BU-412: Charging without Wires


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 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, 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. 

Types of Wireless Charging

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 at a frequency of 915MHz (frequency of some microwave ovens) 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 watts 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 radio charging that operates in a “radio space” much like Bluetooth, but a low power. A4WP has not yet been approved as a standard because of emission concerns. 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
Technology Inductive charging,
coil distance 5mm;
Inductive charging,
similar to Qi
Radio 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 might offer chargers 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 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 the 1/4 wavelength (915Mhz has a wavelength of 0.328 meters). Resonance charging can deliver roughly 3,000 watts at a transfer efficiency of 80–90 percent. Resonance charging is in an experimental stage and no approved standards exist.

Concept of Wireless Charging

In ready mode, the charging mat sends signals that sense the presence of an object. Detection occurs by a change in capacitance or resonance. The mat validates the device for compatibility by sending a packet of data by modulating the placed 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 object and only then will service begin. During charging, the receiver sends control error packets to adjust the power level.

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. 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.

Overview of 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 Qi logo, Chinese word meaning "natural energy"


Pros and Cons of Wireless Charging

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 radiation issues when transmitting high power remain insurmountable challenges.

At a transfer efficiency of 80–90 percent with resonance charging, 10–20 percent of the power is lost. This reflects in a substantial energy cost to the user that must be calculated as a decrease in drivable distance per watt. Applied to a large vehicle population, this counteracts efforts to conserve energy.

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 70–80 percent. Such a loss adds up when considering that an estimated one billion mobile phone chargers remain plugged into an AC outlet worldwide.

Lost energy turns into heat and a wireless charger can get quite warm during charging. 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.  

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. 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.

Birds on a Wire 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 3/16/2015

*** Please Read Regarding Comments ***

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: answers@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.


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.

Home Delivery Restaurants in Chennai

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?

On January 5, 2015 at 10:29pm
kathun wrote:

why we can’t transfer charge from one mobile to another via bluetooth or wifi?

On January 30, 2015 at 7:31am
Phil wrote:

The Yagi-Uda array (TV aerial) was designed as a means of wireless power transmission in the 1920’s.

Although it did work it was impractical and inefficient but it did give us high gain antenna’s

On March 19, 2015 at 9:37pm
S Howell wrote:

I have a $400 wireless option in my 2015 Dodge Dart, an induction mat for a cell phone, my Android, to recharge while I am driving and making voice activated calls. Nice feature right? I thought so, but, WTH, nobody has a clue how to make it work? I was told I could obtain something to match the phone to the wireless mat to activate the transmit/receive for charging, but there isn’t anywhere to find out what I need! Not even the Dodge Homepage has anything about it except to say it is an option- nor do any Dodge Dealers have a clue. So I paid for the technology and cannot use it? Anyone know anything about how I can use this mat charger?

On March 24, 2015 at 8:32am
Steven | Intercel Battery Warehouse wrote:

Interesting development! However, I am always a bit sceptical when it comes to such technologies. Most of the time it will take several years before the technology is ready for mass production and we can use it in our daily products.