Twinbird,
There you go - Buitenzorg got it, and it makes absolute sense although it is not well-described in the manuals. Both the generator control units and the EECs are powered from the essential buses before and during the start, although the power to the starter motor itself comes from the main bus.
Be careful - as Geoffers said, starts below 23v could well be trashing the back end of the engine.
As others have said, if you regularly exercise the batteries by using them instead of GPU to start, they will hold 23v OK. If you always do ground power starts, the batteries rapidly deteriorate. Fortunately, you can fix them by using them, and in fact I have seen batteries recover to normal within several days of being used regularly for starts.
There is also a new-ish modification available (supplement 85 - improved engine starting kit) which gives a bigger aux battery, and connects both the main and aux batteries in parallel for start, which gives a bit more oomph for those operators in situations like those described by Maeroda. It also gives longer endurance following a double DC gen fail.
Last edited by Non-PC Plod; 2nd Dec 2017 at 06:24.