Wireless Inductive Charging

Darian Brooks
December 8, 2015

Submitted as coursework for PH240, Stanford University, Fall 2015


In a world where wireless technology continues to grow and advance, cords become a hassle and annoyance. Advancements with Bluetooth and Wi-Fi have allowed people to transfer information, listen to music on wireless headphones, and even automatically connect to vehicles for calls and other features. However futuristic this sounds, it becomes useless once devices run out of battery. Plugging the device into a power source to charge takes away from the convenience of being wireless, thus wireless charging technology has began to hit bigger markets and become more commercialized for certain products. This allows users to place the device on a surface to charge and makes it easier to just pick the device up to use and put it down to charge at will. With smartphones, charging with a cord just about every day is necessary causing frays or other issues that require users to find or buy yet another cord specifically for the device they are using. On a larger scale, this technology could be used to charge vehicles as well such as a Tesla or Nissan Leaf.

Inductive Charging Basics

Wireless power transfer can be done in a few ways, but one of the most popular wireless power transfer options is through use of inductive charging. This technology stems from the coupled mode theory from the 1950's that fundamentally analyzes waveguides and resonance coupling over space and time. [1] Inductive charging involves a transmission coil in the charging mechanism connected to power and receiver coil in the device with the battery. The receiving side communicates with the transmission side and controls the processing and flow of power. In certain devices such as electric vehicles, AC/DC converters are necessary and attached to the receiving device's onboard charging unit to convert power to charge the battery. [1,2] Through use of electromagnetic waves, a field is created and power transmission is then possible over a short distance. Power runs from the source, to the transmission coil, through the magnetic field, into the receiver coil, is converted, and is absorbed by the battery. This process currently is used at a distance of up to a couple centimeters. [1] A longer distance is possible for inductive charging, but it makes use of large amounts of radiation which is not safe for civilian use. [1] Inductive charging does have intrinsic losses from the magnetic field and the resistance. [2] Distance between the coils is also a variable in calculating the efficiency of the charger with more distance causing less power transfer due to a loss in strength of the magnetic field. [1]


Wireless charging is convenient and fairly efficient, but there has not been enough research done to increase efficiency and distance necessary between the device and charger. Currently, electric toothbrushes and cellular phones need to be in contact with the charger's surface. This makes it easier to charge and saves a few seconds fiddling with cords, but the device is still essentially plugged in and immobile where as having a longer cord would allow use and charging at the same time. The future for wireless charging will include longer ranges between the device and charging mechanism as well as a higher efficiency over a longer distance. At some point, it could be possible to charge multiple devices efficiently and safely from another room of a building solving the issue of battery life.

© Darian Brooks. 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] X. Wei, Z. Wang and H. Dai, "A Critical Review of Wireless Power Transfer via Strongly Coupled Magnetic Resonances," Energies 7, 4316 (2014).

[2] J. I. Agbinya, Ed., Wireless Power Transfer (River Publishers, 2012).