Ridiculous IPhone Charge Usage

I have an IPhone 3GS 16GB and when I went to bed last night it was charged at 40% and when I awoke it was at 4%! It was at half brightness, no apps were running in the background. What happened?

Same thing happened to my iPhone 4 a few months ago. Don't get your hopes up, I still haven't found a way to fix it . All I know is, one day the battery life shrank by half. On standby, the phone loses about 5% an hour, which is absolutely ridiculous. I literally have to charge it every night before I go to bed, otherwise it'll die before the alarm goes off the next morning.

One theory I have is that because I recently moved to china, using power outlets here might put more strain on my charger, because although the output range is within the accepted limits, it's twice what it is in the US. All I know is, my battery is f-ed up either way, and probably reaching the end of its lifespan.