AJ wrote the following extremely useful comment in response to TUAW’ article “USB vs. AC outlet charging on the iPhone 4“:
I’ve seen some misinformation in the comments here, so let me explain how the iPhone charges:
The iPhone charges from a voltage of close to 5V. It does not matter what the source of the power is (like the wall or your car’s cigarette lighter or a computer). The iPhone accepts 5V and uses that 5V to charge its battery. The iPhone’s battery has a nominal voltage around 3.6V, but it actually varies between about 3V when depleted to a maximum of 4.2V. So there is circuitry in the iPhone that essentially converts 5V to whatever voltage at which the battery happens to be at a given time.
Computer USB ports, with the exception of certain ports on MacBooks/Pros/Airs, will provide 5V at up to 500mA to charge an iPhone. Any USB port should be capable of this, as long as it communicates with the iPhone and follows the iPhone’s request for high power (meaning up to 500mA instead of just 100mA). Note that low-power USB ports, like the ones on keyboards and monitors, will provide only 100mA. 100mA charges an iPhone very very slowly.
USB ports on some Macs will offer more than 500mA to charge iPhones, iPods, and iPads. I think the limit on these is around 1A. That’s why those ports will charge an iPhone more quickly than a run-of-the-mill USB port.
iPhone AC chargers and car chargers will output 5V at up to 1A (1000mA). During certain parts of the iPhone’s charge cycle, it will use over 500mA if it detects that it’s connected to a 1A charger instead of a normal 500mA charger or USB port. This allows the iPhone to charge more quickly from an AC or car charger that is capable of 1A, like the one that’s bundled with the iPhone. You will see only a small difference in charge times between different 1A chargers, as those that output a slightly higher voltage than the others may charge just slightly more quickly. But all these chargers are outputting roughly the same 5V at up to 1A.
Chargers for the iPad are capable of outputting about 2A at 5V. However, iPhones will not use this much power to charge. I don’t think iPhones will ever draw more than 1A, because it is dangerous and actually not beneficial to charge a Li-Poly battery at currents higher than its rated capacity in mAh. For example, the iPhone 4′s 1420mAh battery should not be charged with a current over 1420mA. Doing some quick math, 1420mA at 3.7V (roughly the average voltage of the battery during the high-current part of the charge cycle) would mean it draws about 1.2A from a 5V charger (assuming some inefficiency in the iPhone’s battery charging circuitry). So I do not think that the iPhone will charge any more quickly from an iPad charger than from a 1A iPhone charger. I’d bet that the iPhone still maxes out at 1A even when connected to a 2A iPad charger. And if not, the improvement in charge time would be just a few percent.