Qi: Wireless charging and how it works

In short: A standard in which a device charges without a cable, with energy transferred by an alternating magnetic field from a charging coil to a receiver coil.

In more detail: Qi (pronounced “chee”) is developed by the Wireless Power Consortium and used in most smartphones, earbud cases and smartwatches. The principle is inductive coupling: the same idea is in a transformer or an electric toothbrush.

In Depth

Principle

The charging pad uses a coil and alternating current to produce an alternating magnetic field. In the receiver coil in the device this field induces a voltage. A circuit rectifies it and charges the battery. Device and pad communicate through small signals: the pad detects whether a suitable device is on it and regulates the power.

Power and efficiency

Usual levels range from 5 W through 7.5 and 10 W to 15 W; some manufacturers reach more with their own methods. Efficiency is below that of a cable; part of the energy turns into heat. The exact position of the coils matters: if the device sits crooked, it charges more slowly or not at all.

Qi2 and MagSafe

Qi2 adds magnets (Magnetic Power Profile, closely related to Apple’s MagSafe) that align device and coil exactly. This improves efficiency and convenience. New devices from several manufacturers support Qi2.

Notes

Metal between pad and device (coins, magnetic cards, metallic cases) can heat up and disturb charging; some pads then switch off. Heat also harms the battery, so do not charge under pillows. For fast charging the cable usually remains superior; wireless charging wins through convenience, for example in the car or on the bedside table.

See also: Battery and charging correctly, USB-C