What is the function of the main battery regarding the APU?

The primary function of the main battery in relation to the Auxiliary Power Unit (APU) is to start the APU. The APU requires an initial electrical input to engage its start sequence, and the main battery provides this necessary power when the APU is being activated.

This starting process is critical as the APU is used for various purposes such as providing electrical power and bleed air for cabin conditioning. The battery's role is essential to ensure that the APU can begin its operation efficiently and effectively.

While the main battery does support various operational functions within the aircraft's systems, specifically regarding the APU, it is primarily involved in the starting process rather than other tasks like charging during operation, opening or closing doors, or providing backup power for failures, which are handled by different systems.