It also has its own 30-pin connector so you can sync your phone while it's in the case, a feature that other similar cases don't have. The case charges via USB, and there's an LED indicator showing you how much power remains in the lithium polymer battery.
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I'm an advocate of external iPhone batteries, and this case for the iPhone 4 is thin but protective, and also packs additional battery life to keep your phone juiced through the longest days. The Snap Battery Case ($60) is, as its name implies, a snap-on case for the iPhone 4. It protects the phone without obscuring its design; the sides are open but the antenna is protected. And the battery itself doesn't take up too much room, either.