can be generated that are greater than or less than the input voltage, and multiple output voltages can be generated with a single regulator.

Tabs - Flat connectors used on pouch cells.
Tafel equation - The relationship between the internal electrode potentials in a battery and the current which flows. This is an exponential relationship based on empirical results which quantifies the elecrochemical reactions. It is analogous to the Arrhenius equationwhich quantifies the thermochemical process relating the temperature to the rate at which a chemical action progresses.
Taper charge - In quick chargers the charging current is is progressively reduced in a controlled way by controlling the supply voltage. In slow chargers the voltage is fixed and the charging current reduces in an uncontrolled way due to increase in the cell voltage as the charge builds up.
Temperature cut-off - A temperature sensing method which detects heat rise in a cell at overcharge and switches the charger off or to a lower rate of charge.
Temperature sensor - An electronic device which provides a voltage analogue of the temperature of the surface on which it is mounted. A thermistor is an example.
Termination voltage - The maximum voltage which can be tolerated by a cell during charging without damaging the cell. The cell voltage at which the charging process should be terminated.
Tesla (T) - The unit of magnetic flux density. 1 Tesla = 1 Weber / metre2
Thermal Capacity - The amount of energy required to raise the temperature of an object by one degree Celsius. Expressed in Joules/Kg.
Thermal fuse - A safety device which interrupts a circuit when it detects excessive temperature.
Thermal imaging - A photographic technique which displays the range of temperatures of a warm body in the form of a colour spectrum. Used as a design verification tool for detecting hot spots in battery and other equipment designs.
Thermal management - The means by which a battery is maintained within its operating temperature limits during charging and discharging.
Thermal runaway - A condition in which an electrochemical cell will overheat and destroy itself through internal heat generation. This may be caused by overcharge or high current discharge and other abusive conditions.
Thermistor - An electrical device whose resistance varies with temperature. They are used as temperature-measuring devices or in electrical circuits to compensate for temperature variations of other components.
Thyristor - Also called a Silicon-Controlled Rectifier or SCR, it is a solid-state high current semiconductor switching device similar to a diode, with an extra terminal which is used to turn it on. Once turned on, the thyristor will remain on (conducting) as long as there is a significant current flowing through it. If the current falls to zero, the device switches off. See also Power transistor.
Traction battery - A high power deep cycle secondary battery designed to power electric vehicles or heavy mobile equipment.
Transient response - The ability of an electrical or other device to respond faithfully to sudden changes to the input conditions.
Trickle charge - A continuous charge at low rate, balancing losses through local action and/or periodic discharge, to maintain a cell or battery in a fully charged condition. Normally at a C/20 to C/30 rate.
TÜV - TÜV Rheinland Group (TUV - Technical Inspection Association) is an international service company which documents the safety and quality of new and existing products, systems and services.

UL - Underwriters Laboratories Inc - (UL) is an independent, not-for-profit product safety testing and certification organization based in the USA. UL marking indicates that the product conforms with the safety standards laid down by Underwriters Laboratories.
Ultracapacitor - See "Supercapacitor" above.
Ultrasonic welding - Ultrasonic welding involves the use of high frequency sound energy to soften or melt the thermoplastic at the joint. Parts to be joined are held together under pressure and are then subjected to ultrasonic vibrations usually at a frequency of 20, 30 or 40kHz.
UPS - Uninterruptible Power Supply
USABC - The United States Advanced Battery Consortium

Valence - The combining capacity of an atom expressed as the number of single bonds the atom can form or the number of electrons an element gives up or accepts when reacting to form a compound.
Venting - The release of excessive internal pressure from a cell in a manner intended by design to preclude explosion.
Voltage cutoff - A voltage sensing device which will end a charge or discharge at a preset voltage value.
Voltage limit - A voltage value a battery is not permitted to rise above on charge and/or fall below on discharge
Voltage regulator - A circuit which provides a fixed or controlled voltage output from a variable voltage input. Used in power supplies and chargers. Switching regulators , Linear regulators and Shunt regulators are the most common types.
Voltaic efficiency - The ratio (expressed as a percentage) between the voltage necessary to charge a secondary cell and the corresponding discharge voltage.
Volumetric Energy Density (Wh/L) - The energy output per unit volume of a battery
Volumetric Power Density (W/L) -The power output per unit volume of a battery
VRLA battery - Valve Regulated Lead Acid Battery

