The +5V output from a USB hub is permitted to range from 4.75V to 5.25V (5V +-5%). For USB2.0 this range is changed to be 4.4V to 5.25V and for USB3.0 the range starts from 4.0V.
A USB device is permitted to draw up to 100mA (1 unit load) for USB2.0, and up to 150mA for USB3.0. It may do this prior to any USB communication, so this is the current limit a USB device may draw if using a USB connector to provide the charger input without a USB interface to request a higher current from the USB hub (which it may or may not be granted depending on the hub).
USB devices that need more power than this are called high power devices (instead of low power). The maximum load that may be requested for USB2.0 is 500mA and for USB3.0 is 900mA.
The “USB Battery Charging Specification” adds optional new power modes. A host or hub port that supports charging can supply up to 1.5 A when communicating at low speed or full speed, or a maximum of 900 mA when communicating at high speed, and as much current as the connector will safely handle when no communication is taking place. USB 2.0 standard A connectors are typically rated at 1500mA. The Dedicated Charging Port shorts the D+ and D- pins with a resistance of up to 200Ω. This short disables data transfer but allows devices to detect the Dedicated Charging Port and allows simple high current chargers to be made.
