BU-412: Charging without Wires

Wireless charging may one day replace plugs and wires similar to how Wi-Fi and Bluetooth have modernized personal communication. Wireless charging with inductive coupling uses an electromagnetic field that transfers energy from the transmitter to the receiver. Consumers are wild about the convenience of simply placing a portable device on a charging mat. Wireless charging works well with mobile phones, digital cameras, media players, gaming controllers and Bluetooth headsets. Other potential applications are power tools, medical devices, e-bikes and electric cars (EVs).

Wireless transfer of power is not new. In 1831, Michael Faraday discovered induction and stated that electromagnetic forces can travel through space. In the late 1800s and early 1900s, Nicola Tesla began demonstrating wireless broadcasting and power transmission. Early experiments in Colorado Springs in 1899 lead to the Wardenclyffe Tower in New York — Tesla was adamant to prove that electrical power could be transmitted without wires, but a lack of funding halted the project.

It was not until the 1920s that public broadcasting began, and Europe built massive AM transmitters with signal strengths to penetrate many countries. The transmitter at Beromünster in Switzerland (Figure 1) 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.

Description: htSwitzerland's National AM Radio Station Beromünster


Figure 1: Switzerland's National AM Radio Station Beromünster

Constructed in 1931 as an independent voice against the Nazi propaganda of Germany to the displeasure of Adolf Hitler. The station broadcasted AM signals until 2008.

How does wireless charging relate to radio transmission? Both models are similar in that they transmit power by electro-magnetic waves. 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 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 needs only a few microvolt (one millionth of a volt) to rise the signal above the noise level and receive clear intelligence when amplified. 

Types of Wireless Charging

Wireless charging is classified into three categories: Radio charging, inductive charging andresonance charging. Radio charging will serve low-power devices operating within a 10-meter (30 feet) radius from the transmitter to charge batteries in medical implants, hearing aids, watches and entertainment devices. Radio charging can also activate advanced RFID (radio frequency identification) chips through resonantly enhanced induction. The transmitter sends a low-power radio wave at a frequency of 915MHz (frequency of large industrial/commercial microwave ovens, consumer ovens usually use 2.45GH) and the receiver converts the signal to energy. The radio charging method is closest to a regular radio transmitter; it offers high flexibility but has low power capture and exposes people to electro-smog.

Most of today’s wireless chargers use inductive charging featuring a transmit and receive coil in close proximity. Electric toothbrushes were one of the first devices to use this charging method, and mobile phones are the largest growing sector to charge without wires. To retrofit an existing mobile phone for mobile charging, simply attach a “skin” that contains the receiver and provides interconnection to the charger socket. Many new devices will have this feature built in.

For larger batteries such as electric vehicles, resonance charging, or electro dynamic induction,is being developed. Resonance charging works by making a coil ring. The oscillating magnetic field works within a one meter (3 feet) radius; the distance between transmit and receive coil must be well within the 1/4 wavelength (915Mhz has a wavelength of 0.328 meters). Currently, resonance charging in trials can deliver roughly 3,000 watts at a transfer efficiency of 80–90 percent.


The success for wireless charging was subject to adapting a global standard and the WPC (Wireless Power Consortium) accomplished this in 2008. With the “Qi” norm, device manufacturers can now build charger platforms to serve a broad range of compatible Qi devices. The first release limits the power to 5 watts and works as follows:

While in ready mode, the charging mat sends signals that sense the placing of an object. Detection occurs by a change in capacitance or resonance. The mat validates the device for WPC compatibility by sending a packet of data by modulating the load with an 8-bit data string. The receiving device awakens and responds by providing the signal strength. The mat then sends multiple digital pings to identify the best positioning of the placed object. Only then will service begin. During charging, the receiver sends control error packets to adjust the power level. Figure 2 illustrates a Qi compatible charger mat.

Charging mat for a mobile phone

Figure 2: Charging mat for a mobile phone

Wireless charging is most practical for mobile phones and accessories.

Courtesy of Powermat


The charge mat only transmits power when a valid object is recognized. With no load, or when the battery is fully charged, the mat switches to standby mode. The 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. Figure 3 shows a Qi kit representing the transmitter and receiver.


Wireless charging system by Texas Instrument

Figure 3: Wireless charging system by Texas Instruments

Qi-compatible transmitter module (left) and the receiver module. Commercial applications are currently limited to 5 watts.

