wrote on Jan 7^th, 2013 at 6:59pm:

You should be able to start the APU using only the ground power. The battery is always on a standby mode incase of loss of power, I think.

Right..  ground-power would be all you need even IF you let the battery drain..

And the start-sequencer/avionics should see power by any three of the sources (battery, ground-power, APU)..

The problem is that if you let the battery drain.. that's it.. no avionics\start-sequencer/APU startup.. nothing, no matter what you do..

The simplest fix would only need to let the start-sequencer work off APu or ground-power.. so at least you could charge the battery via engine-driven generators..