Define thermal runaway and explain why it is dangerous in EV batteries.

• A. Thermal runaway is an uncontrolled exothermic reaction that can cause rapid heating, gas generation, fire, and potential propagation to adjacent cells.
• B. Thermal runaway is a cooling process.
• C. Thermal runaway is a software feature.
• D. Thermal runaway is a mechanical failure in motor.

Answer: (A)

Thermal runaway is an uncontrolled exothermic reaction inside a battery cell where heat generation outpaces the ability to remove it. Once the temperature climbs enough, the reaction speeds up, creating even more heat in a self‑accelerating loop. In an EV battery pack, this is especially dangerous because many cells are packed close together with shared cooling and electrical paths. Heat from one cell can transfer to neighboring cells, potentially triggering them as well. This can lead to rapid temperature rise, electrolyte decomposition producing flammable and toxic gases, venting, and even fire or explosion. The energy release and the possibility of reaching adjacent cells make the event hard to stop and capable of propagating through the pack, posing serious safety risks to the vehicle and bystanders.

The option describing this as an uncontrolled exothermic reaction with rapid heating, gas generation, fire, and potential propagation to adjacent cells captures exactly what thermal runaway is and why it is so dangerous in EV batteries. The other choices don’t fit: a cooling process would reduce risk, a software feature is unrelated, and a mechanical motor failure describes a different failure mode.