Qi (pronounced chee; IPA: /tʃiː/, coming from the Chinese word meaning "natural energy") is an interface standard developed by the Wireless Power Consortium for inductive electrical power transfer over distances of up to 4 cm (1.6 inches). The Qi system comprises a power transmission pad and a compatible receiver in a portable device. To use the system, the mobile device is placed on top of the power transmission pad, which charges it via resonant inductive coupling.
Mobile device manufacturers that are working with the standard include Asus, HTC, Huawei, LG Electronics, Motorola Mobility, Nokia, Samsung, BlackBerry, and Sony. The Wireless Power Consortium was established in 2008, and is an open-membership cooperation of Asian, European, and American companies in various manufacturing industries. The consortium aims to create a global standard for inductive charging technology.
Under the Qi specification, "low power" for inductive transfers denotes power deliveries below 5 W. Systems that fall within the scope of this standard are those that use inductive coupling between two planar coils to transfer power from the power transmitter to the power receiver. The distance between the two coils is typically 5 mm. It is possible to extend that range to at least 40 mm. Regulation of the output voltage is provided by a digital control loop where the power receiver communicates with the power transmitter and requests more or less power. Communication is unidirectional from the power receiver to the power transmitter via backscatter modulation. In backscatter modulation, the power-receiver coil is loaded, changing the current draw at the power transmitter. These current changes are monitored and demodulated into the information required for the two devices to work together.
The WPC published the Qi low power specification in August 2009. The Qi specification is available as free public download. In 2011, the Wireless Power Consortium began to extend the Qi specification to medium power. The low-power specification delivers up to 5 W (typically used to charge mobile devices), and the medium-power specification will deliver up to 120 W (typically used to power displays and laptops). In 2015, WPC demonstrated a high-power specification that will deliver up to 1 KW, allowing the powering of kitchen utensils among other high-power utilities.
Devices that operate with the Qi standard rely on electromagnetic induction between planar coils. A Qi system consists of two types of devices – the Base Station, which is connected to a power source and provides inductive power, and Mobile Devices, which consume inductive power. The Base Station contains a power transmitter that comprises a transmitting coil that generates an oscillating magnetic field; the Mobile Device contains a power receiver holding a receiving coil. The magnetic field induces an alternating current in the receiving coil by Faraday's law of induction. Close spacing of the two coils, as well as shielding on their surfaces, ensure the inductive power transfer is efficient.