The entry
electric_always_available in the aircraft.cfg will not prevent the battery from draining.
According to the SDK
This flag enables electrical power regardless of the state of the battery or circuit.
The only way to prevent the battery from draining is through a registered version of FSUIPC. (but this would be a cheat
).
The battery is supposed to provide the aircraft with electrical power until you start the APU or the engines and supply electrical power through the generators.
You could also connect to an external ground supply (if available).
Cheers
Thomas