The mophie Juice Pack is the first Apple Certified "Work With iPhone" attached battery in the world. Rather reminiscent of the external battery "sleeves" that we used to use on our old iPaq's (back through the mists of time) it boosts the iPhone to an additional 250 hours standby time and adds an extra 7 hours video playback time...

"The Juice Pack is a rechargeable lithium polymer battery in the form of a non-slip, soft grip case. It extends the time that you can use your iPhone. 

 

Intelligent Power Distribution - Smart Battery Technology instructs the iPhone to always drain out the Juice Pack first. When you're done with Juice Pack, simply detach it and you still have a fully charged iPhone

 

There are 4 light LEDs on the back of the Juice Pack. Simply press the test button and it will tell you how much power is left with the Juice Pack. The LED lights also indicate the charging status and the charging progress."

www.mophie.com