Yes the APU has a starter/generator capable of 90KVA up to 31K and 66KVA up to 41K. Things they are a changin'
Don't forget the battery charger is on line when it's charging and the battery when the charger is cutoff. That makes four sources of DC on the NG with the engines running.
http://www.b737.org.uk/electrics.htm Here's the rest of the story after you look at the schematics.