Milwaukee 18-volt battery packs may contain 15 nickel cadmium cells (NiCad) or five lithium ion (Li-ion) cells. The reason your Milwaukee 18-volt battery won't start charging may vary, depending on the type of cell the battery pack contains. If your battery pack contains Li-ion cells and it is the battery that's gone bad, then you must get it checked out professionally, as they contain power surge breakers that are not suitable for DIY repairs. The first thing you need to find out is whether the battery charger is working.
Check the fuse in the Milwaukee battery charger. Locate the fuse cover -- it usually has a diagram of a fuse or may simply state "fuse."
Remove the cover. Use a small flathead screwdriver to lift off the cover, or if it has a screw, use a suitable screwdriver to loosen the screw and then remove the cover.
Lift out the fuse from the fuse holder. Use your fingers to unclip it. If you can see through the outer casing of the fuse, you can see if the fuse is blown. The thin wire connected to each end of the fuse will be broken. If it's broken, get an identical fuse from an electrical store. If you can't see through the outer casing, it's simpler to purchase a replacement fuse.
Replace the existing fuse, if it's not broken, or insert a new fuse. Push the fuse into the fuse holder using your fingers. Replace the fuse cover. Either push it into place or tighten the screw using the screwdriver.
Put your battery on the charger. If it won't start charging, the culprit was the fuse. If it won't start charging, check the label on the battery pack to determine whether it contains lithium ion or nickel cadmium cells. The label is clearly marked "NiCad" or "Li-ion." If it contains lithium ion cells, get it checked out by your Milwaukee supplier. If it contains nickel cadmium cells, follow the steps in Section 2.
Fully discharge your Milwaukee battery. It's likely to hold a small charge, even if you can't recharge it. Put the battery pack into the power tool and turn it on.
Let it run, even if it's operating on low power. Wait until it stops, then turn it off and leave it for 15 minutes or more. NiCad batteries lose their ability to charge over time as the crystals inside the cells increase in size, meaning they can't charge correctly. Fully discharging the cells breaks the crystals down, enabling it to start charging again.
Repeat the process by turning on the device and letting it run. You find it operates, even though it didn't a few minutes ago. Wait until it stops, let it rest then try again until the device no longer operates. The Milwaukee battery is fully discharged.
Put the battery into the charger. Turn on the charger; you'll find it starts charging as the process of fully discharging the battery has let the cells break down, increasing the surface area, and they can now retain energy.