Cordless 18-volt drills come with a rechargeable battery pack. The battery pack is made up of several rechargeable nickel-metal hydride (NiMH) battery cells. After long periods of repetitive recharging, these battery cells will either hold a weak charge or will not hold a charge at all. In either case, it is possible to replace individual or entire rows of NiMH cells in a drill's battery pack. However, doing this yourself requires dismantling the battery pack, which in some cases, means damaging the pack's plastic housing.
Remove the 18V battery pack from the drill's handle or the external battery charger. Release the battery pack by pressing on the release tabs on either side of the pack.
Relocate the battery pack to to a flat work table, preferably in a garage or workspace that is separate from your home. Determine how the battery's outer housing is held together. In many cases, the battery's outer casing is secured by a few Phillips screws or it is one solid piece of plastic. Remove the screws, as necessary, but if the pack's housing is one piece, cut it open to access the battery cells.
Attach a chisel tip soldering iron to a soldering gun. Plug in the gun's power cable and allow the iron to heat up for about 10 minutes. Firmly touch the chisel tip iron to the crease lining the centre of the battery pack. Press down firmly on the crease with the iron, and run it along the battery pack's crease to cut open the battery pack's housing. Grasp the top section and remove it from the pack to reveal the internal battery cells.
Turn on a voltage meter and touch the red probe to the positive side and the black probe to the negative side of each battery cell. The meter will tell you which cells have a charge and which cells are dead. Note the polarity alignment of the battery cells for when you install new cells.
Touch your heated soldering iron tip where the wire connector's meet each battery cell. Desolder the wire connectors of each dead NiMH cell you want to replace. Remove the desoldered battery cells. In many cases, the cells will be glued together inside the pack. Use a razor blade or similar tool to separate the dead cells from the pack.
Place the replacement NiMH cells inside the cell pack into their appropriate slots. Make sure each battery is aligned properly with the other cells in the pack. Touch the wire connectors to the positive side of each new cell. Use the soldering iron and some fresh solder to attach the wires to each new battery cell.
Place the cell pack back inside the dismantled housing. Place the top section of the battery pack's housing back onto the bottom section of the housing. Replace any retaining Phillips-head screws, as necessary. If you cut open the battery pack's housing, secure the two housing pieces together with a strong plastic adhesive or duct tape.
Performing this procedure yourself is likely to void any warranty that might otherwise be in effect.