How long should the aircraft battery remain off after two 30-second runs during APU start?

The correct answer is based on the operational guidelines for managing the aircraft's battery during APU (Auxiliary Power Unit) startup procedures. After conducting multiple start attempts of the APU, it's important to allow the aircraft battery to rest to prevent overheating and potential damage to the battery.

In this particular scenario, following two 30-second APU start attempts, a rest period of 20 minutes is recommended before any further attempts or uses of the battery. This allows the battery to cool down and recharge slightly, ensuring that it is in good condition for the next use and to maintain overall battery health.

Paying attention to these operational procedures helps ensure safety and reliability in the aircraft's electrical system. Therefore, 20 minutes is the appropriate time frame to let the battery rest under these circumstances.