''3 batteries? An option?
I'm not sure what you mean by starting a 747-400 on batteries.''
2 main batteries in the radio electric bay and 1 in the apu bay. All 3 are the same item and can be interchanged.
With just DC from the main batts running the aircraft, you start the APU from the APU bat. When that is running, you have AC and bleed air to start the main engines. Í only did that once when picking up an aircraft from maintenance on a remote stand and the power cart's frequency was too far out to connect it to the aircraft. The literature stated that the APU batt was good for three starts, but seeing how long it took for the charge rate to come down afterwards, I don't believe it. I would be suprised if we could have got another start out of it.
''APU generators are not interchangeable with IDGs, the IDG is a combined GEN/CSD'''
I thought I remebered reading the gen part of the IDG could be unbolted and fitted in place of the APU gen and vice versa. This was on early GE powered 400s, but I could be getting confused with the Classic.
Of course, all of these could be options. There seem to be a lot in Boeings unlike in Airbus where they believe you should accept the way they build it.