We used to zap nicads with double voltage for a few seconds and that seemed to do the trick. But in your instance where you have a bank in series, I rather doubt if you could get a balanced outcome unless you opened it up and did it one by one. There was also another state they could achieve which I could never understand, that was reading high. In my trade we had a 2,4V memory cell, that could read 2.7V when stuffed and not working.