What triggers thermal runaway and what makes it difficult to contain?

• A. An external heat source alone will trigger thermal runaway, which is easy to contain.
• B. A fault in a neighboring circuit does not affect a cell's heat.
• C. An internal cell fault or overheat leading to an exothermic reaction; heat and gases can spread to neighboring cells, challenging containment without cooling.
• D. Thermal runaway is prevented by any cooling system and never spreads.

Answer: (C)

Thermal runaway begins when a cell experiences an internal fault or overheats, triggering an exothermic reaction that generates heat faster than it can be removed. This self-heating creates a positive feedback loop, pushing the cell temperature higher and causing the reaction to accelerate. In a pack, the heat and released gases don’t stay put; they can move to neighboring cells through conduction and shared materials, which can push adjacent cells into runaway as well. Containing such an event is difficult without cooling because you must remove heat quickly and limit heat transfer to other cells; if cooling is insufficient, heat and gas can propagate and turn a single-cell issue into a pack-wide hazard. External heat sources or faults in neighboring circuits can contribute to the conditions, but the defining factor is the internal fault driving a self-accelerating reaction and its potential to spread when cooling isn’t adequate.