Which of the following best describes the contributing factors to thermal runaway in a nickel-cadmium battery installed in an aircraft?

• A. High internal resistance intensified by high cell temperatures and a high current discharge/charge rate in a constant potential (voltage) charging system.
• B. Low internal resistance intensified by high cell temperatures and a high voltage discharge/charge rate in a constant current charging system.
• C. Low internal resistance intensified by high cell temperatures and a high current discharge/charge rate in a constant potential (voltage) charging system.
• D. High internal resistance intensified by low cell temperatures and a high current discharge/charge rate in a constant current charging system.

Answer: (A)

Thermal runaway happens when the heat generated inside a cell exceeds the amount that can be removed, creating a self-accelerating rise in temperature. In a nickel-cadmium battery used on an aircraft, one of the main ways heat is produced is through high current flow. If the internal resistance of the cell is relatively high, more of the electrical energy that the cell receives is dissipated as heat (I^2R heating). A charging system that maintains a constant voltage can push a relatively large current into the cell when its voltage is below the charger’s voltage, which increases heat production further. As the cell temperature climbs, cooling becomes less effective and the chemistry can respond in a way that generates even more heat, creating a positive feedback loop toward runaway.

So the combination of high internal resistance, high temperatures, a high current rate, and a constant-voltage charging condition is the scenario that most describes the contributing factors to thermal runaway in this context. Lower resistance, lower temperature, or a constant-current charging setup would generally produce less heat and pose a lower runaway risk.