Find out about energy loss and higher temperature when charging on a pad.
Wireless charging may one day replace plugs and wires, similar to how Bluetooth and Wi-Fi have modernized personal communication. The concept rests on inductive coupling using an electromagnetic field that transfers energy from the transmitter to the receiver.
Wireless transfer of power is not new. In 1831, Michael Faraday discovered induction by sending electromagnetic force through space. In the late 1800s and the early 1900s, Nicola Tesla demonstrated wireless broadcasting and power transmission. The experiments in Colorado Springs in 1899 lead to the Wardenclyffe Tower in New York. Tesla wanted to prove that electrical power could be transmitted without wires, but lack of funding halted the project.
It was not until the 1920s that public broadcasting began. Europe built massive transmitters that covered many countries. The station at Beromünster in Switzerland could have transmitted radio signals at 600kW, but legislation on electro-smog and protests from the local population limited the power to 180kW. Smaller FM stations have since replaced these large national transmitters; cellular repeaters and Wi-Fi stations transmit at a fraction of this power and many are in single watt digits.
Wireless charging shares similarities with radio transmission. It sends signals in a near field condition in which the primary coil produces a magnetic field that is picked up by the secondary coil in close proximity. The radio transmitter, on the other hand, works on the far field principle by sending waves that travel through space. While the receiving coil of the wireless charger captures most of the energy generated, the receiving antenna of the radio only needs a few microvolts (one millionth of a volt) to recover a signal that becomes intelligent when amplified.
Wireless charging is classified as inductive charging, radio charging and resonance charging. Most of today’s wireless chargers use inductive charging with transmit and receive coils in close proximity. Electric toothbrushes were one of the first consumer goods to adopt this method.
Radio charging serves low-power devices operating within a 10-meter (30-foot) radius from the transmitter to charge batteries in medical implants, hearing aids, watches, entertainment devices and RFID (radio frequency identification) chips. The transmitter sends a low-wattage radio wave and the receiver converts the signal to energy. Radio charging resembles radio transmission the most; it offers high flexibility but has a low power capture and exposes people to electro-smog. Radio charging is not in common use.
Larger batteries for the electric vehicle use resonance charging by making a coil “ring.” The oscillating magnetic field works within a 1-meter (3-foot) radius. To stay in the power field, the distance between transmit and receive coil must be within a quarter wavelength (915Mhz has a wavelength of 0.328 meters or 1 foot).
Resonance charging is not limited to high-wattage wireless chargers; it is used at all power levels. While a 3kW system for EV charging achieves a reported efficiency of 93–95 percent with a 20cm (8 inch) air gap, a 100W system is better than 90 percent efficient; however the low-power 5W system remains in the 75–80 percent efficiency range. Resonance charging is still in the experimental stages and is not widely used.
Wireless charging needed a global standard and the Wireless Power Consortium (WPC) accomplished this in 2008 by introducing the Qi norm. This opened the door for device manufacturers to offer chargers for Qi-compatible devices with 5 watts of power.
Powermat, a Qi participant, sprung loose over a disagreement and in 2012 started PMA as a new competitive norm. PMA is similar to Qi but runs at a different frequency. Also in 2012, A4WP announced resonance charging that allows freedom of movement while simultaneously charging several devices. A4WP has not yet been approved as a standard. Table 1 summarizes the three norms.
|WPC or Qi (Wireless Power Consortium)||PMA (Power Matters Alliance)||A4WP (Alliance for Wireless Power)|
|Established||2008, Qi was first wireless charging standard||2012, Procter & Gamble and Powermat||2012 by Samsung and Qualcomm|
coil distance 5mm;
similar to Qi
|Resonant charging, loosely coupled; serious emission issues remain.|
|Markets||Qi has widest global use; Over 500 products, more than 60 mobile phones||Tight competition with Qi, gaining ground, 100,000 Powermats at Starbucks,||A4WP and PWA merged, no product available|
|Members & companies||Samsung, LG, HTC, TI, Panasonic, Sony, Nokia, Motorola, Philips, Verizon, BMW, Audi, Daimler, VW Porsche, Toyota, Jeep||Powermat, Samsung, LG, TDK, TI, AT&T, Duracell, WiTricity, Starbucks Teavana, Huawei, FCC, Energy Star, Flextronics||Qualcomm, TediaTek, Intel, LG, HTC, Samsung, Deutsche Telecom. No commercial products|
Table 1: Recognized standards for wireless charging. Qi and PMA are in completion while A4WP has no standard and no commercial products. Emission issues must be solved first.