Ward-Leonard controller - A motor-generator system which uses a AC motor driving a variable voltage DC generator which drives a DC motor to provide a variable power transmission. Used for high power load testing.
Watt - A unit of power, the rate of doing work. Watts = Amps X Volts = One Joule per second.
WattHours (Wh) - A measure of the energy capacity of a battery. The amount of work done in one hour. 
1 Wh = 3.6 kJ.
Weber (Wb) - The unit of the magniude of the magnetic flux. A flux density of one Wb/m2 (1 Weber per square meter) is 1 Tesla
Well to wheel efficiency - The ratio between the mechanical energy ultimately delivered to the road wheels of a vehicle and the chemical energy content of the oil consumed in providing it. It is used to compare the fuel efficiencies of different methods of powering road vehicles and takes into account the refining process, the energy loss in the distribution process (in the case of hydrogen, the energy used to compress it) and the conversion efficiency of the vehicle's power unit.
Wet Cell - A cell with free flowing liquid electrolyte.

X-ray Crystallography - The use of the property of X-ray diffraction by crystals to determine their physical structure.


Zapping - A desperation measure to revive a shorted cell suffering from dendrites. A very high current, low voltage pulse from a large capacitor used in an attempt to vaporise the dendrites.
Zebra battery - A high temperature Sodium Nickel Chloride battery delivering high power.
Galvanic Cell Component

The basic components of a battery are the electrodes with terminals to connect to the external circuit, a separator to keep the electrodes apart and prevent them from shorting, the electrolyte which carries the charged ions between the electrodes and a case to contain the active chemicals and hold the electrodes in place.


Each galvanic or energy cell consists of at least three and sometimes four components

1.         The anode or negative electrode is the reducing or fuel electrode. It gives up electrons to the external circuit and is oxidised during the elecrochemical (discharge) reaction. It is generally a metal or an alloy but hydrogen is also used. The anodic process is the oxidation of the metal reducing agent to form metal ions.

( LEO Lose Electrons - Oxidation)


(OIL - Oxidation is Loss)

2.         The cathode or positive electrode is the oxidising electrode. It accepts electrons from the external circuit and is reduced during the electrochemical (discharge) reaction. It is usually an metallic oxide or a sulfide but oxygen is also used. The cathodic process is the reduction of the oxidising agent (oxide) to leave the metal.

(GER Gain Electrons - Reduction). Remember the mnemonic of the lion growling.


(RIG - Reduction is Gain) Alternative mnemonic - OIL RIG

3.         The electrolyte (the ionic conductor) which provides the medium for transfer of charge as ions inside the cell between the anode and cathode. The electrolyte is typically a solvent containing dissolved chemicals providing ionic conductivity. It should be a non-conductor of electrons to avoid self discharge of the cell.

Metal ions are metal atoms missing electrons and are thus positively charged. Particles missing electrons are called cations and during discharge they move through the electrolyte towards the cathode.

Anions are atoms or particles with excess electrons and thus negatively charged. During discharge they are attracted towards the anode.

4.         The separator which electrically isolates the positive and negative electrodes.



5.         Electrodes: The electrodes material may be a rigid metallic grids as in Lead acid batteries or the active electrode material may impregnated into or coated onto a spiral rolled metallic foil which simply acts as a current collector as in many Nickel and Lithium based cells. See also Battery Manufacturing

6.         Separator: The separator may be a mechanical spacer, fibreglass cloth or a flexible plastic film made from nylon, polyethylene or polypropylene. It must be porous and very thin to permit the charged ions to pass without impediment and it should take up the minimum of space to allow for the maximum use of the available space for the active chemicals. At the same time it must be resistant to penetration by burrs or dendrite growths on the electrode plates or from contamination of the electrode coating to prevent the possibility of short circuits between the electrodes. These characteristics should be maintained at high operating temperature when softening of the plastic material could clog the pores or reduce its resistance to penetration. The breakdown or penetration of the separator is a potential area of weakness in high power cells and special separator materials have been developed to overcome this problem.

7.         Terminals: There are many ways of connecting to the electrodes ranging from spring contacts, through wires or tags to mechanical studs. The main requirement is that the terminals should be able to handle the maximum current without overheating, either the terminal itself or the electrode connected to it. This needs careful design of the connection to the electrodes to take off the current through the maximum possible area of electrode material so as not to cause any hot spots. See also notes about external connections in the section on Battery Pack Design.

8.         Electrolyte: For many years all electrolytes were in aqueous or gel form. Recently solid polymer electrolytes have been developed which do not suffer from leakage or spillage. As well as being safer in case of an accident and they also bring new degrees of freedom to cell design allowing mechanical designs to be shaped to fit into odd shaped cavities. Polymer electrolytes are typically used in Lithium batteries.

