If you have a favourite old cordless drill that just does not seem to have the "oomph" it once had, it is probably due to a failing battery pack. Since most cordless drills outlive the effective life of their battery packs by quite a bit, it is a good idea to know how you can get like-new power back into your drill by rebuilding the battery.
Check the battery pack for small screws that hold the case together. Use a small Phillips head screwdriver to remove them, and open the case.
Attach a chisel tip to a soldering iron if the battery pack has no screws and is a solid one-piece case. Use the hot chisel tip to cut open the pack in a line around the top of the base of the pack, and then open the case by separating the two pieces.
Take note of the rechargeable battery cells within the battery pack. They will be a simple grouping of size "C" batteries in a generic cardboard wrapping. Write down the arrangement of the cells and the way that the wiring connects them for future reference when you reassemble the pack. Use the soldering iron to warm the soldered connections and to make it easier to remove the batteries. Remove the batteries, and set them aside for recycling.
Brush the contacts firmly with a wire brush to clean them and get a good conductive surface.
Look to your notes and arrange the new rechargeable battery cells in the same way. Solder the connections to the new batteries.
Put the two pieces of the battery pack case back together. Fasten with screws if you have them, or make up a batch of epoxy glue, and glue the two pieces together. Allow the glue to dry completely. Recharge the pack for at least 12 hours before using it for the first time.