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Battery Chargers and Charging Method

Charging Schemes

The charger has three key functions

·                     Getting the charge into the battery (Charging)

·                     Optimising the charging rate (Stabilising)

·                     Knowing when to stop (Terminating)


The charging scheme is a combination of the charging and termination methods.


Charge Termination

Once a battery is fully charged, the charging current has to be dissipated somehow. The result is the generation of heat and gasses both of which are bad for batteries. The essence of good charging is to be able to detect when the reconstitution of the active chemicals is complete and to stop the charging process before any damage is done while at all times maintaining the cell temperature within its safe limits. Detecting this cut off point and terminating the charge is critical in preserving battery life. In the simplest of chargers this is when a predetermined upper voltage limit, often called the termination voltage has been reached. This is particularly important with fast chargers where the danger of overcharging is greater.


Safe Charging

If for any reason there is a risk of over charging the battery, either from errors in determining the cut off point or from abuse this will normally be accompanied by a rise in temperature. Internal fault conditions within the battery or high ambient temperatures can also take a battery beyond its safe operating temperature limits. Elevated temperatures hasten the death of batteries and monitoring the cell temperature is a good way of detecting signs of trouble from a variety of causes. The temperature signal, or a resettable fuse, can be used to turn off or disconnect the charger when danger signs appear to avoid damaging the battery. This simple additional safety precaution is particularly important for high power batteries where the consequences of failure can be both serious and expensive.


Charging Times

During fast charging it is possible to pump electrical energy into the battery faster than the chemical process can react to it, with damaging results.

The chemical action can not take place instantaneously and there will be a reaction gradient in the bulk of the electrolyte between the electrodes with the electrolyte nearest to the electrodes being converted or "charged" before the electrolyte further away. This is particularly noticeable in high capacity cells which contain a large volume of electrolyte.

There are in fact at least three key processes involved in the cell chemical conversions.

·                     One is the "charge transfer", which is the actual chemical reaction taking place at the interface of the electrode with the electrolyte and this proceeds relatively quickly.

·                     The second is the "mass transport" or "diffusion" process in which the materials transformed in the charge transfer process are moved on from the electrode surface, making way for further materials to reach the electrode to take part in the transformation process. This is a relatively slow process which continues until all the materials have been transformed.

·                     The charging process may also be subject to other significant effects whose reaction time should also be taken into account such as the "intercalation process" by which Lithium cells are charged in which Lithium ions are inserted into the crystal lattice of the host electrode. See also Lithium Plating due to excessive charging rates or charging at low temperatures.

All of these processes are also temperature dependent.


In addition there may be other parasitic or side effects such as passivation of the electrodes, crystal formation and gas build up, which all affect charging times and efficiencies, but these may be relatively minor or infrequent, or may occur only during conditions of abuse. They are therefore not considered here.


The battery charging process thus has at least three characteristic time constants associated with achieving complete conversion of the active chemicals which depend on both the chemicals employed and on the cell construction. The time constant associated with the charge transfer could be one minute or less, whereas the mass transport time constant can be as high as several hours or more in a large high capacity cell. This is one of the the reasons why cells can deliver or accept very high pulse currents, but much lower continuous currents.(Another major factor is the heat dissipation involved). These phenomena are non linear and apply to the discharging process as well as to charging. There is thus a limit to the charge acceptance rate of the cell. Continuing to pump energy into the cell faster than the chemicals can react to the charge can cause local overcharge conditions including polarisation, overheating as well as unwanted chemical reactions, near to the electrodes thus damaging the cell. Fast charging forces up the rate of chemical reaction in the cell (as does fast discharging) and it may be necessary to allow "rest periods" during the charging process for the chemical actions to propagate throughout the bulk of the chemical mass in the cell and to stabilise at progressive levels of charge.


See also the affects of Chemical Changes and Charging Rate in the section on Battery Life.


A memorable though not quite equivalent phenomenon is the pouring of beer into a glass. Pouring very quickly results in a lot of froth and a small amount of beer at the bottom of the glass. Pouring slowly down the side of the glass or alternatively letting the beer settle till the froth disperses and then topping up allows the glass to be filled completely.



The time constants and the phenomena mentioned above thus give rise to hysteresis in the battery. During charging the chemical reaction lags behind the application of the charging voltage and similarly, when a load is applied to the battery to discharge it, there is a delay before the full current can be delivered through the load. As with magnetic hysteresis, energy is lost during the charge discharge cycle due to the chemical hysteresis effect.


The diagram below shows the hystersis effect in a Lithium battery.



Allowing short settling or rest periods during the charge discharge processes to accommodate the chemical reaction times will tend to reduce but not eliminte the voltage difference due to hysteresis.

The true battery voltage at any state of charge (SOC) when the battery is in its "at rest" or quiecent condition will be somewhere between the charge and discharge curves. During charging the measured cell voltage during a rest period will migrate slowly downwards towards the quiescent condition as the chemical transformation in the cell stabilises. Similarlly during discharging, the measured cell voltage during a rest period will migrate upwards towards the quescent condition.


Fast charging also causes increased Joule heating of the cell because of the higher currents involved and the higher temperature in turn causes an increase in the rate of the chemical conversion processes.


The section on Discharge Rates shows how the effective cell capacity is affected by the discharge rates.

The section on Cell Construction describes how the cell designs can be optimised for fast charging.


Charge Efficiency

This refers to the properties of the battery itself and does not depend on the charger. It is 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.


Charge acceptance and charge time are considerably influenced by temperature as noted above. Lower temperature increases charge time and reduces charge acceptance.


Note that at low temperatures the battery will not necessarily receive a full charge even though the terminal voltage may indicate full charge. See Factors Influencing State of Charge.


Basic Charging Methods

·                     Constant Voltage A constant voltage charger is basically a DC power supply which in its simplest form may consist of a step down transformer from the mains with a rectifier to provide the DC voltage to charge the battery. Such simple designs are often found in cheap car battery chargers. The lead-acid cells used for cars and backup power systems typically use constant voltage chargers. In addition, lithium-ion cells often use constant voltage systems, although these usually are more complex with added circuitry to protect both the batteries and the user safety.

·                     Constant Current Constant current chargers vary the voltage they apply to the battery to maintain a constant current flow, switching off when the voltage reaches the level of a full charge. This design is usually used for nickel-cadmium and nickel-metal hydride cells or batteries.

·                     Taper Current This is charging from a crude unregulated constant voltage source. It is not a controlled charge as in V Taper above. The current diminishes as the cell voltage (back emf) builds up. There is a serious danger of damaging the cells through overcharging. To avoid this the charging rate and duration should be limited. Suitable forSLAbatteries only.

·                     Pulsed charge Pulsed chargers feed the charge current to the battery in pulses. The charging rate (based on the average current) can be precisely controlled by varying the width of the pulses, typically about one second. During the charging process, short rest periods of 20 to 30 milliseconds, between pulses allow the chemical actions in the battery to stabilise by equalising the reaction throughout the bulk of the electrode before recommencing the charge. This enables the chemical reaction to keep pace with the rate of inputting the electrical energy. It is also claimed that this method can reduce unwanted chemical reactions at the electrode surface such as gas formation, crystal growth and passivation. (See also Pulsed Charger below). If required, it is also possible to sample the open circuit voltage of the battery during the rest period.


The optimum current profile depends on the cell chemistry and construction.


·                     Burp charging Also called Reflex or Negative Pulse Charging Used in conjunction with pulse charging, it applies a very short discharge pulse, typically 2 to 3 times the charging current for 5 milliseconds, during the charging rest period to depolarise the cell. These pulses dislodge any gas bubbles which have built up on the electrodes during fast charging, speeding up the stabilisation process and hence the overall charging process. The release and diffusion of the gas bubbles is known as "burping". Controversial claims have been made for the improvements in both the charge rate and the battery lifetime as well as for the removal of dendrites made possible by this technique. The least that can be said is that "it does not damage the battery".

