The batteries for these things are pretty costly and my experience, mainly with Ryobi, but with several other manufacturers on a smaller scale, has been mixed. The one thing they all seem to have in common is that their battery pack life, not the charge-life, but the period over which the battery pack is usable, is disappointingly short. I recently bought four of the RYOBI batteries, as I have garden tools and shop tools that use them, last July, IIRC, and one of those is already useless. The charger, I believe, is at fault. Now, I've had RYOBYI 18-volt-battery-powered tools for seven years, and find that, now, I have more permanently-dead RYOBI batteries than useable ones.

By contrast, I have a Gardner-Denver Wire-Wrap tool that uses the same sort of sub-C ni-cd batteries that I bought in 1979, and I still use the first battery that came with it. Of course, I have dozens of other batteries for it, too, as I build 'em out of defunct power-tool batteries. What I've found is that if one has a reasonably high-current 100-volt supply available one can gate a pulse of current through a NiCd battery to "resurrect" it. That 30-year old battery went bad, finally, in January (?) of this year, but when I High-Current-Pulse charged it for about 3 minutes, it retained about an hour's charge in full use and was subsequently rechargeable in its original charger. It remains useable to this day. I've tried this with power-tool batteries, with some limited success, albeit that I've found I have to repeat the current-pulse-charge method with the particular batteries I do that with about every fourth time I attempt to charge them. There's a lesson in there somewhere ...

Perhaps you'll be able to design a better charger circuit for them, but I'd be wary of the commercial power-tool batteries. I somehow don't supsect the batteries themselves so much as the chargers. Now, they do have Lithium-Ion batteries, at somewhat higher cost, and I have no experience with them. Maybe they're better. I haven't bought a lithium battery/charger combination ... yet ... because of the poor performance I've gotten from notebook batteries, etc that use them. Oddly enough, cellphones don't suffer from the ailments that plague notebook batteries. With these power tool batteries, IMHO, all bets are off!

RE