Which statement correctly describes the root cause of thermal runaway in nickel-cadmium cells?

• A. High internal resistance causing overheating
• B. Balanced cells
• C. Low internal resistance allows large current between unbalanced cells causing heating
• D. External heat source triggers

Answer: (C)

Thermal runaway happens when the heat generated inside a cell or pack cannot be removed fast enough, so temperature climbs and exothermic reactions accelerate, producing even more heat. In nickel-cadmium packs, a common path to this is when cells aren’t at the same state of charge. The pack can develop a low-resistance path between cells with different voltages, so a higher-charged cell can push a large intercell current into a lower-charged one. That current, flowing through a low-impedance path, wastes power as heat (P = I^2R) and heats the cells. As the temperature rises, the chemistry can change in a way that allows even more current to flow, creating a self-reinforcing loop—the essence of thermal runaway. So the idea that a low internal resistance permits large intercell current between unbalanced cells, leading to heating, correctly describes the root cause. A high internal resistance would limit current and reduce heating; balanced cells wouldn’t create large intercell currents; an external heat source can initiate heating, but the root mechanism is the internal, self-amplifying heat from intercell currents.