The iPWR SuperPack is a compact and affordable juice pack that works well as an emergency extended battery, but it is not something you'll want attached to your iPhone or iPod all the time.
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This battery pack works with most Apple handheld devices, including the iPhone and many models of the iPod. It can double the iPhone 3G's battery life.
If you look for an extended battery that works with basically all the iPhones and iPods that share similar connectors, the iPWR SuperPack is for you. This juice pack makes a good emergency backup battery and it works with the iPhone, iPhone 3G, iPod Touch (first and second generation), iPod video, iPod Nano (third and fourth generation), and iPod Classic.
For about the price of a pair of lithium AA batteries, you can nab this travel-friendly battery pack--which just so happens to come with a pair of lithium AA batteries.
Battery power: there's never enough. Energizer's pricey but pretty new power pack gives you twice as much runtime. It's great for long trips, emergencies, and the like.