While the A4WP format may not be available soon in a charging station, a war is being fought over Qi and PMA. To accommodate both systems, some manufacturers offer chargers and mobile devices that serve both standards. This is a repeat of when Columbia Records released the 33 rpm LP (Long Play) in 1948 and RCA Victor promoted the 45 rpm record featuring a large hole. Dual-speed gramophones and an insert solved the problem.
Modern wireless charging follows a complex handshake to identify the device to be charged. When placing a device onto a charge mat, the change in capacitance or resonance senses its presence. The mat then transmits a burst signal; the qualified device awakens and responds by providing identification and signal strength status. The signal quality is often also used to improve the positioning of the receiver or enhance magnetic coupling between mat and receiver.
The charge mat only transmits power when a valid object is recognized, which occurs when the receiver fulfills the protocol as defined by one of the interoperability standards. During charging, the receiver sends control error signals to adjust the power level. Upon full charge or when removing the load, the mat switches to standby.
Transmit and receive coils are shielded to obtain good coupling and to reduce stray radiation. Some charge mats use a free moving transmit coil that seeks the object placed for best coupling, others systems feature multiple transmit coils and engage those in close proximity with the object.
WPC calls the transmitter the TX Controller, or Base Station, and the receiver on the mobile device the RX Controller, or Power Receiver. There is a resemblance to a transformer with a primary and secondary coil. Figure 2 illustrates an overview of a Qi wireless charging system.
Figure 2: Overview of Qi wireless charging system.
Several systems are competing that may not be compatible. The three most common are Qi, PMA, A4WP.
|Qi logo, Chinese word meaning "natural energy"|
Wireless charging offers the ultimate convenience for consumers and enables safe charging in a hazardous environment where an electrical spark could cause an explosion. It further permits charging where grease, dust or corrosion would prevent a good electrical contact. Eliminating electrical contacts also helps doctors in sterilizing surgical tools. Wireless charging is durable and does not wear out the contacts on multiple insertions.
Makers of electric vehicles seek convenience in charging, and this is elegantly solved by parking the vehicle over a transmit coil. Engineers talk about embedding charging coils into highways for continuous charging while driving or when waiting at a traffic light. This is technically feasible, but high cost, low efficiency and field emission when transmitting high power remain insurmountable challenges.
For household and business use, the California Energy Commission (CEC), Level V, mandates that AC adapters must meet a minimum efficiency of 85 percent; Energy Star Level V requires 87 percent (European CE uses CEC as a base). Adding the losses of the AC adapter to wireless charging brings the overall efficiency down further as the inductive transfer efficiency of inductive charging is only 75–80 percent. Such a loss adds up when considering that an estimated one billion mobile phone chargers are plugged into AC outlets worldwide. To improve efficiency and comply with the Energy Star requirements, WPC combines the power needs into a single power conversion.
Lost energy turns into heat, and a wireless charger can get quite warm during charging. This causes stress on the device’s battery as it sits on the mat. It should be noted that the heat buildup only occurs during charging; the charging pad cools down once the battery is fully charged.
WPC was very careful when releasing Qi; the first version has a power limit of 5 watts. A medium-power version of up to 120 watts is in the works, but this norm must meet stringent radiation standards before release. Radiation prompts health concerns and these are raised by folks living next to mobile phone towers and Wi-Fi stations.
Electromagnetic energy from radio towers, mobile phones, Wi-Fi and now wireless charging are categorized as non-ionizing radiation and are said to be harmless. Ionizing rays from x-rays, on the other hand, can cause cancer. As the number of non-ionizing devices grows, folks begin to question the safety of this form of radiation as well. Regulatory authorities are observing possible health risks and will impose restrictions if harm can be proven.
Health problems caused by electromagnetic waves are inconclusive; however, carrying a mobile phone close to the body is a concern. In standby mode the device is constantly seeking contact with a tower by transmitting signal busts. The transmit power is adjusted to the proximity of the tower and is higher in remote areas.
Going wireless demands a 25 percent cost premium on the charging station, a burden that also affects the receiver. For consumers who don’t want to pay the price, charging by wires remains a workable alternative. Birds looking for the missing wires will appreciate this move.
Figure 3: Pros and cons of wireless charging
Last Updated 2016-02-25
Comments are intended for "commenting," an open discussion amongst site visitors. Battery University monitors the comments and understands the importance of expressing perspectives and opinions in a shared forum. However, all communication must be done with the use of appropriate language and the avoidance of spam and discrimination.
If you have a question, require further information, have a suggestion or would like to report an error, use the "contact us" form or email us at: BatteryU@cadex.com. While we make all efforts to answer your questions accurately, we cannot guarantee results. Neither can we take responsibility for any damages or injuries that may result as a consequence of the information provided. Please accept our advice as a free public support rather than an engineering or professional service.