Originally Posted by EconomyClass
Airliners don't primarily depend on batteries for electric power do they? Aren't these just like the computer UPS, there to insure power continuity in emergencies?
You are right. The main source is engine-driven generators. The second source is APU driven generator. The third source is RAT driven generators. The fourth source is battery.
However, to start the plane with no external power source, you need the battery to start the APU.
Also, in flight, if you have a dual engine failure, you need the battery (or the RAT) to start the APU before you can attempt an engine restart (unless you are in the right zone of the envelope to attempt a windmilling restart).