·                     IUI Charging This is a recently developed charging profile used for fast charging standard flooded lead acid batteries from particular manufacturers. It is not suitable for all lead acid batteries. Initially the battery is charged at a constant (I) rate until the cell voltage reaches a preset value - normally a voltage near to that at which gassing occurs. This first part of the charging cycle is known as the bulk charge phase. When the preset voltage has been reached, the charger switches into the constant voltage (U) phase and the current drawn by the battery will gradually drop until it reaches another preset level. This second part of the cycle completes the normal charging of the battery at a slowly diminishing rate. Finally the charger switches again into the constant current mode (I) and the voltage continues to rise up to a new higher preset limit when the charger is switched off. This last phase is used to equalise the charge on the individual cells in the battery to maximise battery life. See Cell Balancing.

·                     Trickle charge Trickle charging is designed to compensate for the self discharge of the battery. Continuous charge. Long term constant current charging for standby use. The charge rate varies according to the frequency of discharge. Not suitable for some battery chemistries, e.g. NiMH and Lithium, which are susceptible to damage from overcharging. In some applications the charger is designed to switch to trickle charging when the battery is fully charged.

·                     Float charge. The battery and the load are permanently connected in parallel across the DC charging source and held at a constant voltage below the battery's upper voltage limit. Used for emergency power back up systems. Mainly used with lead acid batteries.

·                     Random charging All of the above applications involve controlled charge of the battery, however there are many applications where the energy to charge the battery is only available, or is delivered, in some random, uncontrolled way. This applies to automotive applications where the energy depends on the engine speed which is continuously changing. The problem is more acute in EV and HEV applications which use regenerative braking since this generates large power spikes during braking which the battery must absorb. More benign applications are in solar panel installations which can only be charged when the sun is shining. These all require special techniques to limit the charging current or voltage to levels which the battery can tolerate.


Charging Rates

Batteries can be charged at different rates depending on the requirement. Typical rates are shown below:

·                     Slow Charge = Overnight or 14-16 hours charging at 0.1C rate

·                     Quick Charge = 3 to 6 Hours charging at 0.3C rate

·                     Fast Charge = Less than 1 hour charging at 1.0C rate


Slow charging

Slow charging can be carried out in relatively simple chargers and should not result in the battery overheating. When charging is complete batteries should be removed from the charger.

·                     Nicads are generally the most robust type with respect to overcharging and can be left on trickle charge for very long periods since their recombination process tends to keep the voltage down to a safe level. The constant recombination keeps internal cell pressure high, so the seals gradually leak. It also keeps the cell temperature above ambient, and higher temperatures shorten life. So life is still better if you take it off the charger.

·                     Lead acid batteries are slightly less robust but can tolerate a short duration trickle charge. Flooded batteries tend to use up their water, and SLAs tend to die early from grid corrosion. Lead-acids should either be left sitting, or float-charged (held at a constant voltage well below the gassing point).

·                     NiMH cells on the other hand will be damaged by prolonged trickle charge.

·                     Lithium ion cells however can not tolerate overcharging or overvoltage and the charge should be terminated immediately when the upper voltage limit is reached.


Fast / Quick Charging

As the charging rate increases, so do the dangers of overcharging or overheating the battery. Preventing the battery from overheating and terminating the charge when the battery reaches full charge become much more critical. Each cell chemistry has its own characteristic charging curve and battery chargers must be designed to detect the end of charge conditions for the specific chemistry involved. In addition, some form of Temperature Cut Off (TCO) or Thermal Fuse must be incorporated to prevent the battery from overheating during the charging process.


Fast charging and quick charging require more complex chargers. Since these chargers must be designed for specific cell chemistries, it is not normally possible to charge one cell type in a charger that was designed for another cell chemistry and damage is likely to occur. Universal chargers, able to charge all cell types, must have sensing devices to identify the cell type and apply the appropriate charging profile.


Note that for automotive batteries the charging time may be limited by the available power rather than the battery characteristics. Domestic 13 Amp ring main circuits can only deliver 3KW. Thus, assuming no efficiency loss in the charger, a ten hour charge will at maximum put 30 KWh of energy into the battery. Enough for about 100 miles. Compare this with filling a car with petrol.

It takes about 3 minutes to put enough chemical energy into the tank to provide 90 KWh of mechanical energy, sufficient to take the car 300 miles. To put 90 KWh of electrical energy into a battery in 3 minutes would be equivalent to a charging rate of 1.8 MegaWatts!!


Charge Termination Methods

The following chart summarises the charge termination methods for popular batteries. These are explained in the section below.




Charge Termination Methods





Slow Charge

Trickle OK

Tolerates Trickle


Voltage Limit

Fast Charge 1




Imin at Voltage Limit

Fast Charge 2

Delta TCO




Back up Termination 1





Back up Termination 2






TCO = Temperature Cut Off

Delta TCO = Temperature rise above ambient

I min = Minimum current


Charge Control Methods

Many different charging and termination schemes have been developed for different chemistries and different applications. The most common ones are summarised below.


Controlled charging

Regular (slow) charge

·     Semi constant current Simple and economical. Most popular. Low current therefore does not generate heat but is slow, 5 to 15 hours typical. Charge rate 0.1C. Suitable for Nicads

·     Timer controlled charge system Simple and economical. More reliable than semi-constant current. Uses IC timer. Charges at 0.2C rate for a predetermined period followed by trickle charge of 0.05C. Avoid constantly restarting timer by taking the battery in and out of the charger since this will compromise its effectiveness. The incorporation of an absolute temperature cut-off is recommended. Suitable for Nicad and NiMH batteries.


Fast charge (1 to 2 hours)

·      Negative delta V (NDV) Cut-off charge system

This is the most popular method for rapid charging for Nicads.


Batteries are charged at constant current of between 0.5 and 1.0 C rate. The battery voltage rises as charging progresses to a peak when fully charged then subsequently falls. This voltage drop, -delta V, is due to polarisation or oxygen build up inside the cell which starts to occur once the cell is fully charged. At this point the cell enters the overcharge danger zone and the temperature begins to rise rapidly since the chemical changes are complete and the excess electrical energy is converted into heat. The voltage drop occurs regardless of the discharge level or ambient temperature and it can therefore be detected and used to identify the peak and hence to cut off the charger when the battery has reached its full charge or switch to trickle charge.

This method is not suitable for charging currents less than 0.5 C since delta V becomes difficult to detect. False delta V can occur at the start of the charge with excessively discharged cells. This is overcome by using a timer to delay the detection of delta V sufficiently to avoid the problem. Lead acid batteries do not demonstrate a voltage drop on charge completion hence this charging method is not suitable forSLAbatteries.


·     dT/dt Charge system NiMH batteries do not demonstrate such a pronounced NDV voltage drop when they reach the end of the charging cycle as can be seen in the graph above and so the NDV cut off method is not reliable for ending the NiMH charge. Instead the charger senses the rate of increase of the cell temperature per unit time. When a predetermined rate is reached the rapid charge is stopped and the charge method is switched to trickle charge. This method is more expensive but avoids overcharge and gives longer life. Because extended trickle charging can damage a NiMH battery, the use of a timer to regulate the total charging time is recommended.


·     Constant-current Constant-voltage (CC/CV) controlled charge system. Used for charging Lithium and some other batteries which may be vulnerable to damage if the upper voltage limit is exceeded. The manufacturers' specified constant current charging rate is the maximum charging rate which the battery can tolerate without damaging the battery. Special precautions are needed to maximise the charging rate and to ensure that the battery is fully charged while at the same time avoiding overcharging. For this reason it is recommended that the charging method switches to constant voltage before the cell voltage reaches its upper limit. Note that this implies that chargers for Lithium Ion cells must be capable of controlling both the charging current and the battery voltage.

In order to mainain the specified constant current charging rate, the charging voltage must increase in unison with the cell voltage to overcome the back EMF of the cell as it charges up. This occurs quite rapidly during the constant current mode until the cell upper voltage limit of the cell is reached, after which point the charging voltage is maintained at that level, known as the float level, during the constant voltage mode. During this constant voltage period, the current decreases to a trickle charge as the charge approaches completion. Cut off occurs when a predetermined minimum current point, which indicates a full charge, has been reached. See also Lithium Batteries - Charging and Battery Manufacturing - Formation.


