Several years ago, I bought a set of Milwaukee power tools. I love the tools, but one of the batteries seemed to die after a few years (but very little usage.) The batteries are like $80 and since I don't use the tools very often, I've never bothered replacing the one that seemed to be dead (since I have 2 batteries.)
However, today I came across a very cool tip.
Apparently these "dead" batteries are a known issue with Milwaukee and often the batteries aren't dead, but just need to be jump started. To awaken one of these dead batteries, place a standard 9-volt battery against the leads on your dead Milwuakee battery for 30 seconds (single lead on your Milwaukee is positive, the double lead is negative.)
Sure enough after doing this little tip, I was able to plug my battery in to the charger and it started charging it!