This has happened on several occasions:

    With External power and Air active, I fuel the plan (GSX) and then load the passengers before starting the APU.  But when I then attempt to start the APU, there seems to be no power available.  The meters show power and connection and the battery shows that it is being charged but I am not able to start the APU (nothing activates on APU panel).  As a test, I disconnected the EXT power and all the lights on the engineers panel went dark whereas normally they stay lit even on battery power alone.  That test seems to indicate that the battery has lost charge even though the meters show that it is in positive territory.  I find no way of recharging the battery except by connecting to external power!  What am I missing here?  The battery (if charged) and/or external power should be sufficient power to startup APU.  External power should even be sufficient to start the engines.  Is there some logic behind this that I am missing? 

Can someone shed some light on this?  I know I can start the APU right away and it works but why not after EXT power has been connected for just a brief time?

Craig