Note 1: When Fast Charging rates are specified, they usually refer to the constant current mode. Depending on the cell chemistry this period could be between 60% and 80% of the time to full charge. These rates should not be extrapolated to estimate the time to fully charge the battery because the charging rate tails off quickly during the constant voltage period.

Note 2: Because it is not possible to charge Lithium batteries at the charging C rate specified by the manufacturers for the full duration of the charge, it is also not possible to estimate the time to charge a battery from empty simply by dividing the AmpHour capacity of the battery by the specified charging C rate, since the rate changes during the charging process. The following equation however gives a reasonable approximation of the time to fully charge an empty battery when the standard CC/CV charging method is used:

Charging time (hrs) = 1.3 * (Batterycapacity in Ah) / (CC mode charging current)


·     Voltage controlled charge system. Fast charging at rates between 0.5 and 1.0 C rate. The charger switched off or switched to trickle charge when predetermined voltage has been reached. Should be combined with temperature sensors in the battery to avoid overcharge or thermal runaway.

·     V- Taper controlled charge system Similar to Voltage controlled system. Once a predetermined voltage has been reached the rapid charge current is progressively reduced by reducing the supply voltage then switched to trickle charge. Suitable forSLAbatteries it allows higher charge level to be reached safely. (See also taper current below)

·     Failsafe timer

Limits the amount of charge current that can flow to double the cell capacity. For example for a 600mAh cell, limit the charge to a maximum of 1,200mAH. Last resort if cut off not achieved by other means.

·     Pre-charging

As a safety precaution with high capacity batteries a pre-charging stage is often used. The charging cycle is initiated with a low current. If there is no corresponding rise in the battery voltage it indicates that there is possibly a short circuit in the battery.

·     Intelligent Charging System
Intelligent charging systems integrate the control systems within the charger with the electronics within the battery to allow much finer control over the charging process. The benefits are faster and safer charging and battery longer cycle life. Such a system is described in the section on Battery Management Systems.



Most chargers provided with consumer electronics devices such as mobile phones and laptop computers simply provide a fixed voltage source. The required voltage and current profile for charging the battery is provided (or should be provided) from electronic circuits, either within the device itself or within the battery pack, rather than by the charger. This allows flexibility in the choice of chargers and also serves to protect the device from potential damage from the use of inappropriate chargers.


Voltage Sensing

During charging, for simplicity, the battery voltage is usually measured across the charger leads. However for high current chargers, there can be a significant voltage drop along the charger leads, resulting in an underestimate of the true battery voltage and consequent undercharging of the battery if the battery voltage is used as the cut-off trigger. The solution is to measure the voltage using a separate pair of wires connected directly across the battery terminals. Since the voltmeter has a high internal impedance there will be minimal voltage drop in the voltmeter leads and the reading will be more accurate. This method is called a Kelvin Connection. See also DC Testing.


Charger Types

Chargers normally incorporate some form of voltage regulation to control the charging voltage applied to the battery. The choice of charger circuit technology is usually a price - performance trade off. Some examples follow:

·     Switch Mode Regulator (Switcher) - Uses pulse width modulation to control the voltage. Low power dissipation over wide variations in input and battery voltage. More efficient than linear regulators but more complex.
Needs a large passive LC (inductor and capacitor) output filter to smooth the pulsed waveform. Component size depends on curent handling capacity but can be reduced by using a higher switching frequency, typically 50 kHz to 500kHz., since the size of the required transformers, inductors and capacitors is inversely proportional to the operating frequency.
Switching heavy currents gives rise to EMI and electrical noise.

·     Series Regulator (Linear) - Less complex but more lossy - requiring a heat sink to dissipate the heat in the series, voltage dropping transistor which takes up the difference between the supply and the output voltage. All the load current passes through the regulating transistor which consequently must be a high power device. Because there is no switching, it delivers pure DC and doesn't need an output filter. For the same reason, the design doesn't suffer from the problem of radiated and conducted emissions and electrical noise. This makes it suitable for low noise wireless and radio applications.
With fewer components they are also smaller.

·     Shunt Regulator - Shunt regulators are common in photovoltaic (PV) systems since they are relatively cheap to build and simple to design. The charging current is controlled by a switch or transistor connected in parallel with the photovoltaic panel and the storage battery. Overcharging of the battery is prevented by shorting (shunting) the PV output through the transistor when the voltage reaches a predetermined limit. If the battery voltage exceeds the PV supply voltage the shunt will also protect the PV panel from damage due to reverse voltage by discharging the battery through the shunt. Series regulators usually have better control and charge characteristics.

·     Buck Regulator A switching regulator which incorporates a step down DC-DC converter. They have high efficiency and low heat losses. They can handle high output currents and generate less RF interference than a conventional switch mode regulator. A simple transformerless design with low switch stress and a small output filter.

·     Pulsed Charger. Uses a series transistor which can also be switched. With low battery voltages the transistor remains on and conducts the source current directly to the battery. As the battery voltage approaches the desired regulation voltage the series transistor pulses the input current to maintain the desired voltage. Because it acts as a switch mode supply for part of the cycle it dissipates less heat and because it acts as a linear supply part of the time the output filters can be smaller. Pulsing allows the battery time to stabilise (recover) with low increments of charge at progressively high charge levels during charging. During rest periods the polarisation of the cell is lowered. This process permits faster charging than possible with one prolonged high level charge which could damage the battery since it does not permit gradual stabilisation of the active chemicals during charging. Pulse chargers usually need current limiting on the input source for safety reasons, adding to the cost.

·     Universal Serial Bus (USB) Charger

The USB specification was developed by a group of computer and peripheral device manufacturers to replace a plethora of proprietary mechanical and electrical interconnection standards for transferring data between computers and external devices. It included a two wire data connection, a ground (earth) line and a 5 Volt power line provided by the host device (the computer) which was available to power the external devices. An unintended use of the USB port has been to provide the 5 Volt source not only to power peripheral devices directly, but also to charge any batteries installed in these external devices. In this case the peripheral device itself must incorporate the necessary charge control circuitry to protect the battery. The original USB standard specified a a data rata of 1.5 Mbits/sec and a maximum charging current of 500mA.

Power always flows from the host to the device, but data can flow in both directions. For this reason the USB host connector is mechanically different from the USB device connector and thus USB cables have different connectors at each end. This prevents any 5 Volt connection from an external USB source from being applied to the host computer and thus from possibly damaging the host machine.

Subsequent upgrades increased the standard data rates to 5 Gigabits/sec and the available current to 900 mA. However the popularity of the USB connection has led to a lot of non standard variants paricularly the use of the USB connector to provide a pure power source without the associated data connection. In such cases the USB port may simply incorporate a voltage regulator to provide the 5 Volts from a 12 Volt automotive power rail or a rectifier and regulator to provide the 5 Volts DC from the 110 Volts or 240 Volts AC mains supply with output currents up to 2100 mA. In both cases the device accepting the power has to provide the necessary charge control. Mains powered USB power supplies, often known as "dumb" USB chargers, may be incorporated into the body of the mains plugs or into separate USB receptacles in wall mounted AC power socket outlets.

See more about USB connections in the section on battery Data Buses.

·     Inductive Charging

Inductive charging does not refer to the charging process of the battery itself. It refers to the design of the charger. Essentially the input side of charger, the part connected to the AC mains power, is constructed from a transformer which is split into two parts. The primary winding of the transformer is housed in a unit connected to the AC mains supply, while the secondary winding of the transformer is housed in the same sealed unit which contains the battery, along with the rest of the conventional charger electronics. This allows the battery to be charged without a physical connection to the mains and without exposing any contacts which could cause an electric shock to the user.


