Sometimes your iPhone just needs a little boost in the energy department. whether it’s an extra-long day or a forgot-to-charge-last-night pick-me-up, Mophie’s new Juice Pack plus for iPhone 4 can extend your device’s battery power, all in a convenient package that slips on your phone.

As opposed to the company’s $80 Juice Pack Air, which features a 1500 mAh battery, the $100 Juice Pack plus packs a high-capacity 2000 mAh battery inside its hardshell case, supplementing it with a protective, shock-proof rubberized band that encircles the iPhone’s edge–and it only adds an additional millimeter of thickness to your phone’s sleek profile.

To switch to the backup battery, all you need to do is flip a switch on the case. Mophie says the power cell can more than double the phone’s battery life; it’s rated for an additional 8 hours of 3G talk time (or 16 hours on 2G), 7 additional hours of 3G Internet, 11 hours of Wi-Fi Internet, 44 hours of audio playback, or 11 hours of video playback. A handy four-LED status indicator lets you know how much charge is left in the battery at the touch of a button.

An included USB cable lets you charge and sync your phone while it’s in the Juice Pack plus and the case is constructed with a special acoustic chamber that Mophie says enhances the sound quality of the iPhone 4’s speaker.

While the current shipping version only comes in black, Mophie says additional color choices of cyan, magenta, and yellow are coming soon.