Your Makita 18-volt battery may contain 15 nickel cadmium (NiCad) 1.2-volt cells or five lithium-ion (Li-ion) 3.6-volt cells. You can easily find out the type of cell your Makita battery contains by looking on the battery label as they are clearly labelled. It's important to find out the type of cells in your battery as battery packs containing Li-ion cells should only be revived professionally since the battery pack contains a power surge breaker that ensures the cells are not over charged, or fully discharged. If your Makita 18-volt battery contains NiCad cells, then reviving them is easy.
Put your Makita 18-volt NiCad battery on charge. Let it charge until the charging unit indicates it's fully charged. Some chargers have a light that illuminates once charged; others have a light that illuminates during charging and turns off once the battery is charged.
Turn off the charger and remove the battery from the charging unit. Insert your Makita battery into the device it powers.
Select the lowest power setting on your device. You need to discharge the Makita battery as slowly as possible.
Turn on the device and let it run continuously until it stops. Once it's stopped, turn off the device and let the device and battery cool completely. Let it rest for about half an hour.
Turn on the device. It operates again, although slower than before. Let it run until it stops. Turn off the device and let it rest for about 15 minutes.
Repeat the process by turning it on, letting it run until it stops, then turning it off and allowing it to rest for 15 minutes. After several times of repeating the procedure you find the device won't operate. Your Makita battery is fully discharged. The process has enabled crystals in the cells to break down into smaller crystals. This means there is a much greater surface area and the cell can retain more energy: almost the same as when it was new.
Remove the battery from the device and put it on charge. Allow it to charge until it's full. It will take much longer than you are used to. Turn off your charger and remove the battery from the charger. Never leave a battery in a charger once it's charged as it increases the chances of the crystals growing and your battery will need to be revived again.
Put your revived Makita battery into the device. It's ready to use in the usual way and will last much longer between charges.