There is meant to be a method of recovering ni-cads, it involves connecting across a car battery to break the whiskers that grow between the cells. Try googling for the method (ebay sellers also sell these instructions for a couple of quid).
I haven’t tried it, and it sounds like a risky strategy to me (definitely one to do outside with a lot of space around you), but it is meant to work.
I have 2 Bosch batteries that I need to recover, but as they are 14.4V, it means connecting 2 car batteries up which is both a pain in the butt, and twice the firepower, hence I haven’t done it yet….
Or spend £20-£30 on 2nd hand ebay ones.