the problem that you are having is about the battery calibration settings.
Oviously this particular phone was created for the chinese market and as such calibrated on the chinese electricity voltage.
The Problem you got is this: even using the original charger with a plug adaptor does not necessarily charging the battery fully, moreover you are putting the battery under undue stress and whats happening you are slowly frying the battery.
www.themobilephonesurgery.com
Alex
August 2008