I got interested in this subject due to my old Makita 9.6v drill. I have 2 charger stations for it, both refuse to work. I'm still not sure if it's the chargers or the batteries. I surfed through a few Youtube vids on the subject myself. ( Youtube vid link above is "private" now...)
I saw a few techniques. One was to zap the battery with a welder, either straight or reverse the polarity first, then zap + to + and - to - . I'm not sure how to do this with my stick welder. It has DC straight and DC reverse ,and high and low leads.
Also saw one where the guy just hooks up two good batteries in series to double the voltage and zaps that way.
I have one good genuine Makita battery from Ebay, and an older one. Unfortunately I threw another one away that might have been a prime candidate.
Anyway, I ended up zapping both batteries with a battery charger that has a 10 amp 12v setting. Then I jammed two 4d nails into the battery leads and just put the 12 charger on it, on 1.5 amp and let them charge for around 1/2 hr. Kept safety glasses on and felt the batteries for overheating.
Unscientifically, it worked! Still don't know if it's the chargers or the batteries. In my total cheapness, I know there's many many Makita knock-off chink batteries for sale on Ebay- cheapest around $19.
That drill has served me well for many years, really reluctant to give it up.