Courtesy of Texas Instruments

Drawbacks of Wireless Charging

Inductive charging is not without disadvantages. The California Energy Commission (CEC), Level V, mandates that AC adapters 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 charger circuit to the AC adapter brings the overall efficiency for a hardwired charger to about 70 percent. Wireless charging has a transfer efficiency of 70–80 percent; coupled with their own AC power conversion the overall charge efficiency hovers between 60 and 70 percent. In addition to efficiency losses, the wireless charger includes the “readiness” mode to identify the placement of an object, a feature that adds to power consumption.

Charger manufacturers, including Cadex Electronics, make great efforts to meet regulatory requirements. Losses incurred through less efficient charge methods go against the government-backed Energy Star program, and exceptions may need to be made to allow more energy use to support convenience. With roughly one billion chargers on standby or in charge mode, the extra power consumed is significant. The number of mobile phones is estimated at over five billion in the world; in 2008, 3.2 billion power supplies were manufactured globally; most are plugged into the main drawing power.

Lost energy turns into heat and a wireless charger can get quite warm during charge. Any temperature increase to the battery causes undue stress, and batteries charged on wireless devices may not last as long on a mat as on the regular plug-in charger. It should be noted that the heat buildup only occurs during charging; the Qi wireless charger will cool down when the battery is fully charged.

The 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. There are health concerns because the devices operate in close proximity to human activity at a radio frequency ranging from 80–300kHz. Some stations transmit at 915MHz, the frequency used to heat food in microwave ovens.

Electromagnetic energy from radio towers, mobile phones, Wi-Fi, routers, 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 safety. Regulatory authorities are waiting for evidence and will only impose restrictions if a health risk can be scientifically proven. Meanwhile, parents object to schools installing Wi-Fi, and homeowners protest about electric meters that communicate data without wires. Radiation from wireless chargers may be seen as harmless because they do not transmit intelligence. In most cases, the household radiation is low enough not to worry, but it is the field strength and close proximity to the source that could add to potential harm.

Charging EVs without plug and cable offers the ultimate in convenience as the 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. While this is technically feasible, cost, efficiency and radiation issues at these higher powers are insurmountable challenges.

At a transfer efficiency of 80–90 percent, 10–20 percent of the power is lost. This reflects in a substantial energy cost to the user and should be calculated as a decrease in drivable distance per watts. Applied to a large vehiclepopulation, this goes against the efforts to conserve energy. Daimler’s Head of Future Mobility, Professor Herbert Kohler, says that inductive charging for EVs is at least 15 years away and cautioned about safety. The potential radiation of EV charging is higher than Wi-Fi or talking on a mobile phone; it could also endanger people wearing a pacemaker.

Besides low efficiency and radiation concerns, wireless charging offers decisive advantages in industry. It allows safe charging in a hazardous environment where an electrical spark through charge contacts could cause an explosion, or where heavy grease, dust and corrosion would make electrical contacts impractical. Wireless charging also helps when multiple insertions would wear out the battery contacts too quickly. There is, however, a cost premium and this is especially apparent in custom devices that cannot take advantage of cost reductions through mass production.

Currently, a wireless charging station will cost roughly 25 percent more than a regular charger. A 25 percent premium also applies to the receiver. If the portable device cannot be charged with the battery installed, as is possible with a mobile phone, then each battery would need its own receiver and the battery pack would bear the added cost. Unless wireless charging is necessary for convenience or environmental reasons, charging through battery contact continues to be a practical alternative.


On January 27, 2011 at 1:24pm
Serge wrote:

typo: the word in asterisks needs to be removed from the text:

Detection occurs by noticing a change in capacitance or resonance when placing an object ***is*** on the mat.

On February 2, 2011 at 7:14pm
Lalit Singh Rawal wrote:

nice article. Yes in future wireless power transmission could be the most important achievement of engineering

On April 1, 2011 at 11:04pm
amol wrote:

i m using compaq latop.. my latop battery discharges very early .. wen i remove power it discharges is half hr…. 5 years to finish… wat i should do

On October 17, 2011 at 12:17am
Moustafa Magar wrote:


Just wondering if this technology could be used in a high temperature environment.

Thank you

Moustafa MAGAR

On January 11, 2012 at 11:47pm
Hashan Gayasri wrote:

Any explanation why the antenna of the mobile phone doesn’t induct the transmitted power and burn the signal amplifier?
Is it because resonance frequencies are different ?

