Are you spending time getting ready for the flight, then when you are ready for pushback, you try to start the APU, so you can start the engines during pushback? Therefore quite a bit of time passes before you actually try to start the APU.
It's possible, that if there is some time passing, between the time you load the 737 into the sim, and then get around to starting the APU, that the main battery is losing power, even though you're connected to the External Power supply. You could try adding the line electric_always_available=1 to the aircraft.cfg, under the [electrical] section. Like this:
[electrical] max_battery_voltage = 24.0 generator_alternator_voltage = 30.0 max_generator_alternator_amps = 400.0 avionics_bus=0,1,17 avionics=1,1,17 flap_motor=0,1,17 gear_motor=0,1,17 autopilot=0,1,17 pitot_heat=0,2,17 additional_system=0,1,17 marker_beacon=1,1,17 gear_warning=0,1,17 fuel_pump=0,1,17 starter1=0,17,17 starter2=0,17,17 standby_vacuum=0,1,17 hydraulic_pump=0,1,17 fuel_transfer_pump=0,1,17 light_nav=0,1,17 light_beacon=0,1,17 light_landing=0,1,17 light_taxi=0,1,17 light_strobe=0,1,17 light_panel=0,1,17 light_recognition=0,1,17 light_wing=0,1,17 light_logo=0,1,17 light_cabin=0,1,17 directional_gyro=0,1,17 directional_gyro_slaving=0,1,17 electric_always_available=1
That way, the main battery, should never run low.
Note. I open the aircraft.cfg with Notepad. After making the changes, select Save (not Save As...). Also. Always backup the aircraft.cfg BEFORE you make any changes to it!
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