We at 100milligrams have been on the lookout for quite some time for a spare battery for iPhone that charges the phone rather than simply providing additional power. We keep hearing complaints about the battery life in the iPhones and so we think this external battery and dock is one of the better solutions available at the moment. A lot more than just an external battery it also charges your phone and achieves additional functions and all the while is lightweight and non-intrusive. This external battery does more than just supply power to your Iphone - it charges it while on the go so you dont have to keep the battery attached until the phone and external battery are dead. Just attach the for an hour or so and you'll get an additional 2-3 hours use on your phone. We also liked the size of this external battery. Too many before it have been too chunky, intrusive heavy and ugly but this one is compact, light and fits snugly on your iphone. Another great feature is the Dock. You can charge both your phone and the dock at the same time, so you have options whether to take your spare battery or not. Features Get 4-5 hours additional operarting time out of your Iphone 3G Mobile power keeps your phone charged Ext Battery Charges your Internal Iphone Battery Double the battery life of your phone Works with standard iPhone 3G model Output Voltage: DC5V Output Current: USB DC5V