Manage the Battery
An iPhone runs on battery power when it is not connected to a power source. You can recharge the battery by connecting the iPhone either to a computer or to an electrical outlet (via the USB power adapter that comes with the iPhone).
An icon indicating the charge level of your iPhone appears in the upper right, on the iPhone's status bar. It also appears in iTunes, by the iPhone icon under Devices in the sidebar. When the battery charge falls to 20 percent or less, a warning message pops up on your iPhone. If you ignore the message, the iPhone will eventually shut down (see Charging Tips, next page, for more details).
Apple claims that a properly maintained iPhone battery should retain "up to 80% of its original capacity at 400 ...