Fixing your 18 volt Milwaukee battery pack depends on the type of cells the pack contains. Milwaukee use three types of cells in its 18 volt packs: nickel cadmium (NiCad), nickel metal hydride (NiMH) and increasingly lithium ion (Li-ion). The fairly recent change to Li-ion cells is because each cell produces three times the voltage and milliamps than a nickel-based cell. It makes battery packs two-thirds lighter and they last longer. Problem is little can be done to fix a Li-ion battery pack as they require specialized knowledge and tools. Check out the label on your 18 volt Milwaukee battery pack; if it contains NiCad or NiMH cells, you can fix it.
Charge your 18 volt Milwaukee battery pack in the charger as usual. Wait until the charge light indicates it's charged. You've probably found it doesn't take long to charge and then goes dead again quickly. This is because NiCad cells suffer from internal crystal growth over time, reducing their ability to hold a charge while NiMH cells suffer from voltage reduction for a similar reason, although it's not as common.
Turn off the charger and place your 18 volt Milwaukee battery pack into the power tool. Fixing your battery pack requires you to completely discharge the battery and it needs to be done systematically.
Turn on the Milwaukee power tool using the highest setting it has. The faster the power tool operates the better the fix. The concept is to break down the nickel crystals in the cells to the size they were when you first purchased your Milwaukee battery.
Allow your power tool to run until it stops. It may take a while, depending on the condition of the Milwaukee battery. During this time the battery gets hot, which is good, as the cells start to break down.
Turn off the power tool once it's stopped and let the battery and tool cool for an hour. As the battery cools it regains energy so you can start it again.
Turn on the power tool and let it run as before. Wait until it stops automatically. Let it rest for half an hour, then repeat the process. You need to keep repeating the process until you find the tool doesn't operate. This means the battery pack is totally discharged, the crystals have broken down and your Milwaukee battery is fixed.
Charge your Milwaukee battery in the usual way. Be prepared to wait quite a while for the battery to fully charge. You've become used to it charging for a short time, but it's repaired, so it will take much longer.
Turn off the charger once the battery is fully charged. Put your battery into the power tool and experience the extra power and endurance from your fixed Milwaukee battery.