Battery Energy Density

The energy density is a measure of the amount of energy per unit weight or per unit volume which can be stored in a battery. Thus for a given weight or volume a higher energy density cell chemistry will store more energy or alternatively for a given storage capacity a higher energy density cell will be smaller and lighter. The chart below shows some typical examples.

In general higher energy densities are obtained by using more reactive chemicals. The downside is that more reactive chemicals tend to be unstable and may require special safety precautions. The energy density is also dependent on the quality of the active materials used in cell construction with impurities limiting the cell capacities which can be achieved. This is why cells from different manufacturers with similar cell chemistries and similar construction may have a different energy content and discharge performance.


Note that there is often a difference between cylindrical and prismatic cells. This is because the quoted energy density does not usually refer to the chemicals alone but to the whole cell, taking into account the cell casing materials and the connections. Energy density is thus influenced or limited by the practicalities of cell construction.

Battery Definition

Lithium-ion Polymer

Lithium-ion polymer batteries use liquid Lithium-ion electrochemistry in a matrix of ion conductive polymers that eliminate free electrolyte within the cell. The electrolyte thus plasticizes the polymer, producing a solid electrolyte that is safe and leak resistant. Lithium polymer cells are often called Solid State cells.


Because there's no liquid, the solid polymer cell does not require the heavy protective cases of conventional batteries. The cells can be formed into flat sheets or prismatic (rectangular) packages or they can be made in odd shapes to fit whatever space is available. As a result, manufacturing is simplified and batteries can be packaged in a foil. This provides added cost and weight benefits and design flexibility. Additionally, the absence of free liquid makes Lithium-ion polymer batteries more stable and less vulnerable to problems caused by overcharge, damage or abuse.


Solid electrolyte cells have long storage lives, but low discharge rates.


There are some limitations on the cell construction imposed by the thicker solid electrolyte separator which limits the effective surface area of the electrodes and hence the current carrying capacity of the cell, but at the same time the added volume of electrolyte provides increased energy storage. This makes them ideal for use in high capacity low power applications.


Despite the above comments there are some manufacturers who make cells designated as Lithium polymer which actually contain a liquid or a gel. Such cells are more prone to swelling than genuine solid polymer cells.



Pouch cell - Also known as Lipo cells

Vulnerable, Inexpensive, Design freedom on dimensions, Difficult packaging, High energy density but reduced by support packaging needed, Prone to swell and leak, Less danger of explosion (cell bursts), Good heat dissipation, Made in very high volumes, Economical for small volumes, Sizes up to 240 Ah


Pouch casings are typically used for Lithium Polymer cells with solid electrolytes, providing a low cost "flexible" (sometimes in unintended ways) construction. The electrodes and the solid electrolyte are usually stacked in layers or laminations and enclosed in a foil envelope. The solid electrolyte permits safer, leak-proof cells. The foil construction allows very thin and light weight cell designs suitable for high power applications but because of the lack of rigidity of the casing the cells are prone to swelling as the cell temperature rises. Allowance must be made for the possibility of swelling when choosing cells to fit a particular cavity specified for the battery compartment. The cells are also vulnerable to external mechanical damage and battery pack designs should be designed to prevent such possibilities.

The Honcell example illustrated uses spiral wound electrodes and a solid polymer electrolyte.


This construction, using stacked electrodes is suitable for making odd shaped cells but few applications make use of this opportunity.

Battery Cost

The price of Lithium cells continues to fall as the technology gains more acceptance.

The target price for high power cells is around $300/kWh but cell makers are still quite some way from achieving that.

Although Lithium secondary batteries may cost two or three times more than the cost of equivalent Lead acid batteries and even more when the necessary battery management electronics are thaken into account, this is more than compensated for by their longer cycle life which may be five to ten times the life of Lead acid batteries. Valid cost comparisons should therefore take into account the lifetime costs as well as the initial capital costs.

Battery Connection in Series or Parallel

Serial and Parallel Battery Configurations

Battery packs achieve the desired operating voltage by connecting several cells in series, with each cell adding to the total terminal voltage. Parallel connection attains higher capacity for increased current handling, as each cell adds to the total current handling. Some packs may have a combination of serial and parallel connections. Laptop batteries commonly have four 3.6V Li-ion cells in series to achieve 14.4V and two strings of these 4 cells in parallel (for a pack total of 8 cells) to boost the capacity from 2,400mAh to 4,800mAh. Such a configuration is called 4S2P, meaning 4 cells are in series and 2 strings of these in parallel. Insulating foil between the cells prevents the conductive metallic skin from causing an electrical short. The foil also shields against heat transfer should one cell get hot.

Most battery chemistries allow serial and parallel configuration. It is important to use the same battery type with equal capacity throughout and never mix different makes and sizes. A weaker cell causes an imbalance. This is especially critical in a serial configuration and a battery is only as strong as the weakest link.

