What does the term "thermal runaway" indicate concerning aircraft batteries?

The term "thermal runaway" specifically refers to a condition in which a battery experiences a rapid increase in temperature and pressure, often due to an internal short circuit. This is particularly critical in lithium-ion batteries, which are commonly used in aviation applications. During thermal runaway, the heat generated by the battery can lead to further chemical reactions that produce even more heat, thus creating a cycle that can result in catastrophic failure, such as fire or explosion.

In this context, when a battery is internally shorted, it creates an excessive amount of heat that can trigger thermal runaway. Understanding this phenomenon is essential for maintaining the safety and reliability of aircraft battery systems. The other options do not relate to the specific and dangerous condition of thermal runaway, as they refer to a fully charged state, leaking acid, or a general need for maintenance unrelated to the internal issues of a battery.