Open the aircraft.cfg file to edit. Goto the [electrical] section and make the following change.
[electrical]
;BusType, MaxAmpLoad, MinVoltage BusTypes:0=MainBus,1=AvionicsBus,2=BatteryBus,3=HotBatteryBus,4-7=Generator/AlternatorBus(1-4)
flap_motor = 0, 5 , 17.0
gear_motor = 0, 5 , 17.0
autopilot = 0, 5 , 17.0
avionics_bus = 0, 5 , 17.0
avionics = 1, 5 , 17.0 //<--- change the 1 to 0
pitot_heat = 0, 2 , 17.0
additional_system = 0, 2 , 17.0
...................
i.e. avionics = 0, 5 , 17.0
This will tie your avionics to the main bus so that they get power from the main battery switch.




