Peace guys,
Was suffering from the same problem, phone kept dying - eventually, it got really bad, rang o2 and they said it was not a battery issue but a software one, and said they would need to send the phone off, which as a result would reset everything and would take about a week. I had no success with Samsung Kies. Eventually, after using one of my wife's old phones for two months, I decided to do some research on the internet, and lo and behold - I looked at my battery and it wouldn't fit back into my phone because it was so fat, despite pushing it in. Finally, I purchased another battery, and phone came on after two months. I have had no problems (Alhamdulillah) for one month now.
Tip: Don't leave your phone on charge over night as apparently, this phone (maybe others) can get over-charged and as a result battery becomes faulty. Thus, charge it to perhaps 95% and take it off charge.
Peace guys
[Ps - research the true Islam for true satisfaction]