Theory
Once connected, all USB devices must identify themselves to the host. This is called “enumeration.” In the identification process, the host determines the power needs of the USB devices and allows or denies the device to increase its load from 100-mA maximum to 500-mA maximum.
Theory vs. Reality
USB ports do not limit current. Although the USB spec provides details about how much current a USB port must supply, there are mile-wide limits on how much it might supply. Though the upper limit specifies that the current never exceed 5 A, a wise designer shouldn't rely on that. In any case, a USB port can never be counted on to limit its output current to 500 mA, or any amount near that. In fact, output current from a port often exceeds several amperes since multiport systems (such as PCs) frequently have only one protection device for all ports in the system. The protection device is set above the total power rating of all the ports. Therefore, a 4-port system may supply more than 2 A from one port if the other ports are not loaded. Furthermore, while some PCs use 10% to 20% accurate IC-based protection, others use less accurate polyfuses (fuses that reset themselves) that will not trip until the load is 100% or more above the rating.
USB ports rarely, if ever, turn off power. The USB spec isn't specific about this, but it's sometimes believed that USB power may be disconnected as a result of failed enumeration, or other software or firmware problems. In actual practice, no USB host shuts off USB power for anything other than an electrical fault (such as a short). Most notebook and mother-board makers are unwilling to pay for fault protection, let alone smart power switching. So no matter what dialog takes place between a USB peripheral and host, 5 V (at either 500 mA or 100 mA, or even maybe 2 A or more) will be available. This is born out by the appearance in the market of USB-powered reading lights, coffee mug warmers, and other items that have no communication capability. They may not be “compliant,” but they function.
Source:
http://powerelectronics.com/mag/powe...attery_faster/
For the pdf version of the article, including diagrams:
http://powerelectronics.com/ar/Maxim.pdf
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