A coil with an alternating current flowing generates a shifting magnetic field, when a secondary coil approaches it, there will be an electric current in the secondary coil, and export the electric current then we can do something useful like charging the battery. In the image below, the left is a transmitter coil inside a charging pad and the right is a receiver coil inside a mobile phone. When the mobile phone is placed on the top of the charging pad connected to the power – through electromagnetic induction – the mobile is charged. Basically, an AC current passes through the coil in the charging pad, which produces a magnetic field that makes a voltage in the coil of the receiver, which can be directly used to charge the battery of the mobile phone.

In order to popularize the magnetic inductive wireless charging technology, The Wireless Power Consortium (WPC) was formed in 2008 and released the first international standard of wireless power transfer, Qi, in the end of 2012 to ensure devices of different manufacturers are compatible. Main chip firms and power device manufactures joined the WPC. So far, there have been 334 member companies in WPC, Qi says on its website.

The advantages of magnetic inductive coupling include ease of implementation, convenient operation, high efficiency in close distance (typically less than a coil diameter) and safety. Therefore, it is applicable and popular for mobile devices. A normal electronics engineer is able to design a wireless charger, and that’s why there are all sorts of cheap phone wireless chargers on Taobao.com which is the largest online shopping site in China. If you’ve bought the cheap phone wireless charger from Taobao, your experience may be not good – a) sometimes you need to reposition your phone on the charging pad to get the phone pointed at the charging area on the pad; b) the allowed separation between the phone and the charging pad is small. The reason for the low spatial usage freedom of such wireless chargers is that the power transfer efficiency in magnetic induction has a lot to do with the overlap degree between the transmitter coil and the receiver coil – the higher the overlap degree, the more efficiently the power energy transfer, whereas the power transfer efficiency is less, additionally, the magnetic field generated by the primary coil spreads so the energy attenuates rapidly as the transfer distance increases, thus the device to be charged has to get very close to the charging pad to gain the maximum of the power energy.

The energy efficiency depends on the tightness of coupling between two coils and their quality factor. The tightness of coupling is determined by the alignment and distance, the ratio of diameters, and the shape of two coils. The quality factor mainly depends on the materials, given the shape and size of the coils as well as the operating frequency.