Apparently many USB charger units—you've probably got at least one that came free with a gizmo: I've got three—won't work to charge the iPhone. That's 'cause Apple's lovely design engineers made the phone's electronics look for the standard USB D+ and D- lines, which lots of USB chargers omit. Thanks to a simple hack, however, you can add these lines in by spoofing reference voltages instead. All it takes is a few resistors, some wire cutting and soldering, and the ability to have no fear that you're precious iPhone is going to go up in an expensive puff of smoke. The magic resistor formula is revealed below.
The project was designed to work on 12V car USB units, and it's pretty neat, hey? Interestingly, with just a few extra tweaks you can adapt it so the circuit works with high-capacity batteries. That'll let you run your iPhone almost anywhere in the world away from power sockets. [Tzywen via Makezine]