A low power example is the electric toothbrush. The toothbrush and the charging base form the two-part transformer, with the primary induction coil contained in the base and the secondary induction coil and the electronics contained in the toothbrush. When the toothbrush is placed into the base, the complete transformer is created and the induced current in the secondary coil charges the battery. In use, the appliance is completely separated from the mains power and since the battery unit is contained in a sealed compartment the toothbrush can be safely immersed in water.


The technique is also used to charge medical battery implants.


A high power example is a charging system used for EVs. Similar to the toothbrush in concept but on a larger scale, it is also a non-contact system. An induction coil in the electric vehicle picks up current from an induction coil in the floor of the garage and charges the vehicle overnight. To optimise system efficiency, the air gap between the static coil and the pickup coil can be reduced by lowering the pickup coil during charging and the vehicle must be precisely placed over the charging unit.

A similar system has been used for electric buses which pick up current from induction coils embedded beneath each bus stop thus enabling the range of the bus to be extended or conversely, smaller batteries can be specified for the same itinerary. One other advantage of this system is that if the battery charge is constantly topped up, the depth of discharge can be minimised and this leads to a longer cycle life. As shown in the section on Battery Life, the cycle life increases exponentially as the depth of discharge is reduced.

A simpler and less expensive alternative to this opportunity charging is for the vehicle to make a conductive coupling with electric contacts on an overhead gantry at each bus stop.

Proposals have also been made to install a grid of inductive charging coils under the surface along the length of public roadways to allow vehicles to pick up charge as they drive along however no practical examples have yet been installed.


·    Electric Vehicle Charging Stations

For details about the specialised, high power chargers used for EVs, see the section about Electric Vehicle Charging Infrastructure.


Charger Power Sources

When specifying a charger it is also necessary to specify the source from which the charger derives its power, its availability and its voltage and power range. Efficiency losses in the charger should also be taken into account, particularly for high power chargers where the magnitude of the losses can be significant. Some examples are given below.


Controlled Charging

Easy to accommodate and manage.

·  AC Mains

Many portable low power chargers for small electrical appliances such as computers and mobile phones are required to operate in international markets. They therefore have auto sensing of the mains voltage and in special cases the mains frequency with automatic

switching to the appropriate input circuit. Higher power applications may need special arrangements. Single phase mains power is typically limited to about 3 KW. Three phase power may be required for charging high capacity batteries (over 20 KWh capacity) such as those used in electric vehicles which may require charging rates of greater than 3 KW to achieve reasonable charging times.

·  Regulated DCBatterySupply

May be provided by special purpose installations such as mobile generating equipment for custom applications.

·  Special Chargers

Portable sources such as solar panels.


Opportunity Charging

Opportunitycharging is charging the battery whenever power is available or between partial discharges rather than waiting for the battery to be completely discharged. It is used with batteries in cycle service, and in applications when energy is available only intermittently.

It can be subject to wide variations in energy availability and wide variations in power levels. Special control electronics are needed to protect the battery from overvoltage. By avoiding complete discharge of the battery, cycle life can be increased.

Availability affects the battery specification as well as the charger.

Typical applications are:

·  Onboard vehicle chargers (Alternators, Regenerative braking)

·  Inductive chargers (on vehicle route stopping points)

·  Solar power

·  Wind power


Mechanical charging

This is only applicable to specific cell chemistries. It is nor a charger technology in the normal sense of the word. Mechanical charging is used in some high power batteries such as Flow Batteries and Zinc Air batteries. Zinc air batteries are recharged by replacing the zinc electrodes. Flow batteries can be recharged by replacing the electrolyte.


Mechanical charging can be carried out in minutes. This is much quicker than the the long charging time associated with the conventional reversible cell electrochemistry which could take several hours. Zinc air batteries have therefore been used to power electric buses to overcome the problem of excessive charging times.


Charger Performance

The battery type and the application in which it is used set performance requirements which the charger must meet.

·  Output Voltage Purity

The charger should deliver a clean regulated voltage output with tight limits on spikes, ripple, noise and radio frequency interference (RFI) all of which could cause problems for the battery or the circuits in which it is used. For high power applications, the charging performance may be limited by the design of the charger.

· Efficiency

When charging high power batteries, the energy loss in the charger can add significantly to the charging times and to the operating costs of the application. Typical charger efficiencies are around 90%, hence the need for efficient designs.

· Inrush Current

When a charger is initially switched on to an empty battery the inrush current could be considerably higher than the maximum specified charging current. The charger must therefore be dimensioned either to deliver or limit this current pulse.

· Power Factor

This could also be an important consideration for high power chargers.


Battery Characteristics

Lithium is the lightest of metals and it floats on water. It also has the greatest electrochemical potential which makes it one of the most reactive of metals. These properties give Lithium the potential to achieve very high energy and power densities in high power battery applications such as automotive and standby power.


Many variations of the basic Lithium chemistry have been developed to optimise the cells for specific applications or perhaps in some cases to get around the patents on the original technology. Lithium metal reacts violently with water and can ignite into flame. Early commercial cells with metallic lithium cathodes were considered unsafe in certain circumstances, however modern cells don't use free Lithium but instead the Lithium is combined with other elements into more benign compounds which do not react with water.

The typical Lithium-ion cells use Carbon for its anode and Lithium Cobalt dioxide or a Lithium Manganese compound as the cathode. The electrolyte is usually based on a Lithium salt in an organic solvent.


Lithium batteries have now taken their place as the rechargeable battery of choice for portable consumer electronics equipment. Though they were expensive when introduced, volume production has brought the prices down.


See more details below.

Cell Nomenclature

There is considerable confusion about naming standards for cells with different systems used in Europe, the USA and Japan as well as manufacturers ' own standards.

One convention is two letters followed by a series if numbers.

The first letter represents the cell chemistry. The second letter represents the shape of the cell.

The numbers represent the dimensions of the cell in millimetres. For cylindrical cells the first two digits are the diameter and the remaining digits the length. For prismatic cells the first two digits represent the thickness, the second pair the height and the last pair the width.

Because of the plethora of "standards" the only safe course in identifying a cell is to consult the manufacturers' data sheets.


Common Primary Cells

See Battery Case Sizes for dimensions of common primary cells.

Cylindrical Cells

LC18650 is a common Li-ion cell in a Cylindrical can   Size (diameter18mm height 65.0mm)

See Cylindrical Cell Sizes for a listing of typical cylindrical cell sizes and capacities

Prismatic Cells

LP083448 is a Li-ion cell in a Prismatic can   Dimensions( thickness 8mm height 48mm width 34 mm)

See Prismatic Cell Sizes for a listing of typical prismatic cell sizes and capacities.


See Power Cell Sizes for examples of high power prismatic cells. (High power cylindrical cells are also available)


See also Battery Pack Design

Cell Balancing

Cell Balancing


In multi-cell batteries, because of the larger number of cells used, we can expect that they will be subject to a higher failure rate than single cell batteries. The more cells used, the greater the opportunities to fail and the worse the reliability.

Batteries such as those used for EV and HEV applications are made up from long strings of cells in series in order to achieve higher operating voltages of 200 to 300 Volts or more are particularly vulnerable. The problems can be compounded if parallel packs of cells are required to achieve the desired capacity or power levels. With a battery made up from n cells, the failure rate for the battery will be n times the failure rate of the individual cells.


All cells are not created equal

The potential failure rate is even worse than this however due to the possibility of interactions between the cells. Because of production tolerances, uneven temperature distribution and differences in the ageing characteristics of particular cells, it is possible that individual cells in a series chain could become overstressed leading to premature failure of the cell. During the charging cycle, if there is a degraded cell in the chain with a diminished capacity, there is a danger that once it has reached its full charge it will be subject to overcharging until the rest of the cells in the chain reach their full charge. The result is temperature and pressure build up and possible damage to the cell. With every charge - discharge cycle the weaker cells will get weaker until the battery fails. During discharging, the weakest cell will have the greatest depth of discharge and will tend to fail before the others. It is even possible for the voltage on the weaker cells to be reversed as they become fully discharged before the rest of the cells also resulting in early failure of the cell. Various methods of cell balancing have been developed to address this problem by equalising the stress on the cells.


