My phone battery drains very fast after a full charge even though I don't
use it constantly. I typically surf the net, check email on wifi or 3g for 5 minutes every hour and the battery almost drains after 3-4 hours.
Anyone has the same problem?
There are several facts about lithium ion batteries that many people are unaware of. One which would potentially affect what you're experiencing is this. The discharge cycle of a lithium ion battery is continuous, meaning, it will slowly lose charge even when it's not in use. However, after a few complete usage cycles this idle discharge is significantly reduced.
What this means to you is this. Please use up your whole battery down to about 10%, then, allow the device to charge to 100% all at once, do not unplug it during this time. Repeat this process several times preferably with discharges happening during normal usage. This should noticeably extend the performance of your battery.
Other concerns include high temperatures, and failure to regularly discharge all cells. With high temperatures the contents of the battery degrade and are permanently destroyed. Please pay attention to the listed operating temperatures for your device, these temperatures most likely reflect the optimal temperature ranges for the battery over any other temperature sensitive component of the device.
As for not discharging the battery completely regularly, this will result in portions of the physical cell to become incapable of complete discharge, essentially crystalizing their state. It's important to make a complete discharge occasionally. I'm not sure of the recommended frequency for an iPhone 3GS, however, with laptop batteries it is once per week.
Personally I let my phone drain down every couple of weeks with some hearty adventure gaming or watching a movie or similar. My iPhone 3G battery has withstood repeated long sessions of intense usage and still has good battery performance even nearly a year later. I'm sure you will see your battery life improve over time.