Third Rail removable battery on the case — 5 green lights indicate the battery has 100 percent power (lights measure power in 20 percent increments).

Third Rail removable battery on the case — 5 green lights indicate the battery has 100 percent power (lights measure power in 20 percent increments). (Third Rail photo / June 12, 2012)

Name: Third Rail Slim Case for iPhone 4/4S with Removable Smart Battery

What it is: A slender protective case and removable backup battery that won't turn your iPhone into a chunky monkey. The 1250 mAh lithium ion battery is ultrathin and weighs only 1.4 ounces. When your iPhone 4/4S is low on power, you slide the ultrathin smart battery onto the back of the case, and your phone starts charging. Even with the battery on the back of the Third Rail case, the phone is still sleek. After charging, you can use the on/off switch to stop charging or slip the battery off the case and drop it in a purse or pocket until needed again. The battery has five indicator lights to show how much power remains, in 20 percent increments. A micro-USB cable and a USB adapter cable are included to charge the phone through the case's micro-USB port and to use the battery with hundreds of other devices. The battery has an out port to use when charging other devices.

The good: I tend to use Wi-Fi, Bluetooth and run several apps simultaneously, so my iPhone is a power hog. I tested the case and battery along with a spare, and some days needed the extra battery. I recommend carrying a spare if your phone usually needs more juice not long after lunch. One of the coolest features is that the batteries stack. I can stack both batteries on the case and charge my phone and additional batteries in a few hours using a single micro-USB cable.

The bad: The iPhone battery is about 1440 mAh, so at 1250 mAh, the slim, lightweight Third Rail battery doesn't fully recharge a drained phone but would get most users through a day. (Third Rail is developing a compatible higher capacity battery, along with introducing several color options.)

Cost: $89.99 (a spare battery costs an additional $59.99)

Available from: thirdrailmobility.com, AT&T, Amazon, BestBuy, Target, etc.