If everything is shut including the APU and the engines, how long can the batteries supply the power to the necessary DC and AC equipments for? Are these the same batteries that power the electric starter motor for the APU too?
Target requirement is usually 1/2 hour and the equipment supplied is very minimal but enough for safe flight. The APU battery is usually dedicated. The EEC computers have their own PM generators and critical flight control modules have their own powerpacks on some aircraft.
Re:Limitations etc...
A 727 APU can only be operated on the ground. Many 707s had no APU so a huffer truck was needed to start the engines. Buick V8s were commonly used as the motive source if I recall correctly.