Most rechargeable drill battery packs contain nickel cadmium or nickel metal hydride cells wired in a series to increase voltage: single cell NiCad and NiMH cells produce 1.2 volts. Rechargeable battery packs start to lose their charge quickly once they have been charged and discharged a certain number of times: the number of times depends on the overall use of the drill. If this happens, you need to get your rechargeable drill battery to hold a charge again.
Plug your rechargeable drill battery into the charging unit and turn on the charger. Let your battery charge until the charger indicates the battery is full. Turn off the charger and remove the drill battery.
Let the battery rest for an hour. Insert the drill battery in the drill. You need to run your drill on its fastest speed setting until it stops operating. This may take a while, so it's best to clamp your drill in a vice so you don't have to hold it. Make sure the drill is securely held in place. Turn on your drill and let it run. Some drills have a button you press to keep the drill running, other drills need to have the switch held in to operate. If you need to hold the switch in the "on" position, then use electrical insulating tape wrapped around the handle and switch to hold the button in place.
Let the drill run until it stops. Turn off the drill, but take care: the drill will be very hot. Wait for 10 to 15 minutes for the drill to cool and then turn on the drill. It will run very slowly. Let it run until it stops. Repeat the process every 15 minutes until the drill won't run. The battery is fully drained.
Remove the battery from the drill. Plug in your charger and turn on. Allow the drill to charge fully. It will take longer than usual. Once charged, your drill battery holds a charge again.
NiCad battery packs in particular suffer from what is called the "memory effect." This is when your charger indicates the battery pack is fully charged, but goes dead very quickly. The reason this occurs is due to changes in the chemical structure of the battery cells. Rechargeable battery cells contain tiny crystals that retain energy. If a battery is not fully drained before getting recharged, the cells grow. This change reduces the surface area of the crystals meaning they cannot retain as much energy and although the charger indicates the battery is charged its capacity is reduced.