Update May 2011: It's been five years since I posted this instructable - and I have another battery pack to work on. This one is a 19.2 volt pack, but...
I hate it when my cordless drill gives up in the middle of a job. It seems that the supplied battery packs just don't cut it. So I opened up the battery pack and replaced the NiCads with much longer lasting NiMH batteries. Now I get a much longer use between charges.
My battery pack had 4 Torx screws (I later replaced with regular Phillips screws for easier access). Inside you'll find 10 NiCad batteries. They appear to be just a bit smaller than 'C' cells, and all attached together in series. 1.2 volts X 10 cells = 12 volts. The cells are marked as 1300 mAH which is why they don't last very long. You will probably want to photograph the arrangement of batteries since you will need to make a new pack look just like this. Also notice the thin insulator that keeps the top battery from shorting against the batteries on the bottom.
Step 2: Assemble the new pack of batteries
Aquire a new set of identically sized batteries. In my case the size is called 'Sub-C'. I got them at BatterySpace.com for $24.00. They come with tabs attached. Carefully assemble the new cells into the same arrangement as the original, and solder the tabs appropriately. Be careful not to short out a cell while assembling it - even briefly! You will also need to scratch the surface of the leads before soldering and put a thin coat of solder on each tab before assembly (called tinning). Re-assemble and test. Be sure to re-use the fusable link that came with the original battery pack