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 transmitting power 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 devices 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 a radio transmitter most; it offers high flexibility but has a low power capture and exposes people to electro-smog.
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 watt of power; 10W is in preparation.
In 2012, Powermat, a Qi participant, sprung loose over a disagreement and 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 for more spatial freedom and parallel charging of multiple devices. A4WP has not yet been approved as a standard. Table 1 illustrates 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, the war will be fought over Qi and PMA. Manufacturers offer chargers and mobile devices that serve both standards as it was the case when the LP (Long Play) was released by Columbia Records in 1948. This market was disrupted by RCA Victor bringing out the 45 rpm record running at a different speed and featuring a larger hole than the LP. Dual-speed gramophones and an insert solved the problem. This could not be done with VHS and Betamax, or with HD-DVD and Blu-Ray. While mobile manufacturers are backing one wireless charging standard, coffee shops and food joints are supporting another. Ultimately, the war will be won by the consumer.
Larger batteries for the electric vehicles adopt 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 the 1/4 wavelength (915Mhz has a wavelength of 0.328 meters).
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 an efficiency of better than 95 percent, a typical 100W system exceeds 90 percent; the low-power 5W systems remains in the 75–80 percent efficiency range. Resonance charging is in experimental stages and no approved standards exist.
In standby mode, the charging mat may send signals that sense the presence of an object. Detection occurs by a change in capacitance or resonance. Upon detecting an object, the mat transmits a burst signal, which transfers sufficient energy to power up the receiving device. It awakens and responds by providing identification and signal strength signals which can be used to improve the positioning of the receiver or to 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 by engaging only 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 of 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 allows safe charging in a hazardous environment where an electrical spark could cause an explosion; it permits charging where grease, dust and corrosion prevent a good electrical contact. Eliminating contacts also helps doctors sterilizing surgical tools. Wireless charging is durable and does not wear out the contacts on multiple insertions.
An EV driver simply parks 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 charger 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.
State-of-the art wireless power transmission (WPT) combines the AC adapter providing regulated DC and isolating the AC mains into a single power conversion. This amalgamation results in better efficiencies that is 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 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 shared with folks living in the mid of cell phone towers and Wi-Fi stations. This could be at the center of the delay to develop the medium power standard, but interoperability and backwards compatibility to 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 believed to be harmless. Ionizing rays from x-rays, on the other hand, have been shown to cause cancer. As the number of non-ionizing devices increases, people begin to question the safety. Regulatory authorities are observing the health risk and will impose restrictions if a danger can be proven.
A larger risk, if any, may be carrying a mobile phone close to the body. The device in standby mode 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 adds roughly 25 percent to the charging station, a cost increase that also increases the cost of the receiver by about the same amount. This price should come down with volume but for many everyday applications, charging with wires through battery contact continues to be a practical alternative. This question is also being asked by birds looking for wires in Figure 3.
Figure 3: Wireless charging is not without shortcomings
Unlike wireless communications where only minute levels of power is transmitted, wireless charging sends power through the air. Losses contribute to unwanted heat and radiation.
Last Updated 3/30/2015
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