The specifications of all USB interfaces also covered power management. For example, the specifications of USB 1.0 and USB 2.0 already defined 5 V and 500 mA (high power), or 5 V and 100 mA (low power) for the supply of connected USB devices. 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 this. The Battery Charging Specification distinguishes between three different ports: the Standard Downstream Port (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, can be achieved by 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 a current draw of 1.5 A. The CDP can be used for battery charging. The removable power is 7.5 W. A device connected to the port recognizes it by means of a handshake. And the Dedicated Charging Port (DCP), which is also designed for 1.5 A, is recognized by the short circuit of pins D+ and D-. It is designed for plug-in power supplies. Higher powers from 10 W up to 100 W are covered by USB Power Delivery (USB-PD).