It sounds like a mighty big promise: double your iPhone 4 battery life. MiPow are very good at keeping promises and have designed the Maca Air Power Case to charge your iPhone when the dreaded 9% battery alert appears. It's the slimmest power case out there and features an On-Off button so that you can save your juice until the make-or-break moment. It's like that Push Only In Emergency type of situation, only this time you won't block any city escalators or make a cross-country train unnecessarily screech to a halt.
Charging status and battery level indicators. Made for iPhone certified. On / Off Switch for Output voltage 5V. Li-polymer batter with a 1200 mAh capacity. For iPhone 4 and iPhone 4S. Charging time 3-4 hours. USB charging cable included.