Bad news: The battery in the Plantronics M2500 is NOT replaceable under ordinary circumstances. And unfortunately, this trend of non-replaceable batteries seems to be catching on with manufacturers, like with Apple IPods. But here it's even worse, because you can't even open the battery cover unless you're willing to DESTROY the case in the process!
But when my Plantronics M2500's batteries stopped taking a charge, I figured it was as good as destroyed anyway. So if you're a lunatic like I, and are DETERMINED to revive your dead M2500 rather than recycle this rather expensive bluetooth headset, it is possible -- though very difficult -- to replace the batteries.
First you have to find two 1/4 AAA NiMH batteries (10mmX14mm), preferably with solder tabs. These are not easy to find, especially when you just want two of them. So if you can't get these, this is the end of the story. But if you can get the batteries and have successfully soldered them together, you then "simply" (ha ha!) pry off off the battery case (under the dark gray rubbery stuff) by cracking the glue line and pulling it off.
Be sure to keep the mangled bit of plastic case that WAS covering the battery compartment. No, you can't use it again, but it will remind you never again to buy electronics with non-replaceable batteries.
Solder in your new battery pack, and see if the M2500 will charge up and then turn on. If it won't, THIS may be the end of the story. But if it does, congratulations! You have replaced the batteries! You're finished except you just have to figure out how to rebuild the case. I used epoxy clay, which can be molded into any shape, and then hardens into a rock-hard chunk of plastic. That works pretty well, but once you do that, you'll not be able to do all this again. Which may be a good thing. :-)