A CDE Definition
Qi wireless charging
Pronounced "chee" from Chinese, meaning "life force," Qi is a wireless charging method from the Wireless Power Consortium (www.wirelesspowerconsortium.com). Introduced in 2009, Qi is based on "closely coupled" electromagnetic induction that requires the device to be aligned on top of the coil on the charging pad. Hundreds of devices are Qi certified, including Google Nexus and Samsung phones, and loosely coupled versions of Qi are expected in the future. See wireless charging.
Medium and Loosely Coupled
Under development at the Wireless Power Consortium (WPC) are a "medium-coupled" version of Qi that operates at a distance of up to approximately 15 mm and a loosely-coupled version (Qi 3D) that can be up to 50 mm. Typical applications are in vehicles and for mounting under table and desktop surfaces. Loosely coupled products not certified by the WPC have also been announced.
Three Qi Coils
Transferring electrical power to a portable product by placing it in a cradle or on a flat charging surface. There is no cable to plug in. Wireless charging generally takes longer than wired charging, but it can be convenient. For example, in an office, a smartphone can be dropped onto a desktop charging pad throughout the day when not being used. The Wireless Power Consortium and the AirFuel Alliance govern the major protocols (see Qi wireless charging and AirFuel). Another addition to wireless charging are the products from Energous (see WattUp).
Closely Coupled Induction (Qi and AirFuel Inductive)
Operating at kilohertz frequencies, the Qi and AirFuel Inductive (formerly PMA) technologies create a magnetic field between the transmitting coil in the power source and the receiving coil in the target device. Devices must be within approximately 7 mm of the charging pad. Depending on the number of transmitting coils in the charger, a phone might have to be moved around a bit to align the coils and establish a connection.
Closely coupled charging can cause nearby metal objects to become uncomfortably warm, which is why there is a warning notice on automobile console chargers to not place metal objects such as keys and coins on the pad.
Loosely Coupled Induction (Resonant Coupling)
Operating at megahertz frequencies, the transmitter and receiver coils are highly tuned, and devices can be an inch or more away from the charger. The greater distance is advantageous in automotive and under-table implementations. However, the farther away the two coils, the longer it takes to fully charge.
AirFuel Resonant (formerly Rezence) and Qi 3D are examples of Loosely Coupled chargers. Non-certified loosely coupled Qi chargers are also available. See fast wireless charging, Qi wireless charging and AirFuel.
No Metal Contacts
Charging in a Vehicle
Add-On Wireless Charging in the Car
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