On February 6, 2012 at 5:30pm
Gerry Schreiber wrote:

Ok, so how difficult i it to actually plug in a charger be it for a cell phone or a car…...c’mon, this is getting crazy.  The scary part is that in a world of necessary energy conservation, global warming etc etc etc we are prepared to set our concerns aside to save about 5 seconds of time?  Our drive for convenience really just exposes our laziness.  I think there a lot more important things to spend money on in the world than developing increased convenience.  ...put your minds and money to that for a star.  While I have been writing this about 50 children have died from starvation induced illnesses…meanwhile we wring our hands over the possibility of getting a little extra radiation we think might harm us.  Just because we can, doesn’t mean we should

On February 6, 2012 at 7:27pm
Sara wrote:

Wireless charging is just a waste of energy.  We look for efficiency in electronics, not putting out a large emf for a small pickup.  Not needed as stated elsewhere.

On February 22, 2012 at 10:25am
шкурило wrote:

молодцы это очень хорошо даже. ну можно ж придумать более мошне батарейки с таким внешним видом. например заряд от обычного  батарейка от телефона хватает на один день так вед. если заделать их так чтобы один раз зарядил и забыл на один год ну хотя бы на пол года и ни надо думать и ни надо придумывать всякие способы зарядит телефон. конечно такие штучки хороша когда под рукой нет зарядное устройства или нет электричество. ну все равно это к лучшему.

On March 7, 2012 at 10:44am
patjasiu wrote:

As stated in article great technology for hazardous areas (no spark allowed) and also for 100% water/dust-proof devices (no contacts & no slots = no leaks & no corrosion & no short circuit risk). One could safely get device or EV charged outdoors even in rainy weather or on ships deck in salty corrosive mist - which isn’t so easily done with plug-in technology. Going further I wonder how mass of water affects inductive power transmission - and I guess it hardly does, so You could “push current” fully underwater - try to “plug-in” something this way!
Maybe charging phone on desk this way is luxury, but technology is hard to beat in some areas!

On November 5, 2012 at 10:43am
cody wrote:

Funny how so many of these Comments are based on opinions and are sooooo wrong…lol…...This betters the planet….....technology reaching to be battery free….HOW IS THAT A BAD THING??????

2. waste energy????  in the short small brain it does…....you have to compare a usb adapter plugged in 24/7, YES it uses power…...and many other factors and this wireless charging ends up in the long run using less and cost less to use etc etc….

On December 10, 2012 at 1:29pm
thumar rushik wrote:

please give me information about voltage at mobile usb port

On January 3, 2013 at 9:23am
Wm. Cerniuk wrote:

Nice article.  Great reference and well written.

On January 3, 2013 at 1:35pm
cody wrote:

Funny how that used a product of one of the Huge Companies In Wireless Manufacturing and that Company is Not Qi…LOL…and Qi didn’t start their so called Standard until This year…
And you the Guy talking about Global Warming etc, This has and is Proven, if not by just Common Sense but proven by test and use. The Traditional way of charging with a a/c wall adapter and plugging into phone causes a person to be Lazy when it comes to using it. Do you watch for the charging to reach 100% and unplug the phone then unplug the Adapter? If you do, you are one in a count of less then 1%....before you beat it down, do some research of test etc.
Also when was the Last time you saw a charger Adapter get qualified for Energy Star? Prob haven’t because the ones that do exist are so far out of acceptable cost no one can afford them. These chargers are down to a 0.0001 kwh draw on standby and it has already been proven it will reach down to Zero draw…

On March 5, 2013 at 9:02pm
kaisen wrote:

AD time, wireless induction charger/pad etc…
we are looking for wholesalers, distributors…  we have our own compatible standard. Also we are producing Qi standard charger/pad…  welcome to ask for an inquiry…
thanks for your time


On March 5, 2013 at 9:04pm
kaisen wrote:

oh, forgot to leave my contact info: weinaisong@gmail.com

On March 5, 2013 at 9:20pm
cody wrote:

PowerMat or Duracell Powermat are not Qi . In fact they have their own group which wasn’t mentioned and have the who’s who of Big Corps in their group..
1.Vinton G. Cerf, Honorary Chairman
Vinton G. Cerf is vice president and chief Internet evangelist for Google

