What causes thermal runaway in a NiCad battery?

• A. Excessive rate of discharge
• B. Excessive rate of charge
• C. Short circuit within a cell
• D. High ambient temperature

Answer: (B)

Thermal runaway happens when a cell generates heat faster than it can dissipate. In a NiCad battery, charging too quickly pushes the chemistry into overcharge. During overcharge, exothermic reactions accelerate, gas can form, and internal resistance may rise. All of these effects generate more heat, and as the temperature climbs, reaction rates increase further, creating a self‑accelerating loop that the cell can’t shed fast enough. That’s why an excessive charging rate is the primary trigger for thermal runaway in NiCad cells. High ambient temperature or a short circuit can contribute to the problem, but the root cause in this context is charging the cell too quickly.