As above, the main battery is only the backup source for electric brakes. The normal source of brakes is the 28VDC.
The main battery provides power for:
airplane power-up
APU start (assists APU battery)
refueling operations
towing operations
electric braking (as backup power source)
captains flight instruments (energizes essential instruments until RAT
deployment)
The APU battery provides power on the ground for:
APU start
navigation lights (during battery-only towing operations)
Clearly the 787 isn't short of electric power sources. There are . . .
four variable frequency engine starter/generators
two variable frequency APU starter/generators
three external AC power receptacles
one Ram Air Turbine (RAT)
one main battery
one APU battery
three flight control Permanent Magnet Generators
two EEC Permanent Magnet Alternators
Last edited by whatdoesthisbuttondo; 29th Mar 2013 at 18:14.