2.Mani Parmar, Duracell Powermat, Procter & Gamble

3.James E. Snook, Starbucks
James is the Vice President, Emerging Platforms & Innovation in Stabucks

4.Jeff Howard, AT&T Mobility
Jeff Howard, Vice President, Mobile Device Portfolio in AT&T

5.Katharine Kaplan, Energy Star (Board Observer)
Katharine Kaplan, Chief, ENERGY STAR Product Development and Program Administration Grou

6.James Miller, FCC (Board Observer)

Fcc and Energy Star on the team…lol…that’s the people who matter…lol

On May 23, 2013 at 5:56pm
Thomas Tsoi wrote:


On August 7, 2013 at 10:12pm
nithin wrote:

Why we are not using wireless charging in new mobile phones ?

can we make charge the battery using signals.?
How ?
Pls explane…..

On November 10, 2013 at 5:31am

can you say ? , somebody get pattern for it ?

On November 30, 2013 at 9:15am
gayatri wrote:

i want different modes of charingggg

On December 13, 2013 at 1:19pm
Frank Cohen wrote:

Thanks for the good article and the great Web site. I am designing a new mens luxury watch - see http://votsh.com - that uses epaper displays. Using inductive recharging seems like the best way to go. -Frank Cohen

On February 5, 2014 at 11:37pm
Glenn Crosby wrote:

Microwave ovens operate 2400 MHz, not 915 MHz as stated in the article.  2400 MHz is near the resonant frequency of water molecules so the cooking is efficient.

On March 30, 2014 at 11:53pm
Rashi wrote:

plese give me diagram (wire less charger)

On April 29, 2014 at 10:03pm
Katline wrote:

Just got the Nexus wireless charger from Amazon and they are going absolutely crazy with the discounts with these brands. If you don’t have a promo code, you can use this one: http://amzn.to/1iyFZfq - before they take it down.

On April 30, 2014 at 4:21am
hallary wrote:

First of all let me tell you, you have got a great blog on topic Charging without wire .I am interested in looking for more of such topics and would like to have further information. Hope to see the next blog soon.

I have recently posted on topic SMS Gateway visit link http://www.experttexting.com/sms-gateway/

On May 29, 2014 at 5:08am
RAY wrote:

Hadan gayasri the two interfering waves would have to be in very close frequency..called resonance. This is where as in hearing or signal distorted.  Actually only effects human more than to electronics..aka cancer cause or possible reverse as an cure (if possible… Thats whole different suvject but read fcc snd cell phone manual where legal disclamer printings in tiny letternso peoole dont really read the Fscts)

On May 29, 2014 at 5:19am
RAY wrote:

Amol buy a fresh notevook battery..google the battery part no. You will fine hope there if no longer available then your notebook is tine tilo burry it..but inbthis site there isnabway to re-trigger battery by hot wire the battery side of the nb bsttery terminals using 100ohm resister but look in left index. A link will bring you to show you how to try it (no guarantee)

Here. http://batteryuniversity.com/learn/article/how_to_repair_a_laptop_battery

On September 27, 2014 at 2:59am
Nathan wrote:

Hi I just was wondering this and looked it up!
but it sounds possible for an app to be made with this sort of re-charge ability.
I am also thinking going onto the app at times of low battery to charge while away from charging devices is possible!
Could you please send my email more info of new inventions in this area.
Thanks nathan

On October 13, 2014 at 12:02am
Ray wrote:

GOOGLE = noteobok battery revival
GOOGLE = lithium polymer cell battery circuit diagram
GOOGLE = lithium polymer cell battery circuit chip manufacturer

please be advised that proceed at your own risk..

On November 27, 2014 at 11:04pm
orderplz wrote:

Thank you for the info. It sounds pretty user friendly. I guess I’ll pick one up for fun. thank u.

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On December 1, 2014 at 5:15pm
Kyle Hobbs wrote:

I am wanting to create one, but i need a list of materials to do so. Contact me please at kylejhobbs@gmail.com

On December 2, 2014 at 10:53pm
Nathan wrote:

Good luck Kyle u need cash to make things I got plenty of great ideas myself but I’m a poor guy!

On December 4, 2014 at 12:00am
Puja Pansare wrote:

Great idea!..  Can you plz mail me more information related to it.

On December 12, 2014 at 11:21am
Fred wrote:

Are wireless chargers a hazard to those with cardiac pacemakers, automatic defibrillators and the like?