Wireless charging: Electricity from the air

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The Qi standard has won the wireless charging race. Wireless charging is increasingly found in entertainment and medical electronics, electromobility, and industrial applications.

The principle of contactless energy transfer is not new. It has been researched for as long as transformers and inductive couplings have been around. At the end of the 18th century, electrical engineering pioneer Nikola Tesla considered how to transport energy from Niagara Falls around the world.

Inductive coupling: energy transfer over short distances

Electricity can be transferred wirelessly in various ways. With non-radiative coupling, energy transfer is limited to short distances of just a few centimeters. Induction is the most commonly used principle. It is based on the magnetic flow and has a relatively high level of efficiency of up to 90 percent. In the sender, an oscillator generates an alternating magnetic field. Transfer is by mutual induction between the coils in the sender and receiver.

The alternating current in the sending coil induces an alternating voltage in the receiving coil, which is rectified and fed to the consumer as DC voltage. The induction principle is used, for example, to charge mobile devices, such as smartphones, RFID transponders, medical devices with low current demand (pacemakers, etc.), electric cars, between machine parts or between rail systems and electric vehicles, such as the Transrapid monorail train.

Resonant inductive coupling – known under brand names such as WiTricity – allows energy to be transferred over larger distances. Free oscillating circuits, consisting of a capacitor and a coil, are placed between the sending and receiving coils. The resonance between the oscillating circuits improves magnetic coupling between the sending and receiving coils.

Convenient, but not very efficient

Wireless charging works: Simply place the device on a charging pad, wait for a few hours – done. No messing around with charging cables or wear and tear on plug contacts. Without an external power connection, more compact designs and completely sealed product housings are possible – a bonus for medical and industrial applications in difficult environments.

However, there are various limitations associated with inductive charging: It is slower than with a cable and uses more electricity. In practice, the efficiency of 50 to 85 percent is much lower than charging with a cable, where the level is about 95 percent. In other words, the power source must supply more energy or the charging time is considerably extended. The coils in the sender and receiver must be placed relatively precisely on top of each other. Wireless charging also needs more hardware components, which means additional costs.

Industry standard Qi: winner of the wireless charging race

The battle for supremacy in inductive charging has been won by the standard Qi (Chinese: life force) from the Wireless Power Consortium (WPC). Among others, the consortium includes Samsung, Sony, Nokia, and Apple, as well as battery manufacturers, network operators, automotive components suppliers, and furniture stores. Qi currently supports output power to 5 watts. Other power levels, such as 2 watts and 10 watts, are also possible nowadays. Qi-capable mobile devices include the Apple iPhone X and 8 and Samsung’s Galaxy devices S8 and S9 (Plus).

The current publicly available Qi standard 1.2.3 offers up to 15 watts. However, only a few devices are able to offer fast wireless charging at this time. The strength of the charging current is controlled with the transmission frequency. When the battery is almost fully charged, the charging current is reduced successively by reducing the transmission power of the receiver. When the battery is full, an end-of-charge signal is sent to the charging station, which then changes to stand-by mode.

Qi’s main competitor, PowerMat, which also uses inductive coupling, has thrown in the towel and joined the Wireless Power Consortium. Its own standard, AirFuel, is thus dead. Ultimately, benefits, such as being able to charge several devices at one charging station, and higher powers up to 40 watts were unable to help the technology make the breakthrough. The main reason for joining the WPC was Apple’s decision to use the Qi standard for its current iPhones. The merger of the Power Matters Alliance (PMA) with the Alliance for Wireless Power (A4WP), which uses resonant magnetic coupling, was also unable to reverse the trend.

BMW, Audi, Nissan, and Co.: contactless charging of electric vehicles

The scarcity of EV charging stations and messing around with power cables are factors that hold back electric mobility. Contactless charging could provide some new momentum. However, to date, the industry has not agreed on a common standard. The only manufacturer that offers an inductive charging system ex works is BMW – for its plug-in hybrid 530e iPerformance. The GroundPad generates a magnetic field. Electricity is induced in the CarPad beneath the vehicle, which charges the high voltage battery over a distance of around eight centimeters. The charging power is just 3,2 kW – the level of a household socket. BMW says that the efficiency level is 85 percent.

The battery in certain Audi e-tron models can be charged inductively by Audi Wireless Charging (AWC). When the car is positioned above the charging plate, the charging coil moves upward and starts the charging process with 3.6 kW. The alternating magnetic field induces a DC voltage in the secondary coil beneath the front axle via the air gap. It is rectified by an AC/DC converter and fed into the high voltage on-board power supply. Audi says that the level of efficiency is above 90 percent. Nissan plans to use the wireless charging systems form WiTricity for its electric cars, such as the Leaf. Porsche also backs inductive charging.

In the future, clusters of inductive charging systems can be integrated in the public infrastructure – for example, on the highway. Electric cars could then charge continuously while driving and batteries could be smaller. Contactless charging while driving works; this was demonstrated by semiconductor producer Qualcomm Technologies Inc. (electronica exhibitor, Hall B4, Booth 415) with its inductive charging technology Halo.

Electricity from radio waves

Radiative energy transfer using electromagnetic waves, such as light or radio waves, is hardly relevant in practice. It can be used to bridge much greater distances. However, the free space loss allows only a very low level of efficiency of less than one percent.

A patent application from Apple gave rise to speculation that the manufacturer’s mobile devices could be charged with electricity via radio waves in the future. There is also a rumor that Apple is working together with Energous. The US company is developing the remote charging technology Watt Up, which is said to charge mobile devices with low power consumption within a radius of 4.5 meters, using radio waves.

Ossia is pursuing a similar approach: The Cota Transmitter base station is packed full with thousands of mini antennas. The device to be supplied is equipped with an RF receiving chip, which sends radio frequency signals that are received by the antennas of the base station. This sends back concentrated RF signals that the RF chip converts into energy. The range is said to be three meters. No market-ready products are yet available.


Contactless charging possibilities could be used wherever people need power for their devices: in offices, factories, hospitals, airports, railroad stations, hotels, or trade fairs. For example, charging pads are integrated into the tables in some Starbucks and McDonald´s outlets. It would also be possible to turn entire rooms and halls into wireless power suppliers, for example, with conductive wall paint or copper rods, like Disney Research has implemented with its System Quasistatic Cavity Resonance for Ubiquitous Wireless Power Transfer (QSCR). It is also possible to pass on power from one device to another without contact: Huawei’s in-ear headphones Freebuds 2 Pro tap the energy of the Mate 20 (Pro) – all that’s necessary is to place them on the back surface of the smartphone.

The company is still working on the energy efficiency: “In the future, inductive charging stations for cars, cell phones, and other applications can become much better in terms of energy efficiency if alternating current from the socket is converted directly in to the transmission frequency, in other words, without the detours via direct current,” says Gregor Dürrenberger from the Swiss Research Foundation for Electricity and Mobile Communication (FSM).

electronica 2018

Discover more about wireless technologies at the electronica Wireless Congress: Systems & Applications. Smart energy supply is the focal topic of the Power Electronics Forum.