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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/s2 (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)

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 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)

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.

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

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