Drill batteries usually consist of multiple nickel cadmium (NiCad) or nickel metal hydride (NiMH) cells. Each cell produces 1.2 volts and is wired in series to produce the required voltage to power your drill. Battery cells can only be charged and discharged a certain number of times before their ability to retain a charge deteriorates. Dead drill batteries can be fixed by simply replacing the individual cells. This is a much cheaper option than replacing the complete battery pack.
Remove the dead battery pack from the drill and place it on a suitable workbench. Read the label on the drill pack to determine the type -- NiCad or NiMH -- and the number of battery cells it contains. The label also states the output voltage, which is a multiple of 1.2. Divide the output voltage by 1.2 to work out the number of cells in the pack. For example, if the output is 9.6 volts, dividing it by 1.2 equals 8 cells.
Purchase the correct type and number of battery cells you need to replace the dead cells in your battery pack. Batteries are available at an electrical store.
Remove the cover from the dead battery pack. Battery pack covers have several small screws holding the cover in place, or hinged clips. Use a small screwdriver, usually a Phillips, to remove the screws. For clips, insert a small flathead screwdriver under the clip and twist to unclip it. Once the cover is removed you will see the battery cells.
Check how the battery cells fit into the battery pack. Some simply push in place; others are soldered. If the cells push in place, use a small screwdriver to lift each cell from its compartment. If they're soldered, heat a soldering iron until it reaches operating temperature. Touch the end of the soldering iron to the cell terminal. Allow the solder to melt. Remove the soldering iron and quickly lift the battery away before the solder hardens. Repeat the process to remove each dead cell. Turn off the soldering iron.
Place the dead cells away from the battery drill. You don't want to mix the good cells with the dead ones.
Insert the replacement cells in the battery compartment. Ensure you insert them the correct way. The connections in the battery pack are labelled "+" (plus) and "-" (minus). The cell terminals are labelled the same way. Push the new cells carefully in place; if they need soldering, heat up a soldering iron. Insert a battery cell in the compartment, then touch the soldering iron to the cell terminal. Apply the solder, allowing a little to melt. Remove the soldering iron and solder. Let the solder solidify. Repeat the process on each cell terminal. Turn off your soldering iron.
Replace the battery cover. Screw in the cover screws or use your fingers to push the hinged clips back in place.
Charge the battery pack for an hour. This makes sure all the cells have equal charge. Put the battery pack in your drill and begin using it.
Some drill battery packs are sealed units. Don't attempt to break the pack open. Get a replacement pack, ensuring it has a removable cover.
NiCad and NiMH cells cannot be disposed of in regular trash. Take them to a recycling facility.