Self Balancing

Unbalanced ageing is less of a problem with parallel chains which tend to be self balancing since the parallel connection holds all the cells at the same voltage and at the same time allows charge to move beween cells whether or not an external voltage is applied. There can however be problems with this cell configuration if a short circuit occurs in one of the cells since the rest of the parallel cells will discharge through the failed cell exacerbating the problem.


See Interactions Between Cells for more details.


The problems caused by these cell to cell differences are exaggerated when the cells are subject to the rapid charge and discharge cycles (microcycles) found in HEV applications.

While Lithium batteries are more tolerant of micro cycles they are less tolerant of the problems caused by cell to cell differences.


Because Lead acid and NiMH cells can withstand a level of over-voltage without sustaining permanent damage, a degree of cell balancing or charge equalisation can occur naturally with these technologies simply by prolonging the charging time since the fully charged cells will release energy by gassing until the weaker cells reach their full charge. This is not possible with Lithium cells which can not tolerate over-voltages. Although the problem is reduced with Lead acid NiMH batteries and some other cell chemistries, it is not completely eliminated and solutions must be found for most multicell applications.







No matter what battery management techniques are used, the failure rate or cycle life of a multicell battery will always be worse than the quoted failure rate or cycle life of the single cells used to make up the battery.








Once a cell has failed, the entire battery must be replaced and the consequences are extremely costly. Replacing individual failed cells does not solve the problem since the characteristics of a fresh cell would be quite different from the aged cells in the chain and failure would soon occur once more. Some degree of refurbishment is possible by cannibalising batteries of similar age and usage but it can never achieve the level of cell matching and reliability possible with new cells.


Equalisation is intended to prevent large long term unbalance rather than small short term deviations.


Cell selection

The first approach to solving this problem should be to avoid it if possible through cell selection. Batteries should be constructed from matched cells, preferably from the same manufacturing batch. Testing can be employed to classify and select cells into groups with tighter tolerance spreads to minimise variability within groups.


Large versus small cells

The high energy storage capacities needed for traction and other high power battery applications can be provided by using large high capacity cells or with large numbers of small cells connected in parallel to give the same capacity as the larger cells. In both cases the large cells, or the parallel blocks of small cells, must be connected in series to provide the required high battery voltage.


·                     Using large cells keeps the interconnections between cells to a minimum allowing simpler monitoring and control electronics and lower assembly costs. Until electric vehicles conquer a substantial percentage of the transportation market, the large cells they need will continue to be made in relatively small quantities, often with semi-automatic or manual production methods, resulting in high costs, wide process variability and the consequent wide performance tolerance spreads. When the cells are used in a serial chain, cell balancing is essential to equalise the stress on the cells, caused by these manufacturing variances, to avoid premature cell failures.


There are also safety issues associated with large capacity cells. A single 200 AmpHour Lithium Cobalt cell typically used in EV applications stores 2,664,000 Joules of energy. If a cell fails or is short circuited or damaged in an accident, this energy is suddenly released, often resulting in an explosion and an intense fire, known euphemistically as an “event” in the battery industry. When such an event occurs in a battery pack there is a strong likelihood that the fire and pressure damage resulting from a cell failure will cause neighbouring cells to fail in a similar way, ultimately affecting all of the cells in the pack with disastrous consequences.


·                     Using small cells connected in parallel to provide the same voltage and capacity as the larger cells results in many more interconnections, greater assembly costs and possibly more complex control electronics. Small, cylindrical, 2 or 3 AmpHour cells, such as the industry standard 18650 used in consumer electronics applications, are however made in volumes of hundreds of millions per year in much better controlled production facilities without manual intervention on highly automated equipment. The upside is that unit costs are consequently very low and reliability is much higher. When large numbers of cells are connected in a parallel block, the performance of the block will tend towards the process average of the component cells and the self balancing effect will tend to keep it there. The parallel blocks will still need to be connected in series to provide the higher battery voltage but the tolerance spread of the blocks in the series chain will be less than the tolerance spread of the alternative large capacity cells, leaving the cell balancing function with less work to do.


On the safety front, the more reliable low capacity cells are much less likely to fail and if a failure does occur, the stored energy released by any cell is only one hundredth of the energy released by a 200 AmpHour cell. This lower energy release is much easier to contain and the likelihood of the event propagating through the pack is much reduced or eliminated. This is perhaps the most important advantage of designs using lower capacity cells.


See also What a Joule can do


Pack construction

Another important avoidance action is to ensure at all times an even temperature distribution across all cells in the battery. Note that in an EV or HEV passenger car application, the ambient temperature in the engine compartment, the passenger compartment and the boot or trunk can be significantly different and dispersing the cells throughout the vehicle to spread the mechanical load can give rise to unbalanced thermal operating conditions. On the other hand, if the cells are concentrated in one large block, the outer cells in contact with ambient air may run cooler than the inner cells which are surrounded by warmer cells unless steps are taken to provide an air (or other coolant) flow to remove heat from the hotter cells. After cell selection, equalising the temperature across the battery pack should be the first design consideration in order to minimise the need for cell balancing. See also Thermal Management (Uniform heat distribution)


Cell equalisation

To provide a dynamic solution to this problem which takes into account the ageing and operating conditions of the cells, the BMS may incorporate a Cell Balancing scheme to prevent individual cells from becoming overstressed. These systems monitor the State of Charge (SOC) of each cell, or for less critical, low cost applications, simply the voltage across, each cell in the chain. Switching circuits then control the charge applied to each individual cell in the chain during the charging process to equalise the charge on all the cells in the pack. In automotive applications the system must be designed to cope with the repetitive high energy charging pulses such as those from regenerative braking as well as the normal trickle charging process.


Several Cell Balancing schemes have been proposed and there are trade-offs between the charging times, efficiency losses and the cost of components.


Active balancing

Active cell balancing methods remove charge from one or more high cells and deliver the charge to one or more low cells. Since it is impractical to provide independent charging for all the individual cells simultaneously, the balancing charge must be applied sequentially. Taking into account the charging times for each cell, the equalisation process is also very time consuming with charging times measured in hours. Some active cell balancing schemes are designed to halt the charging of the fully charged cells and continue charging the weaker cells till they reach full charge thus maximising the battery's charge capacity.

·                     Charge Shuttle (Flying Capacitor) Charge Distribution

With this method a capacitor is switched sequentially across each cell in the series chain. The capacitor averages the charge level on the cells by picking up charge from the cells with higher than average voltage and dumping the charge into cells with lower than average voltage. Alternatively the process can be speeded up by programming the capacitor to repeatedly transfer charge from the highest voltage cell to the lowest voltage cell. Efficiency is reduced as the cell voltage differences are reduced. The method is fairly complex with expensive electronics.

·                     Inductive Shuttle Charge Distribution

This method uses a transformer with its primary winding connected across the battery and a secondary winding which can be switched across individual cells. It is used to take pulses of energy as required from the full battery, rather than small charge differences from a single cell, to top up the remaining cells. It averages the charge level as with the Flying Capacitor but avoids the problem of small voltage differences in cell voltage and is consequently much faster. This system obviously needs well balanced secondary transformer windings otherwise it will contribute to the problem.

Passive balancing

Dissipative techniques find the cells with the highest charge in the pack, indicated by the higher cell voltage, and remove excess energy through a bypass resistor until the voltage or charge matches the voltage on the weaker cells. Some passive balancing schemes stop charging altogether when the first cell is fully charged, then discharge the fully charged cells into a load until they reach the same charge level as the weaker cells. Other schemes are designed continue charging till all the cells are fully charged but to limit the voltage which can be applied to individual cells and to bypass the cells when this voltage has been reached.

This method levels downwards and because it uses low bypass currents, equalisation times are very long. Pack performance determined by the weakest cell and is lossy due to wasted energy in the bypass resistors which could drain the battery if operated continuously. It is however the lowest cost option.

Charge Shunting

The voltage on all cells levelled upwards to the rated voltage of a good cell. Once the rated voltage on a cell has been reached, the full current bypasses fully charged cells until the weaker cells reach full voltage. This is fast and allows maximum energy storage however it needs expensive high current switches and high power dissipating resistors.

