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.
source Third Rail Mobility