Which battery is discharged if the DC Service switch is left on while powering the NAV lights?

When the DC Service switch is left on while powering the NAV lights, the battery being discharged is the APU Battery. The APU (Auxiliary Power Unit) battery is designed to provide power for systems when the main engines are not running and can supply power to various electrical components, including the NAV lights.

In this scenario, since the NAV lights are being powered and the DC Service switch is engaged, it indicates that power is being drawn from the APU battery, which is typically utilized for such purposes when the aircraft's main systems are not operational. The other batteries mentioned, such as the Main and Emergency batteries, typically serve different roles, with the Main battery supporting the primary electrical systems and the Emergency battery reserved for critical systems in the event of a main power failure. Therefore, the APU battery is directly tied to providing power when auxiliary functions like NAV lights are in use while the DC Service switch is on.