Cordless drills are useful around the home for repairs and building projects. Most cordless drills run on a rechargeable lithium-ion battery. After a while your drill's battery may start to lose charge quickly, or not sustain a charge at all. In most cases, this problem is caused by the drill's charging unit, or the battery pack itself. While you can rebuild your battery pack yourself, it requires that you take it apart, which may void your drill's warranty.
Remove the battery pack from your drill or charging unit. Relocate the battery pack to a flat work surface. Determine how the battery pack is held together. In many cases you just remove a few Phillips screws. Otherwise, you may have pry apart or cut open the battery pack's housing.
Insert a soldering chisel tip into your soldering gun. Plug in the gun's power cord, and allow the tip to heat up for about five minutes. Locate the crease on the battery pack separating the pack's top section from the bottom section. Firmly press the heated chisel tip to the crease, and run the chisel tip along the battery pack's casing to cut it open. This step is optional if your battery pack is fastened together with screws.
Pull apart the top and bottom sections of the battery pack's housing. This will reveal the battery cells bound together inside the battery pack's casing. Use a utility knife to cut away the adhesive securing the battery cells inside the pack's housing. Lift the bundle of battery cells out of the pack's housing, and set them aside.
Number each battery cell with a permanent marker, and examine the polarity alignment of the battery cells. Determine which side of each battery is positive (+) and which negative (-). Use a battery tester to figure out which battery cells are dead, or going dead. Turn on your battery tester, and touch the positive probe to the positive end of each battery. Make a note of what battery cells are dead.
Remove the dead battery cells from the bundle. Use a utility knife to cut away the adhesive securing each bad battery cell in place. Use your heated soldering iron to desolder the wire connectors from each battery cell. Dispose of the removed cells.
Place the new battery cells into the empty slots in the battery bundle. Make sure the polarity matches the rest of the cells in the bundle. Use your soldering iron and fresh solder to connect the battery wire connectors to the positive end of each battery cell.
Wrap the entire bundle with electrical tape until all cells are secured to the bundle. Place the battery cells back into the battery pack's bottom housing. Replace the top section of the housing, and replace any retaining screws as needed. If you had to cut open the battery pack, wrap the battery pack with duct tape several times until the top and bottom sections of the casing are secured together.