USB Power: Theory vs. Reality

USB ports do not limit current.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.

USB ports rarely, if ever, turn off power. So no matter what dialog takes place between a USB peripheral and host, a 5 V supply (at either 500 mA or 100 mA, or even maybe 2 A or more) will be available.

More on this PDF file by Maxim Integrated Products holds a U.S. patent on all forms of USB Lithium-battery charging.

http://powerelectronics.com/ar/Maxim.pdf

http://powerelectronics.com/mag/power_charge_battery_faster/