The modification that I originally used to solve the loss of battery power was using electric_always_available=1 under the [electrical] section of the aircraft.cfg, but I haven't used it in a long time.

As for the loss of Avionics (not a loss of battery/generator power), to stop that from happening, I use the default C172 in a cold and dark setting, except with the Avionics Switch ON. Doing that prevents a lot of add-on aircraft from losing the avionics during the flight.

Another way of doing it is to create a keyboard shortcut for Avionics master (on/off)