Sorry for the confusion, inmate.. I'm talking about both the battery cart style external power unit, and the generator style (which is what I meant when I said "regular GPU"). It is the generator stye GPU that tends to play havok with the aircraft generator.
I've seen plenty of generator-style GPUs that have very poor regulation, but even good regulation at a slightly high voltage can trip off the generator Our answer to this problem was to force the external power "off-line" when the generator is switched "on-line"... but this would prevent re-charging a battery-style external cart from the aircraft generator. I'm thinking it's a bad idea to be doing that anyway, and I was looking for confirmation.
Goodgrief seems to indicate he'd be counting on it.... His response seems to suggest that after each start you'd sit there for 15 or 20 minutes burning fuel to re-charge the power cart (You'd have to inorder to get a decent recharge). Assuming the aircraft battery was low as well, the generator would be trying to charge that up too (or do you take the battery off line while charging the battery cart.) The extended run at high current can't be doing great things for the generator, and at say $600-$1000 and operating hour - that's a hell of a price to pay for using a helo as a battery charger. Wow! I wouldn't want to be doing that a lot.