Cell phone batteries don't have the best reputation for standing up to the test of time, but the new Third Rail Slim Case for the iPhone 4 and 4S features a removable smart battery, something that could help keep the phone juiced up and ready to go.

"[The phone has] a slender protective case and removable backup battery that won't turn your iPhone into a chunky monkey," the Chicago Tribune said on the case and battery combo. "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."

With phones that like using a lot of apps and Wi-Fi, this case can help turn a power hogging iPhone into something a bit more sensible. While the news source said it could offer more battery, this seems like a decent start.

Third Rail said the device will sync and charge without having to remove the case and also features the thinnest cell phone batteries on the market.