Imagine a chain with strong and weak links. This chain can pull a small weight but when the tension rises, the weakest link will break. The same happens when connecting cells with different capacities in a battery. The weak cells may not quit immediately but get exhausted more quickly than the strong ones when in continued use. On charge, the low cells fill up before the strong ones and get hot; on discharge the weak are empty before the strong ones and they are getting stressed.

Single Cell Applications

The single-cell design is the simplest battery pack. A typical example of this configuration is the cellular phone battery with a 3.6V lithium-ion cell. Other uses of a single cell are wall clocks, which typically use a 1.5V alkaline cell, as well as wristwatches and memory backup.

The nominal cell voltage of nickel is 1.2V. There is no difference between the 1.2V and 1.25V cell; the marking is simply preference. Whereas consumer batteries use 1.2V/cell as the nominal rating, industrial, aviation and military batteries adhere to the original 1.25V. The alkaline delivers 1.5V, silver-oxide 1.6V, lead acid 2V, primary lithium 3V, Li-phosphate 3.3V and regular lithium-ion 3.6V. Li-manganese and other lithium-based systems sometimes use 3.7V. This has nothing to do with electrochemistry and these batteries can serve as 3.6V cells. Manufacturers like to use a higher voltage because low internal resistance causes less of a voltage drop with a load. Read more: Confusion with Voltages

Serial Connection

Portable equipment needing higher voltages use battery packs with two or more cells connected in series. Figure 3-8 shows a battery pack with four 1.2V nickel-based cells in series to produce 4.8V. In comparison, a four-cell lead acid string with 2V/cell will generate 8V, and four Li-ion with 3.6V/cell will give 14.40V. If you need an odd voltage of, say, 9.5 volts, you can connect five lead acid, eight NiMH/NiCd), or three Li-ion in series. The end battery voltage does not need to be exact as long as it is higher than what the device specifies. A 12V supply should work; most battery-operated devices can tolerate some over-voltage.

Figure 1: Serial connection of four NiCd or NiMH cells 
Adding cells in a string increases the voltage; the current remains the same.

Courtesy of Cadex

A higher voltage has the advantage of keeping the conductor size small. Medium-priced cordless power tools run on 12V and 18V batteries; high-end power tools use 24V and 36V. The car industry talked about increasing the starter battery from 12V (14V) to 36V, better known as 42V, by placing 18 lead acid cells in series. Logistics of changing the electrical components and arcing problems on mechanical switches derailed the move. Early hybrid cars run on 148V batteries; newer models have batteries with 450–500V. Such a high-voltage battery requires 400 nickel-based cells in series. Li-ion cuts the cell count by three.

High-voltage batteries require careful cell matching, especially when drawing heavy loads or when operating in cold temperatures. With so many cells in series, the possibility of one failing is real. One open cell would break the circuit and a shorted one would lower the overall voltage.

Cell matching has always been a challenge when replacing a faulty cell in an aging pack. A new cell has a higher capacity than the others, causing an imbalance. Welded construction adds to the complexity of repair and for these reasons, battery packs are commonly replaced as a unit when one cell fails. High-voltage hybrid batteries, in which a full replacement would be prohibitive, divide the pack into blocks, each consisting of a specific number of cells. If one cell fails, the affected block is replaced.

Figure 2 illustrates a battery pack in which “cell 3” produces only 0.6V instead of the full 1.2V. With depressed operating voltage, this battery reaches the end-of-discharge point sooner than a normal pack and the runtime will be severely shortened. The remaining three cells are unable to deliver their stored energy when the equipment cuts off due to low voltage. The cause of cell failure can be a partial short cell that consumes its own charge from within through elevated self-discharge, or a dry-out in which the cell has lost electrolyte by a leak or through inappropriate usage.

Figure 2: Serial connection with one faulty cell
Faulty “cell 3” lowers the overall voltage from 4.8V to 4.2V, causing the equipment to cut off prematurely. The remaining good cells can no longer deliver the energy.

Courtesy of Cadex

Parallel Connection

If higher currents are needed and larger cells with increased ampere-hour (Ah) ratings are not available or the design has constraints, one or more cells are connected in parallel. Most chemistries allow parallel configurations with little side effect. Figure 3 illustrates four cells connected in parallel. The voltage of the illustrated pack remains at 1.2V, but the current handling and runtime are increased fourfold.



Figure 3: Parallel connection of four cells

With parallel cells, the current handling and runtime increases while voltage stays the same.

