I don't trust rechargable batteries for recording, and never had an issue using regular alkaline batteries in either of my Sony MD recorders. (I used 2 recorders, as it was much quicker to change recorders than to change minidiscs.)

You should get at least 2 and a half hours out of a pair of alkaline AA batts, probably more but I wouldn't push it, 2 discs should go, a third disc, maybe not.