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 in 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 to reach many countries. The transmitter at Beromünster in Switzerland could have transmitted 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.
Wireless charging shares similarities with radio transmission sending signals by electro-magnetic fields. Wireless charging operates 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 into 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 and mobile phones might become the largest growing sector because of convenience.
Radio charging serves low-power devices operating within a 10-meter (30 feet) 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 and other uses use resonance charging by making a coil “ring.” The oscillating magnetic field works within a one meter (3 feet) 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 one 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”) air gap, a 100W system is better than 90 percent efficient; however the low-power 5W systems remains in the 75–80 percent efficiency range. Resonance charging is still in experimental stages with an agreed standard.
Wireless charging needed a global standard and the WPC (Wireless Power Consortium) 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; 10W and higher is in preparation.
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 more freedom and parallel charging of multiple 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, Samung, 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 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 Columbia Records releasing the 33 rpm LP (Long Play) in 1948 and RCA Victor competing with the 45 rpm record featuring a larger hole than the LP. Dual-speed gramophones and an insert solved the problem.
Modern wireless charging adheres to a complex handshake to identify the device to be charged. When placing a device onto the charge mat, a 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. These signals are often 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 above for best coupling, others systems feature multiple transmit coils and engage only those that are 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. It also allows 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 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 simplification 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 cost, efficiency and field emission issues 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 further down 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 remain plugged into an AC outlet worldwide. WPC combines the AC adapter providing regulated DC and isolating the AC mains into a single power conversion. This amalgamation improves the efficiency comparable with the Energy Star requirements.
Lost energy turns into heat and a wireless charger can get quite warm during charging. If the generated heat is not controlled properly, temperature increase causes stress to the battery and reduces life. 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 had 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. Interoperability and backwards compatibility with 5W systems also play a role.
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 would impose restrictions if harm can be proven.
A larger risk, if any, is carrying a mobile phone close to the body. In standby mode the device is constantly seeking contact with a tower by transmitting signal busts. The transmit power is adjusted to the proximity to the tower and is higher in fringe areas.
Going wireless demands a 25 percent cost premium on the charging station, a burden that also affects the receiver. For consumers preferring not to pay the price, charging by wires will continue to be the alternative. Birds looking for wires might appreciate this move (Figure 3).
Figure 3: Wireless charging is not without shortcomings
Unlike wireless communications where only minute levels of power are transmitted, wireless charging sends power through the air. Losses reflect in unwanted heat and radiation.
Last Updated 2015-11-12
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