I've just revived 2 NiMH 12V battery packs for a De Walt drill. Neither would charge, but now one's showing 13V and the other 9V after charging. Both will power the drill (but the 9V is obvs on its last legs). Never tried this before, but since the drill was about to go in the recycling, thought it was worth a try.
I found a load of guidance on YT, including for Lithium batteries. Essentially, you connect a good battery of the same voltage to the dead pack for a few seconds, +ve to +ve, -ve to -ve. Had to do the 9V one a couple of times, but it went from showing 0V to 7V then 9V after 2 attempts. Further attempts yielded no further gains.
Clearly, and particularly with Lithium, there are risks, and I approached this with considerable caution - bit it worked for me. I fly r/c aeroplanes, and Li batteries are commonly used, but there is a lot of 'how not to' guidance for these and there have been some unfortunate incidents. So, you pays your money.... FWIW, I only have one Li battery for my 'planes, and that powers the engine starter (essentially a motor with a rubber cup on the shaft, which you press onto the aeroplane motor to spin it and fire it up) so it's outside of the aeroplanes all of the time, and gets charged about once a year!
