Just out of curiosity I tried last night.  Left the APU running, Engines off, battery to battery bus, source swtich to APU GEN and the Generator Power Switches to ON.   All the power indications seem to stay on but I could not start the plane.   Nothing happens even tho I had the APU Bleed Air On, X-over set to both Auto and ON with both packs turned off.    Tried multiple times and I've talked to few people who have experienced the same issue but don't come to the forums.   IF there is a certain place to look that would be great.