Solves one problem, but creates another

Few things are as handy as an iPhone battery case—binder clips, Swiss Army knives, and the iPhone itself. But most iPhone battery cases add bulk, and don’t always have juice when you need it most. Third Rail takes a novel approach by keeping the case and battery separate. Your iPhone stays in a sleek, attractive Slim Case that adds practically no bulk at all, just a centimeter or so at the bottom. Flip it over and you’ll see a contact strip of three gold rails—that’s where you’ll snap on the 1.4-ounce, 1250mAh Smart Battery. Micro-USB in and out ports let you charge the battery from a computer or USB power adapter and juice up other gadgets that charge via USB. If you want extra power, you can buy additional Smart Batteries for $59.99 each, and when they all run dry you can stack them on the Slim Case to charge everything up at once.

Looks good, works fine, but could be better.

It works as advertised, but the battery capacity is a bit lightweight. I attached the Smart Battery to the Slim Case when my iPhone 4 was at 10 percent, but it only boosted it up to around 75 percent before conking out. I did appreciate the switch on the Smart Case that lets you save the battery’s juice for later. And Third Rail intends for the system to be “future proof” by promising the Smart Battery will snap on to any future Third Rail cases—so far the only case is for iPhone 4 and 4S, sold on its own for $39.99 in case you need an extra.

The bottom line. This solves the “bulky iPhone battery case” problem, but creates another—namely that the dock port is covered. For some this won’t matter, but I use enough dock-connecting accessories that I was constantly removing the two-piece case. If you never use your dock port, you can still charge and sync in the case with the included cable.