Intro: Rebuild an 18v Drill Pack for 15 Bucks!
Up until about a year ago I worked at a battery store, often rebuilding drill packs and Roomba batteries and the like, so when a battery at my local hackerspace (JaxHax) went kaput I decided that I could rebuild it far cheaper than buying a new one. You can use this same basic method to rebuild all Ni-Cd and Ni-MH cordless tool batteries as well. Essentially I took a cheap no-brand battery with suitable cells and transplanted them into my other battery's shell.
The pack came from harbor freight on sale for 15 bucks, and for 15 cells at 1500mA that's pretty good. Whether these are long lasting quality cells or whether they crap out before long we'll have to wait and see, but it's worth a shot. Stock DeWalt packs have 2000mA cells, so this is only 75% the original runtime, but for most things around a hackerspace it's suitable.
Anyway, As you can see from the photos, I opened both packs, removed their cell "cores", cut the new cells into 7 pairs with a dremel, then imitated the original cells layout, soldered desoldering wick where I needed connections, and salvaged the casing, screws, thermistor, and connector from the original pack. For the cell connection on the front end of the pack I used a peice of springy nickel coated steel from an RC car's controller switch as the negative cell's contact didn't have a tab to solder to and soldering to rechargeable batteries is not a good idea and difficult even if you try.
The sharpie marks across the tops of the cells are more just as a reminder and guide while you solder your wick tabs in place, but aren't needed. If you have any questions feel free to ask!