What is thermal runaway?

• A. Large current flow caused by high internal resistance
• B. A safe mitigating event
• C. Large current flow allowed by low internal resistance causes middle cells to overheat and possibly destroy the battery
• D. A protective mechanism triggered by temperature

Answer: (C)

Thermal runaway is a self-sustaining, uncontrolled heating process inside a battery where the heat produced cannot be removed quickly enough. When a lot of current can flow because internal resistance is low, the heat generated by I^2R losses becomes significant. If the cooling isn’t able to remove that heat, the temperature climbs, which can drive chemical reactions that release even more heat. That creates a positive feedback loop: more heat accelerates reactions, which produce more heat, and the situation can spiral. In a battery pack, this heating can spread to neighboring cells, causing additional overheating and potentially destroying the battery. The other choices describe protective or mitigating aspects, which aren’t what initiates runaway; they’re mechanisms that aim to prevent it.