Safety is a fundamental requirement and we have to look at this from a chemical, electrical, mechanical, thermal and complete system viewpoint.
The first thing is to look at the specification of the individual battery cell as this will specify the limits of safe operation:
- Maximum and minimum operating voltage
- the voltage needs to be measured for most applications to ensure you do not go beyond these limits
- Electrical shorts
- external shorts could result in very high currents that can then result in cells getting very hot very quickly, a fusing strategy is a good way to protect at this basic level
- Maximum and minimum temperature range
- depending on the application it is likely that you will need to monitor the temperature of the cells and restrict performance in charge and discharge
- Mechanical Limits
- maximum load that can be applied on each axis or cell face
- loads required for long term operation of the cell
- shock limits
- maximum and minimum ambient pressure
Thermal Runaway
If you heat a battery cell to somewhere above 130°C then exothermic chemical reactions inside the cell will increase the temperature and further reactions will take place. The result is an uncontrolled runaway and increase in temperature. The cell should vent in a controlled manner with fire and molten material. In severe cases the cell may explode. The energy released from one cell failing is likely to heat neighbouring cells that again could be triggered into thermal runaway.
EUCAR Hazard Levels
The EUCAR Hazard Levels define the outcome of cell level safety testing. These levels are normally used to describe the outcome of tests such as overcharge as part of the cell specification.
What happens when batteries are abused?
Lithium-ion batteries are an essential component in electric vehicles, however their safety remains a key challenge. This video explores the science behind what happens when batteries are abused and when they fail.
A great introductory presentation by Billy Wu, Dyson School of Engineering, Imperial College.
Thermal Runaway vs Electrical Energy
The energy released during Thermal Runaway versus the electrical energy stored in a battery.
The energy released during Thermal Runaway (TR) versus the stored electrical energy. A number has been bandied around for a long time that the energy released in a TR event was 2 to 6 times the electrical energy stored in the cell.
There are a lot of papers published where the heat release from a cell has been measured or estimated during thermal runaway. There have also been measurements of numerous chemistries and states of charge etc.
References
- UN/DOT 38.3 6th Edition – Recommendations on the Transport of Dangerous Goods
- IEC 62133-2:2017 – Safety requirements for portable sealed secondary lithium cells, and for batteries made from them, for use in portable applications – Part 2: Lithium systems
- UL 2054 2nd Edition – Household and Commercial Batteries