The specifications of all USB interfaces have also covered power management. Thus, there were already two different power specifications in the specifications of USB 1.0 and USB 2.0 for the supply of connected USB devices: High Power with 5 V and 500 mA and Low Power with 5 V and 100 mA. This was not for charging batteries, but for peripherals such as keyboards and mice.
Independently, developers worked on solutions for charging batteries, which led to proprietary solutions and limited interoperability. USB Battery Charging (USB- BC) was specified to ensure interoperability. The Battery Charging Specification distinguishes between three different ports: the Standard DownstreamPort( SDP), the Charging Downstream Port( CDP) and the Dedicated Charging Port( DCP).
The Standard Downstream Port (SDP) corresponds to the one specified in USB 2.0 as found in laptops and desktops. The maximum load current is 2.5 mA when idle, and 100 mA when active. 500 mA, or 2.5 W respectively, can be achieved with appropriate configuration. With USB 3.0 it is 4.5 W. Connected devices recognize the SDP port by the data lines, which are terminated with 15 kOhm.
The Charging Downstream Port (CDP) is a port specified in USB-BC 1.2 for Battery Charging for personal computers and laptops for 1.5 A current draw. The removable power is 7.5 W. A device connected to the port recognizes it via handshake. And the Dedicated Charging Port (DCP) is designed for 1.8 A and a charging power of 10 W. To configure the DCP port, the two lines of pins D+ and D- are shorted. Connecting the two pins prevents data transfer and signals the mobile devices to be charged that the USB port can only be used to charge the batteries. The DCP port is designed for plug-in power supplies. Higher power levels of up to 100 W are covered by USB Power Delivery( USB-PD).
Principle of power source detection
When charging a battery, the key is to ensure that the charging device senses the current demand of the battery and does not overload it. If the USB charging device can only supply a limited charging current, for example 500 mA, but the battery's state of charge is such that it could be charged with a higher charging current, then there is a risk that the USB Battery Charging will shut down. That is why the USB Charging system has a port detection that ensures compliance and thus avoids faulty switching.