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.

Battery Charging v1.2 Spec

Standard Downstream Port (SDP)

A standard USB port.  Supplies 100mA before negotiation and up to 500mA after negotiation of power availability (150mA and 900mA for USB3)

Charging Downstream Port (CDP)

Supports data transfers as well as charging.  Uses USB commands to determine charging current availability.

Specification min rated current: 1.5A (Table 5-2 Currents).  A Charging Downstream Port (CDP) has to provide a minimum of 1500mA, no matter what state it is in.

Specification max rated current: 5.0A (Table 5-2 Currents)

Dedicated Charging Port (DCP)

See the "USB Charging Specification Version 1.2"

The potential to deliver 10 W by supporting current levels of up to 1.8 A.

Unlike the normal Standard Downstream Port (SDP), the D+ and D– wires in the DCP are shorted together (max 200ohms) to prevent data transfer from occurring.  By shorting these lines together, the DCP can inform a portable electronics product that the port it is connected to is fully focused on charging and does not provide host functionality.

Specification min rated current: 0.5A (Table 5-2 Currents)

Specification max rated current: 5.0A (Table 5-2 Currents)

Dumb Devices Detecting USB Power Availability

If a device doesn't talk USB it can only assume 100mA is available from a standard USB connection, or by detecting the D+ and D- pins are shorted together it can assume 500mA is available as the charger must be a USB Dedicated Charging Port (DCP).

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