Final answer:
Thermal runaway in a nickel-cadmium battery is primarily due to high internal resistance, exacerbated by high temperatures and high current discharge/charge rates in a constant voltage charging system.
Step-by-step explanation:
The contributing factors to thermal runaway in a nickel-cadmium (NiCad) battery installed in an aircraft are best described by: High internal resistance intensified by high cell temperatures and a high current discharge/charge rate in a constant potential (voltage) charging system. The key elements contributing to thermal runaway include high internal resistance and high temperatures.
Internal resistance of a NiCad battery increases as the battery is depleted, and it can also vary based on current magnitude, temperature, and usage history. High temperatures can exacerbate this issue, and when combined with high current charging or discharging, particularly under a constant voltage system, it can lead to excessive heat generation. This heat can further increase internal resistance, creating a feedback loop that results in thermal runaway.
The construction of NiCad batteries is such that they have a high discharge current and high capacity due to the optimized distance and surface area between the electrodes. However, because the internal resistance increases with use and recharging cycles, under heavy load and increased temperatures, energy dissipated as heat can raise internal resistance further, reducing terminal voltage and potentially leading to overheating and thermal runaway.