Courtesy of Cadex

A high-resistance cell, or one that is open, is less critical in a parallel circuit than in serial configuration, however, a weak cell reduces the total load capability. It’s like an engine that fires on only three cylinders instead of all four. An electrical short, on the other hand, could be devastating because the faulty cell would drain energy from the other cells, causing a fire hazard. Most so-called shorts are of mild nature and manifest themselves in elevated self-discharge. Figure 4 illustrates a parallel configuration with one faulty cell.


Figure 4: Parallel/connection with one faulty cell

A weak cell will not affect the voltage but will provide a low runtime due to reduced current handling. A shorted cell could cause excessive heat and become a fire hazard.

Courtesy of Cadex

Serial/Parallel Connection

The serial/parallel configuration shown in Figure 5 allows superior design flexibility and achieves the wanted voltage and current ratings with a standard cell size. The total power is the product of voltage times current, and the four 1.2V/1000mAh cells produce 4.8Wh. Serial/parallel connections are common with lithium-ion, especially for laptop batteries, and the built-in protection circuit must monitor each cell individually. Integrated circuits (ICs) designed for various cell combinations simplify the pack design.



Figure 5: Serial/ parallel connection of four cells

This configuration provides maximum design flexibility.

Courtesy of Cadex

Simple Guidelines for Using Household Primary Batteries

  • Keep the battery contacts clean. A four-cell configuration has eight contacts (cell to holder and holder to next cell); each contact adds resistance.
  • Never mix batteries; replace all cells when weak. The overall performance is only as good as the weakest link in the chain.
  • Observe polarity. A reversed cell subtracts rather than adds to the cell voltage.
  • Remove batteries from the equipment when no longer in use to prevent leakage and corrosion. While spent alkaline normally do not leak, spent carbon-zinc discharge corrosive acid that can destroy the device.
  • Don’t store loose cells in a metal box. Place individual cells in small plastic bags to prevent an electrical short. Don’t carry loose cells in your pockets.
  • Keep batteries away from small children. If swallowed, the current flow of the battery can ulcerate the stomach wall.The battery can also rupture and cause poisoning.
  • Do not recharge non-rechargeable batteries; hydrogen buildup can lead to an explosion. Perform experimental charging only under supervision.

Simple Guidelines for Using Household Secondary Batteries

  • Observe polarity when charging a secondary cell. Reversed polarity can cause an electrical short that can lead to heat and fire if left unattended.
  • Remove fully charged batteries from the charger. A consumer charger may not apply the optimal trickle charge and the cell could be stressed with overcharge.
Charging Battery From a USB Port

Charging from a USB Port

The Universal Serial Bus (USB) was introduced in 1996 and has since become one of the most widespread and convenient interfaces for electronic devices. The USB port is a bi-directional data port that provides a supply voltage to power memory sticks, keyboards, mice, wireless interfaces, cameras, MP3 players and chargers.

With 5V and 500mA of available current, the USB bus can charge a small single-cell Li-ion pack, but there is a danger of overloading the USB hub when attaching too many gadgets. Plugging in a charger that draws 500mA along with other devices will exceed the port’s current limit, leading to a voltage drop and a possible system failure. To prevent overload, some hosts include current-limiting circuits that shut down the supply when overdrawn. Another method is limiting the current of all attachments to 400mA to reserve 100mA for housekeeping.

The most common USB chargers are designed for single-cell Li-ion. The charge begins with a constant current charge to 4.20V/cell, at which point the voltage caps and the current begins to decrease. Due to a voltage drop in the cable, which is about 350mV, and losses in the charger circuit, it is possible that the 5V supply cannot supply the battery’s 4.2V charge threshold. This is no problem; the battery does not suffer but will deliver shorter than expected runtimes.

The rectangular Type A USB plug has four connector pins and a shield. The rightmost contact is number 1 and carries 5V; the leftmost contact is number 4 and forms the ground. The two shorter pins in the middle are reserved for data transfer and have no function in the USB charger. Figure 1 illustrates the rectangular Type A USB plug.

Figure 1: Rectangular Type A USB plug

The rightmost contact is number 1 and carries +5VDC; the leftmost pin is number 4 and is the ground. The housing connects to the ground and provides shielding. Pins 2 and 3 carry data.

With the USB supply current limited to 500mAh, doing active work on a laptop or watching a video on a tablet with a bright screen can result in a net discharge. A larger internal load than what the charger can provide will gradually drain the battery; however, the pack will replenish itself when the activity ends. 

Some USB chargers plugging into the AC main or the cigarette lighter of a car deliver higher peak currents than 500mAh. This allows connecting several devices via a USB bar without causing overload.

Note that the USB port is unidirectional and cannot take power from an outside source. In other words, power only flows out.