Some info I looked up for an Amazon review:
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USB Battery Charging Specification 1.2
Chinese Telecom Standard YD/T 1591-2009
Divider Mode, Compliant with Apple iPod, iPhone (1A), and iPad (2A) Mobile Digital Devices
Non-Apple devices that I know of don't use divider mode. They actually use a 2-step process to determine what kind of port they're connected to. For reference, there are 3 USB charge port-type acronyms used:
SDP (Standard Downstream Port), limited to 500mA (900 mA w/USB3)
CDP (Charging Downstream Port), limited to 1500mA
DCP (Dedicated Charging Port), limited to 1500mA
Here's the process that a device being charged uses to determine how fast it can charge:
The device will output 0.6V on pin 3 (D+) and check the voltage returned on pin 2. If the voltage returned on pin 2 (D-) is less than 0.3V, it thinks it's connected to an SDP. If the returned voltage is between 0.3V and 0.8V, it thinks it's connected to a CDP or DCP.
Next, the device will output 0.6V on pin 2 (D-) and check the voltage returned on pin 3. If the voltage returned on pin 3 (D+) is less than 0.3V, it thinks it's connected to a CDP. If the returned voltage is between 0.3V and 0.8V, it thinks it's connected to a DCP. To qualify as a DCP, there's one additional twist: both the USB Battery Charging Specification 1.2 and the Chinese Telecom Standard YD/T 1591-2009 require that pins 2 & 3 be shorted together (USB BCS 1.2 calls for a maximum impedance of 200 ohms, while the Chinese standard actually calls for a dead short).
A so-called "dumb" charger doesn't do anything fancy: it simply shorts together pins 2 & 3 with a maximum impedance of 200 ohms, leaving the device to figure out that it's connected to a DCP, since shorting pins 2 & 3 together means that 0.6V put on pin 2 will appear on pin 3 and vice-versa. This is how a lot of generic and car chargers work.
So... there is certainly a lot of detail in how devices figure out how much current they can get when plugged in. There is actually more going on than what I included above, but that's enough to figure out that USB charging isn't the same as just plugging in a regular power cord.
For those of you with Apple devices, an Apple device determines what kind of port it's connected to by checking the voltages present on pins 2 & 3 of the USB connector (D- and D+, used for data transfer). If pin 2 is a 2.7V and pin 3 is at 2V, max current is 1000mA (1 Amp). If both pins are at 2V, max current is 500mA. If pin 2 is at 2V and pin 3 is at 2.7V, max current is 2000mA (2 Amps).
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Anyway, let me know if I got anything mixed up.
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