Originally Posted by undflyboy06

I thought it didn't matter if you started the APU with the avionics on because the APU is started by using power from Battery 2. During APU starting, battery 1 is isolated from the load buses. While battery 2 provides power for APU start, battery 1 is used to provide stable electircl power to the equipment that can be adversely affected by voltage transients.

So you can safely turn on the batteries, with the avionic switches engaged, let the APU due the self test, and fire it up.