Most likely a bad connector. I worry about damaging mine every time I pull it apart, since it takes a lot of force (and care) to do without damaging anything.

By the way, my battery life went down from 3 hours to about 90 minutes in the dead of winter, and is now back up to something close to 3 hours again in the warmer weather. 9 months, at least 50 charging cycles, still fine.