Flat Batteries
A couple of points about batteries.
When a Ni-Cad battery is being charged it can have thermal run-away, even in fully controlled conditions in a battery charging bay. Results are pretty devastating and I would hate to see it occur on an aircraft.
The second issue is a flight safety one. Ask yourself why you have batteries on aircraft. They are there to run the necessary electrics when all else has failed. Certainly in the UK, there are requirements that an aircraft should be able to operate on emergency systems for 30 minutes on battery power alone.
I would suggest that if an aircraft is dispatched with a virtually flat battery, sooner or later someone is going to find out to their cost that a flat battery is not a good idea. Just imagine the scenario, flat battery, engines on, battery starts charging, taxy, take off, electrics fail, electrics shed to emergency bus-bar, battery runs down, oops!
To my mind that scenario gives only one option, replace flat battery with a fully charged one immediately.
P