One of my favorite mods on my jailbreak iPhone is that I can display the exact battery percentage of my battery on the status bar. Well, Apple has made this feature available on the iPhone 3GS. To enable the Status Bar percentage just go into your stock Settings application, then the General option, then the Usage option. In the Usage option, you will find the Battery Percentage feature…just turn it on.
Once turned on, it will display the exact percentage of charge your battery has to the left of the battery image on your Status Bar. Simple and easy. A cool little feature.