Sometimes if Milwaukee 12V tools that were allowed to die completely your charger will flash that the bat is bad.
There is a fix that you can search the net for to "jump" the batteries to get them to charge, all you need is a 9V battery and a couple of alligator clip leads.
Milwaukee batteries have 3 terminals, the double is the ground, just hit them for about 15 seconds then put in the charger, and they should pick up a charge, assuming the battery is not toast.
Haven't tried it myself, but it is supposed to work. Just waiting for a dead battery