If you’re looking for a little more precision in just how much juice is left in your iPhone battery, the iPhone has a way to display battery life as a percentage next to the battery icon. To turn on this feature, follow these instructions. Note that this works for the iPhone 3GS, iPhone 4, and on.
1. Tap Settings:
2. Tap General:
3. Tap Usage:
4. Slide the Battery Percentage tab to On: