In the past few weeks, we've seen a lot of cases that extend the battery life of an iPhone. Our sister site--BYTE.com--reviewed a gaggle of cases designed for folks whose iPhones can't make it through the day on a single charge. But one of the cases we didn't get to see was Third Rail Mobility's modular design, where the battery snaps onto the back of the case. Compared to other cases with integrated batteries, this design makes it possible to charge the battery separately from the phone without having to remove the case. It also makes it possible for you to carry extra charged batteries around with you. When the one you're using dies, just snap another one on. One downside though; when a battery is attached to this case, it's a lot thicker than the cases with integrated batteries.