Thermal runaway in a NiCad battery is caused by:

• A. Excessive charge rate
• B. Low charge rate
• C. Low ambient temperature
• D. Overdischarge

Answer: (A)

Thermal runaway happens when the heat produced inside a battery during operation exceeds the ability to remove it, so temperature rises and speeds up the chemical reactions, which then produce even more heat in a self‑accelerating cycle. In a nickel–cadmium cell, charging too quickly forces more energy into the cell than it can safely store. The excess energy drives side reactions and gas formation, generating heat. As the temperature climbs, the chemical processes proceed faster, producing more heat, and if cooling can’t keep up, this runaway heating can escalate quickly. The other scenarios—charging at a low rate, being in a very cold environment, or discharging—don’t create that self‑amplifying heat rise, so they aren’t the cause here.