Third Rail iPhone Case With Detachable Battery
The Third Rail System includes an iPhone 4 slim case for $90, it always surrounds your device and a 1250mAh battery that you can slide into place whenever you need a boost. The removable batteries can charge other devices over micro USB, it can be stacked up to four deep for charging multiple gadgets at once. It is also designed to be compatible with future cases, is good solution for upgrading your iPhone. For more details read the press release