One of the downsides to all of the WiFi connectivity, location-sensing, and general usefulness of Apple's iPhone 4S is how fast it draws down its battery. Unlike almost all of the other touch-screen phones on the market, there's no way to swap in a spare battery so you can extend your Netflix viewing another hour or two when you're stuck in an airport, train station, or boring staff meeting. But a set of products from a Florida company called Third Rail Mobility make it possible to give your iPhone—or any other device that can charge from a USB port—a recharge on the go.
Third Rail provided me with a test unit of the company's Third Rail System for iPhone 4 and 4S, a $90 kit which consists of a slimline case for the phone, an external battery that slides onto the back of the case, and a set of cables that can be used to charge the phone and battery, as well as charge other devices off of the battery. I've been testing the case and battery over the last few weeks to see how well they work for the worst-case scenario of usage: a series of untethered workdays and weekends.