Old Engineer: #1624
I continue to be puzzled as the why the APU battery must remain connected
after it has started the APU...
Not quite sure what you mean by "remain connected" in this context, but
from what I understand, the battery must be fast charged after apu start,
so that it is ready for another start in flight at any time.
The only way that this can be done, apu not running, is to power the charger from
the generator bus. The battery / charger must be connected to do this...
-