Cordless drill battery packs have made using power drills substantially easier. The time is gone when you had to search around for a plug socket and more importantly, cordless drills are far safer to use. The average cordless drill battery produces about 18 volts, but due to technological advances in battery and drill design, this is enough energy to power your drill as efficiently as a wired drill. However, battery packs don't last forever, so if you're finding your cordless drill loses energy quickly, even though you've only just recharged the battery, try a DIY battery repair: it's a lot cheaper than getting a replacement.
Remove your cordless battery pack from the drill and place it on a flat work surface. Read the label on the battery pack to find out the type of cells it contains. Make a note as you will need to buy replacement cells to repair your cordless drill battery. If the label says it contains nickel cadmium (NiCad) or nickel metal hydride (NiMH) cells continue to follows these steps. If the label says it contains lithium ion (Li-ion) cells, don't attempt to repair it. Li-ion cells have power surge breakers to protect the cells from overcharging and discharging. Tampering with the power surge wiring could render the battery pack dangerous.
Charge your cordless drill battery until fully charged using the charger unit, if it contains NiCad or NiMH cells. Turn off the charger once charged. Leave the battery overnight. This lets bad cells lose their charge: good cells will retain their charge.
Locate the screws holding the battery cover in place. Use a Phillips screwdriver to remove the screws on the battery cover. There are usually between six and eight. Put the screws somewhere safe as they are very small and can easily get lost. Remove the battery cover and you will see the individual cells.
Remove the cells from the battery compartment. There are three ways they may be fitted. Some simply press into place, in which case you should use a small screwdriver to lever them out. Others have a clamp holding them in place. If this is the case, then you'll find the clamp has small Phillips screws. Unscrew them using a Phillips screwdriver, then slide the cells out of the compartment. If the cells are soldered in place, heat a soldering iron and hot touch the cell terminals once. As the solder melts, pry the cells out using a small screwdriver. Turn off the soldering iron once you have removed all the cells.
Line up the cells on the work surface. Label each cells numerically. For example if you have 10 cells, label them 1 through 10.
Use a voltmeter to check the voltage of the cell you labelled 1. Put the metal prong attached to the end of the red wire that extends from the voltmeter on the positive cell terminal, then put the prong on the end of the black wire on the negative terminal. Both terminals are clearly marked "+" and "-."
Read the output on the voltmeter display. If it reads between 1.1 and 1.2 volts it's a good cell, so put it to one side: you can still use it. If the readout is less than 1.1, the cell is losing its charge and need replacing. Put the bad cell in a separate place so you don't mix up good and bad cells. Repeat the process on all the cells using the voltmeter. Keep good and bad cells separate.
Count the number of bad cells you have found. Purchase the same number of replacement cells from an electrical store. They are widely available. Ensure you take the note you made earlier detailing the battery type along with you to the store.
Replace all the battery cells in the battery pack. Push them in place using your fingers, if they fit this way. If they were clamped in place, then replace the clamp and use the Phillips screwdriver to tighten the screws. If they were soldered, heat a soldering iron. Put the cells in position, then touch each cell's terminal using the soldering and a little solder. Let the solder melt, then remove the soldering iron and allow the solder to solidify. Repeat the process on all the cell terminals. Turn off the soldering iron.
Replace the cover on the drill battery. Replace the Phillips screws and tighten using a Phillips screwdriver.
Put the drill battery on charge for about an hour. This ensures each cell has the same charge. While the battery is charging, put the bad cells into a clear plastic bag. You need to dispose of them in a recycling unit. Don't put them in your regular trash.
Turn off the charger. Put the repaired battery pack into the power drill, so it's ready to use.
Always remove the battery from the charger once fully charged. Leaving it in a charger can damage the chemical composition of the cells meaning you will need to replace more often.