What is one function of the aircraft battery when the generator is not available?

The aircraft battery serves a crucial role in starting the engine, especially during situations where the generator is not operational. When the generator is offline or unavailable, the battery provides the necessary electrical power to crank the engine and initiate the start sequence. This process is essential because, without the battery, there would be no source of power to turn the engine over, making it impossible to begin flight operations.

The other options, while related to the functionality of the electrical system, don't as directly address the primary role of the battery in starting the engine under these circumstances. Charging the auxiliary battery or powering the auxiliary generator are not primary functions when the generator is inoperative, and starting the generator before engine start relies on the engine being operational first, which the battery helps enable.