The USB power specs calls for maximum of 500mA=0.5 Amps, or 2.5 Watts at 5 Volts. This used to work for most low-power devices until the rise of bigger and brighter screens and really power-hungry tablets and spartphones.
Device manufacturers have developed a couple of workarounds to determine whether their device is connected to a charger, computer, or an unknown source. This is necessary to avoid pulling more than 0.5 Amps from a data port on a computer/laptop, which risks damaging the power source. Most devices only draw over 0.5 Amps if they determine that they are connected to a dedicated charger.
This is accomplished using two common methods. The first, and most common method for all non-Apple devices is to simply short the two data pins (pin2 and pin3) together in the charger. Since the data pins wouldn't be shorted in a computer, or any USB data port, the device can assume it is connected to a charger, and it is safe to draw over 0.5A.
The second approach, used by Apple does not fully short the data pins, rather applying different voltages to the Data(-) and Data(+) pins, to indicate to the device exactly how much power it can draw. Below is a table of known D+/D- voltages and the exact power that they can draw:
current: pin2(D-) / pin3(D+)
2.5A: 2.75V / 2.75V (IPad 4 12Watt)
2.0A: 2.0V / 2.75V (old Iphone 3GS)
900mA: 2.75V / 2.0V (charges IPhones/IPads consistently accross all models)
500mA: 2.5V / 2.5V (?)
not charging: 0V / 0V (?)
Using higher current draw than the power source can safely supply is a fire hazard!
The standard USB pinout is as follows:
pin1 (red) +5VDC (VCC)
pin2 (white) Data(-), D-
pin3 (green) Data(+), D+
pin4 (black) Ground, 0V
Notes:
2.75VDC can be achieved by connecting a 10KOhm resistor to +5V (pin1, red)
2.00VDC can be achieved by connecting a 10KOhm resistor to Ground (pin4, black)
USB specs and chargers allow for 0.25V variation, i.e. VCC from 4.75 to 5.25V
