What ever the situation you don't want to be flying around on a battery.
All the comfort features are turned off. All remaining power is directed
to the cockpit for radio - navigation and instruments to find a safe place
to set down ASAP. Someone with a bit more experience maybe could
provide an answer if the APU (Auxillary Power Unit) can be started in the
air to provide a reserve source of basic electrics ?

I flew into ANC on a 707 the last half hour was on battery * no fun !