Battery endurance is only 15 minutes, and needs to be ON even when using Ground Power

[Rudel_chw]

While editing a cockpit familiarization mission, I became aware that the battery run out after 15 minutes of operation when the main engine is off. The manual of the real aircraft says that the endurance should be at least half hour (mentioned on the Generator failure section).

If I connect external electric power, then the battery does not run out, however, the aircraft seems unable to operate with just Ground Power, the battery switch has to be at ON at all times. This does not correspond to what the manual of the real aircraft says:

"Power may be supplied by one of three sources: the 75A Generator, the 24V 17 Ah battery, or the external power receptacle".