Why can thermal runaway damage an EV battery?
Thermal runaway can seriously damage an EV battery because it is a self-accelerating chemical and thermal reaction in which a battery cell generates heat faster than that heat can be removed. Once the temperature becomes high enough, internal reactions can release additional heat, causing the cell temperature to rise even further.
How thermal runaway starts
A lithium-ion cell can experience thermal runaway after certain types of damage or abnormal conditions, such as an internal short circuit, severe overheating, physical damage, or other cell failures.
As the cell heats up, components inside it can begin to break down. These reactions can generate more heat and gases, creating a cycle in which increasing temperature accelerates further chemical reactions.
Why the damage can spread
An EV battery pack contains many individual cells that are connected and positioned close together. If one cell enters thermal runaway, the heat it produces can transfer to neighboring cells.
If the surrounding cells become hot enough, they may also enter thermal runaway. This process, known as thermal propagation, can turn a problem that began in one cell into a larger battery-pack failure.
What damage can occur?
Thermal runaway can cause:
- Severe cell damage
- Internal pressure and gas generation
- Smoke and potentially toxic gases
- Fire
- Damage to neighboring cells and battery components
- Loss of battery capacity and functionality
- In serious cases, damage to the entire battery pack
Lithium-ion battery electrolytes can be flammable, which is one reason a thermal runaway event can become difficult to control.
Why battery management is important
Modern EVs use battery management and thermal-management systems to monitor and control battery operation. These systems help keep cells within appropriate operating conditions and can detect or respond to certain faults.
However, these systems cannot eliminate every possible failure, particularly when a cell develops an internal defect or suffers significant physical damage.
Can thermal runaway affect a healthy battery pack?
Thermal runaway does not mean an EV battery will normally overheat or catch fire. Battery packs are designed with multiple safety measures, including monitoring, thermal management, electrical protection, and structural protection.
The concern is that when a serious cell failure does occur, the heat generated by one failing cell can potentially affect nearby cells. Engineers therefore design battery packs to limit heat transfer and prevent or slow thermal propagation.
The key point
Thermal runaway is dangerous because it can create a positive feedback loop: heat causes chemical reactions, those reactions generate more heat, and the increasing temperature can cause further reactions. In a multi-cell EV battery, the resulting heat can potentially spread from one cell to others.
For this reason, damaged or flooded EVs should not be treated like ordinary vehicles. High-voltage battery systems can remain hazardous, and manufacturers' procedures and qualified technicians should be used for inspection and repair.