Charge limiting

A crude way of protecting the battery from the effects of cell imbalances is to simply switch off the charger when the first cell reaches the voltage which represents its fully charged state (4.2 Volts for most Lithium cells) and to disconnect the battery when the lowest cell voltage reaches its cut off point of 2 Volts during discharging. This will unfortunately terminate the charging before all of the cells have reached their full charge or cut off the power prematurely during discharge leaving unused capacity in the good cells. It thus reduces the effective capacity of the battery. Without the benefits of cell balancing, cycle life could also be reduced, however for well matched cells operating in an even temperature environment, the effect of these compromises could be acceptable.


All of these balancing techniques depend on being able to determine the state of charge of the individual cells in the chain. Several methods for determining the state of charge are described on the SOC page.

The simplest of these methods uses the cell voltage as an indication of the state of charge. The main advantage of this method is that it prevents overcharging of individual cells, however it can be prone to error. A cell may reach its cut off voltage before the others in the chain, not because it is fully charged but because its internal impedance is higher than the other cells. In this case the cell will actually have a lower charge than the other cells. It will thus be subject to greater stress during discharge and repeated cycling will eventually provoke failure of the cell.


More precise methods use Coulomb counting and take account of the temperature and age of the cell as well as the cell voltage.


Redox Shuttle (Chemical Cell Balancing)

In Lead acid batteries, overcharging causes gassing which coincidentally balances the cells. The Redox Shuttle is an attempt to provide chemical overcharge protection in Lithium cells using an equivalent method thus avoiding the need for electronic cell balancing. A chemical additive which undergoes reversible chemical action absorbing excess charge above a preset voltage is added to the electrolyte . The chemical reaction is reversed as voltage falls below the preset level.


For batteries with less than 10 cells, where low initial cost is the main objective, or where the cost of replacing a failed battery is not considered prohibitive, cell balancing is sometimes dispensed with altogether and long cycle life is achieved by restricting the permitted DOD. This avoids the cost and complexity of the cell balancing electronics but the trade off is inefficient use of cell capacity.


Whether or not the battery employs cell balancing, it should always incorporate fail safe cell protection circuits.


Battery Storage

Battery Storage


The optimum storage conditions for batteries depend on the active chemicals used in the cells. During storage the cells are subject to both self discharge and possible decomposition of the chemical contents. Over time solvents in the electrolyte may permeate through the seals causing the electrolyte to dry out and lose its effectiveness. In all cases these processes are accelerated by heat and it is wise to store the cells in a cool, benign environment to maximise their shelf life. The glove compartment of a car does not qualify as a suitable storage location since temperatures may exceed 60°C shortening dramatically the life of the battery. (See Battery Life)

For cells with the same nominal cell chemistry, individual manufacturers may add different additives to optimise their cell performance for a particular parameter and this may affect the behaviour of the cells during storage. It is possible to make some general recommendation about storage but the best guidance for storage is to consult the manufacturers' specifications and recommendations for their products. Some general guidelines for some common cell chemistries follow:



The possible storage temperature range for Lithium-Ion batteries is is -20°C to 60°C but for prolonged storage period -20°C to 25°C is recommended and 15°C is ideal. Cells should be stored with a partial charge of between 30% and 50%. Although the cells can be stored fully discharged the cell voltage should not drop below 2.0 Volts per cell and cells should be topped up to prevent over-discharge. The maximum voltage should not exceed 4.1 Volts


If secondary cells must be for a prolonged period the state of charge should be checked regularly and provision should be made for recharging the cells before the cell voltage drops below the recommended minimum after which the cells suffer irreparable deterioration. ( This is particularly true for battery packs which may have associated electronics which add to the self discharge drain on the cells)

Battery Safety

Battery Safety


Batteries have the potential to be dangerous if they are not carefully designed or if they are abused. Cell manufacturers are conscious of these dangers and design safety measures into the cells. Likewise, pack manufacturers incorporate safety devices into the pack designs to protect the battery from out of tolerance operating conditions and where possible from abuse. While they try to make the battery foolproof, it has often been explained how difficult this is because fools can be so ingenious. Once the battery has left the factory its fate is in the hands of the user. It is usual to provide "Instructions For Use" with battery products which alert the end user to potential dangers from abuse of the battery. Unfortunately there will always be perverse fools who regard these instructions as a challenge.



Subjecting a battery to abuse or conditions for which it was never designed can result in uncontrolled and dangerous failure of the battery. This may include explosion, fire and the emission of toxic fumes.


We are helped in assessing what hazards to protect against, and the degree of protection required, by the publication of national standards. Some of these are listed in the section on Standards. Typical safety test requirements are outlined in the section on Testing.


"Designed in" safety measures

These are not something that the battery applications engineer can control but they could influence the choice of cells to be specified for a particular application.

Cell chemistry - In the quest for ever higher energy and power densities cell makers have utilised ever more reactive chemical mixes, but these highly reactive properties which are needed to provide the higher energy densities are likely to increase the risk of danger in case of cell failures. For safety reasons the cell maker may compromise on the maximum power by using a less reactive chemical mix or by introducing some form of chemical retardant in order to reduce the risk of fire or explosion if a cell suffers physical damage. As an example, the original Lithium-ion cells used cathodes consisting of Lithium Cobalt oxides and these provide maximum power, however Lithium Manganese oxides and Lithium phosphate cells which have slightly lower power ratings are now the preferred choice for many applications because they are inherently safer if damaged. Lithium titanate anodes do not depend on an SEI layer for stability and are inherently safer, though at the cost of lower energy density.

Electrolytes - Chemical inhibitors are often added to electrolytes to make them self extinguishing or flame retarding in case of abuse of the cell which could lead to fire.

Cell construction - Low power cells have relatively simple mechanical structures which have undergone many years of development and cell failures caused by poor mechanical design a very rare. For high power cells however, thermal design can be a source of weakness. Getting the excess heat out of the cell can be a problem and poor designs can result in localised hotspots within the cell which can cause cell failure. Good thermal performance for high power cells requires substantial thermal conduction paths.

Separators - If for any reason a cell overheats, this can cause the separators, which are typically made of plastic, to distort or melt. In the worst case this could lead to a short circuit between the electrodes with even more serious consequences. Internal short circuits can also occur due to dendrite or crystal growth on the electrodes. External circuits can not protect against an internal short circuit and various separators have been designed to avoid this problem. These include

Rigid separators which do not distort even under extreme temperature conditions.

Flexible ceramic powder coated plastic which prevents contact between the electrodes, resists penetration by impurities, reduces shrinkage at high temperatures and impedes the propagation of a short circuit across the separator.

"Shut down separators" with special plastic formulations, similar to a resettable fuse, whose impedance suddenly increases when certain temperature limits are reached. The melting plastic in the shut down separator closes up its pores thus avoiding a short circuit but the action is not reversible.

Once an internal short occurs, there is not much that can be done by external measures to protect the battery. Such an occurrence can be detected by a sudden drop in the cell voltage and this can be used to trigger a cut off device to isolate the battery from the charger or the load. While it does not solve the problem at least it prevents external events from making it any worse. Fortunately an internal short circuit is a rare occurrence.

The likelihood of an internal short circuit occurring can be minimised by keeping the cell temperature within limits and this should be the user's first line of defence.

Robust packaging - As with cell construction this is unlikely to be a source of problems.

Circuit Interrupt Device (CID) - Some cells also incorporate a CID which interrupts the current if the internal gas pressure in the cell exceeds specified limits.

Safety vents - If other safety devices fail and a cell is allowed to overheat, chemical reactions can result in gassing and the active materials will also expand due to the temperature rise. This can cause a dangerous build up of pressure inside the cell which could result in rupture of the case or even an explosion. Safety vents are needed as a final safety precaution to release the pressure before it reaches a dangerous level. Automatic release guard vents prevent the absorption of external air into the cell but allow controlled release of excess internal pressure to avoid leakage and prevent uncontrolled rupture of the cell case.

