For the 4th time in less than 10 years we are buying a new cordless drill kit at work becuase it's cheaper than replacing the batteries on the existing set. The dead set this time is a 18v DeWalt which retailed for $100 new with 2 NiCad batteries plus the drill and charger. The replacement is a Ryobi with 2 lithium cobalt/manganese batteries plus the drill and charger AND an impact wrench that uses the same battery. The Ryobi set new was also $100.
2 replacement 18v DC9098 DeWalt batteries are $100 (on Amazon). The least expensive new DC9098's were $56 for a pair on eBay, but the company will not buy on eBay because of Paypal and the liability of allowing someone using the company name to bid on eBay.
I managed to do LiFePO4 conversion to one of the DeWalt batteries. Radio Shack's 270-407 8AA cell holder ($2.29) is the right length and width to fit inside the battery shell without cutting. I bridged out 2 cells with a wire, leaving six 3.2 lipos in series to give a 19.2v working pack. The amp-hours are only about 1/2 of the original 2/3's sub-C nicad pack but the current from the lipos runs the drill just fine.
The only hitch now is how to tweak the DeWalt charger. It charges the lipo pack and then errors out flashing red. The erroring out isn't a problem, but the pack voltage went above 26v before the charger gave up. A bad nicad pack doesn't do that. The bad packs we have only have 1 and 5 working cells left and the charger seems to know that and quits without overcharging either. The lipo pack didn't get hot and one overcharge didn't seem to ruin it.
The nice thing about having a spring loaded battery holder is if one cell goes bad it's easy to open the shell and replace that one to keep the pack fresh. In theory we should never need to buy another drill if I can get a charger that works properly.
The lipos AA cells are sold at Radio Shack, but they charge $10 for a 2-pack. From a US seller on eBay I get them for 1/2 that and just bought some to donate to the company to fix the drill.