Thermal Runaway
Thermal runaway is a self-accelerating failure inside a battery cell in which heat-generating reactions raise the temperature and trigger still faster reactions.
How the failure develops
An internal short circuit, severe overheating, overcharge, or physical damage can initiate the process. The affected cell may release hot gas, flammable vapour, smoke, and solid material. The response depends on cell chemistry, construction, state of charge, and the initiating fault.
Opening the battery's main contactors can stop external current, but it cannot remove the chemical energy already stored inside the affected cell or guarantee that its reactions stop.
Thermal runaway describes what happens within the failed cell. It is not automatically a whole-pack fire. Thermal propagation occurs only when heat, flame, hot gas, or ejected material from that cell causes neighbouring cells to fail.
For the deeper safety mechanisms, containment measures, and testing context, see EV Battery Safety and Failure Management.