I'm having this problem since a few days ago.
All of a sudden my GS2 started hanging and turning off by itself, I thought it was time foa factory reset. The battery was showing full charge but it was losing it fast and the available time was only a couple on hours.
I was using the latest stock JB rooted and I decided to try CyanogenMod KK. With CM it got worse, now the battery on full chage connected to the charger show 0 seconds available and it I disconnect it, the time starts to increase from 0s to a couple of minutes and the phone turns off without warning.
Is this a signal that the battery is dead?