As iPhone owners know all too well, the battery on Apple’s beloved smart phone only lasts for about a day. Need more juice than that? That’s where the Third Rail Mobility for the iPhone 4 comes in. The Third Rail System ($90) comes with a case you can attach to your phone, along with a portable smart battery for when your normal battery runs out. We really like the case. It’s sturdy and sleek and the battery attachment is not cumbersome. To charge the smart battery, you hook the USB or micro USB cable from the battery to your computer. You can also leave it on the back of your iPhone and plug your phone (with the case on it) into a wall socket. It will charge the phone and then the smart battery, as well as anything attached via the micro USB to the Smart Battery (an iPod, Bluetooth headset, etc.) By our estimate, the battery will double the length of time your iPhone 4 can go without a recharge. Get the Third Rail System here.