The combination of factors that contribute to thermal runaway for nickel-cadmium batteries installed on aircraft can be described as which of the following?

• A. High internal resistance with low temperature
• B. Moderate internal resistance with low current
• C. No relation between temperature and charging current
• D. Low internal resistance intensified by high cell temperatures and a high current discharge/charge rate in a constant potential charging system

Answer: (D)

Thermal runaway happens when the heat generated inside a battery cannot be removed fast enough, so the temperature keeps rising and the reactions that generate heat accelerate. In nickel-cadmium batteries used on aircraft, charging under a constant voltage means the current depends on the cell’s internal impedance. If the impedance is low, that fixed voltage pushes a large current, and the heat produced is P = I^2R. With a small R, P becomes very large for the high current forced by the constant-potential charger (P ≈ V^2 / R). As the cell heats, reaction rates and possible parasitic processes increase, generating even more heat and potentially gases, which can hinder cooling and create a positive feedback loop. The result is a dangerous cycle where heat production outpaces dissipation, leading toward thermal runaway.