What is most likely to cause thermal runaway in a nickel-cadmium battery?

Thermal runaway in a nickel-cadmium battery is most likely caused by excessive current draw from the battery. When a battery is subjected to a load that exceeds its designed capacity to deliver current, it can generate more heat than it can dissipate. This excessive heat can increase the internal temperature of the battery, leading to further chemical reactions that produce even more heat. If this cycle continues, it can result in thermal runaway, potentially damaging the battery and posing safety risks.

In contrast, low voltage output, high ambient temperature, and insufficient charging time can contribute to battery issues but are not the primary drivers of thermal runaway. Low voltage output may indicate a problem with the battery's ability to deliver power but does not directly create the conditions for thermal runaway. High ambient temperature can certainly affect battery performance and longevity, but it does not inherently cause the rapid increase in heat associated with thermal runaway unless combined with excessive current draw. Insufficient charging time can lead to inadequate battery performance and capacity, yet it does not produce the rapid temperature escalation seen in thermal runaway scenarios.