Third Rail iPhone case has detachable battery, probably won't electrocute you

For humans, coming into contact with the third rail will likely void your personal lifetime warranty, but one accessory manufacturer hopes to apply a similar concept to your iPhone -- in much smaller doses, of course. For $90, the Third Rail System includes an iPhone 4 Slim Case, which always surrounds your device, and a 1250 mAh Smart Battery that you can slide into place whenever you need a boost. The removable batteries can charge other devices over micro USB, and can be stacked up to four deep for charging multiple gadgets at once (at $60 a pop). It's also designed to be compatible with future cases, negating the need to purchase another complete solution each time you upgrade your phone. We'd love to see a similar device that includes inductive charging as well, but for now, this is one of the most intuitive external battery solutions we've seen.

TUAW

Third Rail Mobility