[ame=http://vimeo.com/13835359]Reverse engineering Apple's secret charging methods on Vimeo[/ame]
The engineer/artist/hacker behind the Minty Boost do-it-yourself USB recharger has figured out Apple's weird and arcane way of identifying which chargers are "supported" or not, allowing any reasonably talented modder to make their own 1-amp recharger for iPhones or anything that uses USB to charge. The technical analysis on the Minty Boost site is probably interesting only to serious wireheads; the video above explains it even more succinctly. By taking apart Apple's charger as well as a third-party battery-powered recharger, Limor "ladyada" Fried found out that supported chargers just clip different sized resistors on the USB data lines.
Using the first Minty Boost instructions and kits, hobbyists found that newer devices simply displayed the "charger not supported" warning and wouldn't charge. Looking into this, ladyada took into account that smarter USB devices will alert the computer or charger that they are about to draw current in excess of the 500 mA limit of standard USB. Most dumb devices don't do that, though, and ladyada figured that chargers were going to generally not be too smart. So she tore apart chargers for different Apple devices and found that the resistors were placed on the data lines to limit voltage to 2.8V on one line and 2V on the other. Using the same setup on a Minty, it drew 1A immediately. Experimenting further with a TuneJuice battery backup from Griffin, she found resistors limiting voltage to 2V on both lines when she tore that one apart. This forces the iDevice to draw only 500 mA, a much more reasonable draw for the AAA batteries used in the TuneJuice.
This information was previously available only to charger manufacturers who had signed Apple's restrictive non-disclosure agreement. Hobbyists and smaller manufacturers are now free to make more powerful chargers, and iDevice owners can look forward to an end to the annoying "charging is not supported with this accessory" messages.