My Ryobi cordless drill developed a bad battery, and the price of new replacement batteries is so high that it's usually cheaper to buy another drill with batteries. I wanted a better solution, but I don't use cordless drills enough to warrant buying a better more expensive model.
So I noticed that at American Science & Surplus they were selling item #16970, a 7.2 volt 1200mah C-size NiCad battery packs at $5 each. I bought a couple and took apart my dead Ryobi battery and replaced the cells. It fit perfectly, I only had to cut & fit a couple of connections and those I was able to do easily by soldering to the nickle strips - I never had to solder to the battery itself. I did have to put about 1/2 amp into the pack for about 5 min to get the voltage up to the point where the automatic battery charger would take over.
The installation seems to be sucessful, I'll try to give you more feedback in a year or 2, after the surplus batteries are long gone & sold out.