Flat batteries on a Nimrod render it a museum piece, which I guess is what it is. Without power to the Battery Busbar, which supplies the GPU switching circuit (for both AC and DC GPUs), the aircraft is dead and cannot be powered up. I suggest you disconnect both aircraft batteries, and get two 12V car batteries in series in to produce 24V and hot wire them to one pair of the now available battery terminations to power the battery busbar until you get the 200V GPU on line. Then you can reconnect the aircraft batteries and charge them up.
I suggest you disconnect the batteries every time you leave the ac for any significant time. There is one single unswitched lamp (sec jacks locked) that will be always be illuminated while the batteries remain connected and it will drain them in due course.
I once flattened the batteries while on det (refuel master switch, Doh!). Fortunately, it was a 2 ac det so we had 2 spares, not far away....
Kev