Find it hard to get through the day on a single charge but not going to be anywhere near an outlet? Consider the versatile Third Rail iPhone 4 / 4S Slim Case w/ Smart Battery! A slim hard shell for protection against scratches and scrapes on its own, the Third Rail iPhone 4 / 4S Slim Case w/ Smart Battery has the added benefit of an included smart battery, a slide-on cordless charging battery, so you can power up on the go without a wall or car outlet. With us as your witness, your iPhone's battery will never go hungry again!