This method can work for NiCad and lead-acid batteries:
http://hackaday.com/2008/11/27/zap-new-life-into-ni-cads/
High amperage in addition to high voltage can help. Of course this is very dangerous, the target battery and the source battery could explode, so you should wear goggles and put the batteries in places where you can afford for a bunch of hot acid (and possibly fire) to fly. The safest way to do this would be to have a bunch of long cables running to each battery in an unpopulated non-wooded area, and you standing in the middle with the wires to touch together. It only takes a fraction of a second but multiple "touches" can help.
I think I just threw out a battery that could have been fixed this way :(
