Thanks!
I got the part about the paralleling when I looked at the schematic again. It's definitely not allowed.
However as I was reading through the AMM chapter that you mentioned, I found that one of the steps requires the Batteries to be in Auto (On) and a Note saying "If the APU is to be started on EXT POWER, make sure that the batteries are switched ON. If not, the APU may not start and shut down."
But you're right about the DC BAT BUS supplying power to start the APU. Just can't figure out why it still needs both batteries to start.
Edit: Re-format
Last edited by sarius; 7th Jun 2012 at 17:28.