Keyed and shrouded connectors or terminals - These are designed to protect the operator, to prevent accidental short circuits and the connection of incorrect loads or chargers to the battery. See Battery Pack Design (External connections)

All the designed-in safety precautions can be worthless if the manufacturing processes are not controlled properly. Burrs on the electrodes, misaligned or out of tolerance components, contaminated electrode coatings or electrolytes can all cause short circuits or penetration of the separator. A short circuit caused by a microscopic metallic particle may simply cause cause local overheating or an elevated self discharge rate due to a relatively high impedance current path between the electrodes, but a direct short circuit due to penetration of the separator by a burr on the electrode can lead to excessive overheating and eventually thermal runaway of the cell.


External Safety Devices

Protecting the cell from out of limits operating conditions, either from the loads imposed by the intended application or abuse by the user or from unsuitable charging regimes, is the job of the battery pack designer.


Heat is the biggest killer of batteries and this is most likely to be due to unsuitable charging methods or procedures. But chargers are not the only culprits. Overloading the cell during discharge also causes overheating. Many safety devices are therefore based on sensing the cell temperature and isolating the cell from its load or from the charger if the temperature reaches dangerous levels.

See the section on Chargers for more information about safe charging.

Heat damages a cell no matter what its source and a cell will suffer the same damage by being placed in a high ambient temperature environment as it would from improper use. There is no practical way to protect the cell from this kind of abuse. (Sounding an alarm could be a possibility)


Apart from damage from overheating, a battery may be damaged from excessive currents and from over and under voltage. Suitable protection methods and how they are implemented are described in detail in the section Battery Protection Methods


Short Circuits and their Consequences (What can a Joule do?)

Short circuiting a capacitor or a battery is definitely not recommended as the destructive power unleashed is often seriously under estimated.


As an example, a 0.1Farad capacitor charged to 14 Volts will store 10 Joules of energy (E = ½ CV2 ). This may not seem very much, it is only 10 Watt seconds, but it is enough to punch a hole through aluminium foil creating a lot of sparks. 30 Joules is enough to weld a wire to a ball bearing. This is because the discharge period is very short, almost instantaneous, resulting in a power transfer of hundreds of watts.


Batteries store even more energy. For comparison, a fully charged 3.6 Volt, 1000 mAh mobile phone battery has a low internal impedance and contains 12,960 Joules of energy. Short circuiting these cells can cause extremely high currents and temperatures within the cell resulting in the breakdown of the chemical compounds from which it is made. This in turn can cause the rapid build up of pressure within the cell resulting in its catastrophic failure, with unpredictable consequences including the uncontrolled rupture of the cell or even fire.

By the same token, a single, fully charged 200 Ah, 3.6 Volt Lithium Ion automotive cell (or the similar capacity from any other cell chemistry) contains 2,592,000 Joules of energy. Don't wait around to see what happens if you drop a wrench on the terminals!!




Battery (and User) Protection System

The diagram below summarises the types of problems which can occur in Lithium energy cells and their consequences together with the actions which may be taken by the Battery Management System (BMS) to address the problems and the results of the actions.


Cell Protection Mechanisms



See also Why Batteries Fail


Multi Level Battery Safety Plan

The responsibility for battery safety starts at the cell maker's premises and continues through to the design of the battery application. A multi-level safety plan should include consideration of at least the following components.

Begin with intrinsically safe cell chemistry

Designed in safety measures (See above)

Supplier and production audit

Cell design audit

Manufacturer's technical capability

Staff (Engineering, Management)

Facilities (Materials analysis capability)

Manufacturer's quality systems.

Process controls. (In place and being implemented)

Cell level safety devices

CID (Circuit Interrupt Device)

Shut down separator

Pressure vent

External circuit devices

PTC resistors (Low power only)


Cell and battery isolation to prevent event propagation

Electrical (Contactors)

Physical (Separation, barriers)

BMS Software

Monitoring of all key indicators coupled to control actions.

(Cooling, Power disconnect)

BMS Hardware

Fail safe back-up hardware switch off in case of software failure. Set to slightly higher limits than the software controls.

Battery switch off in case the low voltage BMS power supply or other system component fails.


Use multiple low capacity cells which release less energy in case of an event.

Design in physical barriers to heat and flame propagation between cells.


Automotive High Voltage High Capacity Batteries

Concerns are often expressed about the safety of high power automotive batteries if they are damaged or crushed in an accident. Such batteries are normally subject to stringent safety testing before they may be approved for use and a range of International Standards has been developed for this purpose.

Nevertheless batteries in general present a lower hazard in the case of an accident than a full tank of petrol.

The dangers don't just come from the chemical content of the batteries. High capacity batteries store an immense amount of energy which can cause enormous damage if the battery is short circuited.


See also the trade-off between High and Low Capacity Cells and the consequences for safety.




Handling Instructions


User Safety Precautions

These are intended to protect the user as well as the battery. Detailed recommendations for handling and using batteries are given in the section on User Safety Instructions


MSDS Material Safety Data Sheets

Material Safety Data Sheets are designed to provide safety information about any physical or chemical hazard associated with a particular product and procedures for handling or working with hazardous material content. They are intended for employers, employees and emergency services responsible for dealing with fire or medical emergencies.

MSDS's are specific to individual products or classes of products and include information such as the chemical composition of each of the chemicals used and physical data ( melting point , boiling point , flash point etc.) as well as the reactivity, toxicity, flammability, health effects, recommended first aid, storage, disposal, protective equipment and procedures to follow in case of a fire, spill or leak.

In the case of batteries, the information is usually provided by the cell manufacturer since they control the contents of the product.

See MSDS for an example of a typical data sheet for Lithium cells used in mobile phones.


See also Battery Death for dangerous operating practices which could damage the battery and Electric Shocks for an outline of potential hazards to the user when working with batteries.

Battery Abbreviations


$                      Dollar in US currency

18650             Li-ion cylindrical cell format measuring 18mm times 65mm

A                    Ampere (electrical)

AC                  Alternating current

ADAC            Allgemeiner Deutscher Automobil-Club (German automobile club)

AFC                Alkaline fuel cell

AGM              Absorbent Glass Mat (battery)

AGV               Automatic Guided Vehicle

Ah                  Ampere-hour

APU                Auxiliary Power Unit

BAPCO           Business Applications Performance Corporation

Bar                 Unit ofpressure; 1 bar = 100kPa; 1 bar = 14.503psi

bbl                  Measurements of liquid, 1 barrel = 42 US gallons (35 Imperial gallons), 159 liters

BCG                The Boston Consulting Group

BCI                 Battery Council International

BMS               Battery management system

BMW              Bavarian Engine Works (Bayerische Motoren Werke) 

BTU                British Thermal Unit; 1 BTU = 1,054 joules; 1 BTU = 0.29Wh

C                     Celsius, Centigrade (temperature) 

cal                  Calorie; 1cal = 4.18 joules; 1cal = 4.18 watt/s; 1,000 joules = 0.277Wh

CARB            California Air Resources Board

CCA                Cold cranking amps at –18°C (0°F). The norms differ as follows:

                        BCI discharges battery at CCA-rate for 30s; battery at or above 7.2V passes

                        IEC discharges battery at CCA-rate for 60s; battery at or above 8.4V passes

                        DIN discharges battery at CCA-rate for 30s and 150s; battery at or above 9V and 6V respectively passes

CCCV             Constant current constant voltage (charge method)

CCV                Closed circuit voltage (battery under charge or discharge)

CDMA            Code Division Multiple Access (cell phones)

CEC                Certificate of Equivalent Competency (International regulations)

CID                 Circuit interrupt device

CIPA               Camera and Imaging Products Association

CL                   Current limiting (as in charging a battery)

CNG                Compressed natural gas

CNT                Carbon nanotube

CPU                Central processing unit

Co                   Cobalt (metal)

