Power supplies, in which the middle two pins of the USB port (D + and D +) are short-circuited, be of some smartphones recognized (in my Samsung Galaxy Nexus) as AC adapter and your smartphone uses a higher charge current for charging (the Nexus ~ 700mA ). Are D- and D + is not shorted, in a USB-compatible mode is loaded (charge current ~ 500mA). There are two reasons - firstly, the USB port of a PC specifies a maximum current of 500mA, the other one need D- and D + for a functioning USB transfer to the PC. There are power supplies (especially 12V car adapter), where D and D + are not bridged - Smartphones are slow to load. The adapter bridged D and D +, thus ensuring even in these power supplies that the smartphone an AC adapter recognizes (and no USB data connection) and so uses a higher charging current - provided that the power supply provides this also. Alternatively you can also mod the power supply, which in some cases (in my case an external battery pack) not going ;-)
