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.
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.
- PTC (Pressure, Temperature, Current) Switch - A slightly more complex safety device which inhibits current surges and interrupts the circuit in case of excess pressure or over temperature. It resets and does not permanently disable the battery when triggered, however tripping may irreversibly increase their electrical resistance by up to a factor of two increasing the possibility of a later catastrophic failure. Typically used in small cylindrical cells such as the 18650.
- 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!!
YOU HAVE BEEN WARNED
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.
HANDLE WITH CARE.
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.