which version of USB to use, but as the hair-tearing message above shows, third-party renditions of Apple's proprietary dock connector haven't always been universal solutions for iPods and iPhones. Now we know another important reason why -- secret resistors placed on the data lines in connectors for each iDevice. Minty Boost creator ladyada recently reverse-engineered the chargers for a variety of Apple gadgets, and discovered that iPhones in particular don't draw electricity until they detect 2.8V and 2V signals when they attempt to charge. At that voltage, the handsets suck down about one amp, leading to a rapid filling of your device's Li-ion belly, but by adding additional resistance to drop the voltage further, the iPhone can be coerced into accepting 500mA instead -- perfect for the set of AAA batteries you stashed away in that Altoids tin. Video after the break, full explanation at our source link.