COC                Certificate of Competency

CO2                 Carbon dioxide

CPR                Cardiopulmonary resuscitation

C-rate             Discharge rate of a battery

DC                   Direct current

DGP                Dangerous Goods Panel

DIN                 Deutsches Institut für Normung (German Institute for Standardization)

DLC                Double-layer capacitor

DMFC             Direct Methanol Fuel Cell

DoD                 Depth of discharge

DOE                Department of Energy (US)

DOT                 Department of Transportation (US)

DSP                 Digital signal processor

dT/dt                Delta Temperature over delta time (charge method)

EBM                Electronic battery monitor

ect.                 Et cetera. Latin: And so forth

EDTA              Crystalline acid

EIS                  Electrochemical Impedance Spectroscopy

ELC                 Equivalent lithium content

EMF                Electromagnetic field

EMF                Electromotive force

EPA                Environmental Protection Agency (US)

EV                   Electric vehicle

F                      Fahrenheit (temperature)

f                      Farad (unit of capacitance)

FAA                Federal Aviation Administration

FC                   Fuel cell

FCVT              FreedomCAR & Vehicle Technologies (US Department of Energy)

Foot/’             Foot (dimension) 1’= 12”; 1’ = 0.3048m; 1’times 3.28 = 1m

g                     Gram; 1g = 0.035oz; 1g times 28.35 = 1oz

GSM               Global System for Mobile Communications (cell phones)

h                     Hour (time)

HEV                Hybrid electric vehicle

hp                   Horsepower (power) 1hp = 745.7 watts

Hz                   Hertz (electrical frequency)

I                       Current(electrical)

i.e.                  Id est. Latin: That is

IATA               International Air Transport Association

IC                    Integrated circuit (chip)

IC                    Internal combustion (engine)

ICAO              International Civil Aviation Organization

IEC                 International Electrochemical Commission

Inch/“             Inch; 1”= 25.4mm; 1” = 0.0254 meter; 1”times 39.3 = 1m

IPF                  Interfacial protective film

IPP                  IECaircraft battery rating (0.3/15s power discharge)

IPR                  Aircraft battery rating according to IEC (15s power discharge)

IS                     Intrinsic safety (used on batteries)

J                      Joule, 1J = 1A at 1V for 1s = 1 watt/s; 1J = 0.238calorie/s

kg                    Kilogram; 1kg = 0.45 pound; 1kg times 2.2 = 1 pound

kJ                    Kilo-Joule; 1kJ = 0.277Wh

km                   Kilometer; 1km = 0.621 miles; 1km times 1.60 = 1 mile

kN                    Kilo-Newton (law of motion) 1N = 1kg m/s2

kPa                  Kilo-Pascal (pressure); 1kPa = 0.01 bar; 1kPa = 0.145psi

kW                  Kilowatt (electrical energy); 1kWh = 3.6MJ; 1MJ = 860kcal = 238cal/s 

kWh                Kilowatt-hour (electrical power)

L                      Inductance (electrical coil)

lb                     Pound (weight, from Roman libra) 1 lb times 0.45 = 1kg

LCD                Liquid crystal display

LCO                Lithium cobalt oxide

LED                Light emitting diode

LFP                 Lithium-iron-phosphate

LFPT               Low frequency pulse train (method to test a battery)

LiCoO2            Lithium-ion-cobalt-oxide

LiFePO4          Lithium-iron-phosphate-oxide

Li-ion               Lithium-ion battery (short form)

LiMn2O4          Lithium-ion-manganese-oxide

LiNiCoAlO2     Lithium-ion-nickel-cobalt-aluminum-oxide

LiNiMnCoO2   Lithium-on-nickel-manganese-cobalt-oxide

Li5Ti5O13       Lithium-titanate-oxide

L/km                Liter per kilometer

LMO                Lithium-manganese-oxide

LTO                 Lithium-titanate

m                     Meter (dimension) 1m = 3.28 feet; 1m times 0.30 = 1 foot

mAh                Milliampere-hours

MCFC              Molten carbonate fuel cell

Microfarad [µF]     Capacitor rating, one-millionth 10-6 of a farad)

Min                 Minute (time)

mm                  Millimeter (dimension) 1mm = 0.039”; 1mm times 25.4 = 1”

Mn                  Manganese(chemical element used in batteries)

mpa                 Mega-Pascall unit of pressure

Mpg                Miles per gallon

ms                   Millisecond, one-thousand of a second (10-3).

MW                 Megawatt (power)

N                      Newton (law of motion) 1N = 1kg m/s(force required to accelerate 1kg at 1m/s)

NaS                 Sodium-sulfur (battery)

NASA              National Aeronautics and Space Administration

NCA                Lithium-ion battery with nickel, cobalt, aluminum cathode

NCV                Net calorific value (1 food calorie = 1.16 watt-hour)

NDV                Negative delta V (full-charge detection)

NG                   Natural gas, consumption measured in joules (1,000 joules = 0.277Wh)

NiCd                Nickel-cadmium (battery)

NiFe               Nickel-iron (battery)

NiH                 Nickel-hydrogen battery

NiMH              Nickel-metal-hydride (battery)

NiZn                Nickel-zinc (battery)

NMC               Lithium-ion with nickel, manganese, cobalt cathode

NRC               National Research Council

NTC                Negative temperature coefficient

OCV                Open circuit voltage

OEM               Original equipment manufacturer

Oz                    Ounce; 1 oz = 28 grams; 1 oz times 0.035 = 1 gram 

PAFC              Phosphoric acid fuel cell

PC                   Personal computer

PEM                 Proton exchange membrane (fuel cell), also PEMFC

PEMFC            Proton exchange membrane fuel cell, also PEM

pf                     Pico-farad (capacitor rating, one-trillionth 10-12 of a farad)

pf                     Power factor (ratio of real power to the apparent power on AC)

PHEV              Plug-in hybrid electric vehicle

PRBA              Portable Rechargeable Battery Association

psi                   Pound per square inch (pressure) 1psi = 0.145kPa; 1psi times 6.89 = 1kPa

PTC                 Positive temperature coefficient

PTC                 Over-voltage protection (batteries, motors, speakers)

QA                   Quality assurance

Qi                    Standard on inductive charging by Wireless Power Consortium (WPC)

Q-Mag™        Quantum magnetic battery analysis (Cadex trademark)

R                     Resistor (electrical)

RBRC             Rechargeable Battery Recycling Corporation

RC                   Remote control (hobbyist)

RC                   Reserve capacity of starter battery. Conversion formula:. RC divided by 2 plus 16 = Ah

R&D                Research and development

RPM               Revolution per minute

s                      Second (time)

SAE                 Society of Automotive Engineers, founded early in 1900 by US auto manufacturers

SBS                 Smart Battery System

SEI                  Solid electrolyte interphase (Li-ion)

SG                   Specific gravity (acid density of electrolyte)

SLA                 Sealed lead acid (battery)

SLI                  Starter-light-ignition (battery), also knows as starter battery

SMBus           System Management Bus (smart battery)

SoC                 State-of-charge

SoF                  State-of-function

SOFC              Solid oxide fuel cell

SoH                 State-of-health

UL                    United Laboratories (product safety testing and certification)

UPS                 Uninterruptible power supply

USB                 Universal Serial Bus (data)

V                      Voltage (electrical)

VA                   Volt-ampere (similar to watt with true current flow in a reactive load)

VAC                Voltage with alternating current (grid)

VL                   Voltage limiting (as in charging a battery)

VRLA              Valve regulated lead acid (battery)

W                     Watt (electrical energy; voltage times current = watts)

Wh                   Watt-hour (electrical power; watts times h = Wh); 1Wh = 860 cal/h = 0.238cal/s

Wh/kg             Watt-hour per kilogram (measurement of specific energy)

Wh/km            Watt-hour per kilometer
Wh/l                 Watt-hour per litter (measured in energy density)

Wi-Fi               Wireless fidelity (network)

W/kg               Watt per kilogram (measurement of specific power)

WPC               Wireless Power Consortium

WW                World War

Z                      Impedance (reactance-based resistance, frequency dependent)

ZEBRA            Zeolite Battery Research Africa Project (battery)