Wireless Charging

Andrew Liang
November 11, 2017

Submitted as coursework for PH240, Stanford University, Fall 2017


Fig. 1: Wireless Charging for Car. (Source: Wikimedia Commons)

Since the introduction of electricity in the nineteenth century, the methods of transferring this source of power have evolved. The current, most widely used method of electric cords to provide power has started to give way to wireless charging. Although wireless charging has recently been making headlines in mainstream society through the introduction of wireless charging for phones, the idea of wireless charging can be traced more than a century and a half back to 1831 by Michael Faraday when he demonstrated that electromagnetic energy could be transmitted through space. [1] In the late nineteenth century, Nikola Tesla built upon that and conducted experiments on wireless transmission of power. In 1891, Tesla built the first device that could transmit electricity without the use of cords with the invention of the Tesla coil. [2] Currently, the two most popular methods of wireless charging are inductive charging and resonance charging.

Inductive Charging

Inductive charging, shown in Fig. 2, is a technique used to transfer power wirelessly over short distances, often 5 to 7mm, using sub-radio frequencies or low radio frequencies of the electromagnetic wave spectrum. It takes place when energy is transferred from an external coil, often the charger, to a secondary coil, often a device such as a mobile phone, using an alternating magnetic field. The external coil produces an electromagnetic field that is alternating in polarity which the secondary coil then uses by drawing power from the alternating electromagnetic field and transforming it into an electrical current. This current is then rectified to charge the battery of the device. [1]

Resonance Charging

Resonance charging is a technique used to transfer power wirelessly over longer distances, often 7 to 40mm, using sub-radio frequencies or low radio frequencies of the electromagnetic wave spectrum. This takes place using resonance coupling through an inductance capacitance tuned circuit. Energy is transferred from an external coil, often the charger, to a secondary coil, often a device, when coils undergo oscillations at the same frequency. [1] This power will not be picked up unless the frequencies are the same. Resonance charging is relatively newer than inductive charging and thus is not as developed.

Fig. 2: Wireless Charging System Diagram. (Source: Wikimedia Commons)


The current applications of wireless charging are still limited but rapidly expanding. Electric toothbrushes, mobile phones, tablets, and other electronics are all making use of wireless charging. Electric vehicle charging, as seen in Fig. 1, and medical devices are other applications.

Current methods call for the device needing to be placed on a charging pad. While this frees up the need for wires, this limits spatial freedom, as the device must be kept on the charging pad. This makes simultaneous usage of device and charging difficult.

Efficiency is of another concern. Plugless, a wireless EV charging company, currently states that their wireless charging systems are approximately 12% less efficient than corded L2 30amp 240V charging systems. This means that if one owned a 2016 Nissan Leaf, which the EPA rates with energy consumption of 30kWh/100miles, and drove 10,000 miles per year, electricity consumption would increase 360kWh each year just by using a Plugless wireless charger. As such, the benefits of using a wireless charging system may not justify the current costs.

© Andrew Liang. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] V. K. Khanna, Implantable Medical Electronics: Prosthetics, Drug Delivery, and Health Monitoring (Springer, 2015).

[2] C. T. Rim and C. Mi, Wireless Power Transfer for Electric Vehicles and Mobile Devices (Wiley-IEEE, 2017).