There's no denying the iPhone 4 is really nice, but all that surfing and gaming can surely take its toll on the battery so the extra 1600mAh that the MiLi Power Spring iPhone4 Battery Case packs could really come in handy.

The concept is simple enough as the MiLi Power Spring is actually a hard plastic case for your iPhone that also packs a battery.

Measuring just 18mm at its thickest point, the manufacturer says this is the thinnest iPhone case with a included battery charger in existence.

This means that won't have to worry about turning your sleek iPhone 4 into a brick as the case doesn't add more then 9mm to your phone's depth.

And since the built in battery is recharged trough a microUSB connector you won't have to carry a special charger around just for the MiLi Power Spring.

Although the main purpose of this device is to prolong your battery life, the hard plastic case will also add some degree of protection to your iPhone and, since it covers that all too familiar antenna spot, you may also get much stronger signal, although the manufacturer doesn't advertise this in any way.

Available right now in black (another four colors are soon to follow), the MiLi Power Spring will set you back $149.99, so that's not an easy buy by any means. (USBfever via geardiary)

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