Charge the electric car while you drive

STANFORD (US) — New technology could lead to wireless charging of electric vehicles while they cruise down the highway.

The long-term goal of the high-efficiency charging system—that uses magnetic fields to transmit large electric currents between metal coils placed several feet apart—is to dramatically increasing the driving range of electric cars and trucks and develop an all-electric highway.

“Our vision is that you’ll be able to drive onto any highway and charge your car,” says Shanhui Fan, associate professor of electrical engineering at Stanford University. “Large-scale deployment would involve revamping the entire highway system and could even have applications beyond transportation.”


A wireless charging system would address a major drawback of plug-in electric cars—their limited driving range. The all-electric Nissan Leaf, for example, gets less than 100 miles on a single charge, and the battery takes several hours to fully recharge. A charge-as-you-drive system would overcome these limitations, researchers say.

“What makes this concept exciting is that you could potentially drive for an unlimited amount of time without having to recharge,” says study co-author Richard Sassoon, managing director of the Stanford Global Climate and Energy Project (GCEP), which funded the research. “You could actually have more energy stored in your battery at the end of your trip than you started with.”

As reported in the journal Applied Physics Letters, the wireless power transfer is based on a technology called magnetic resonance coupling. Two copper coils are tuned to resonate at the same natural frequency—like two wine glasses that vibrate when a specific note is sung. The coils are placed a few feet apart. One coil is connected to an electric current, which generates a magnetic field that causes the second coil to resonate. This magnetic resonance results in the invisible transfer of electric energy through the air from the first coil to the receiving coil.

“Wireless power transfer will only occur if the two resonators are in tune,” Fan notes. “Objects tuned at different frequencies will not be affected.”

In 2007, researchers at the Massachusetts Institute of Technology used magnetic resonance to light a 60-watt bulb. The experiment demonstrated that power could be transferred between two stationary coils about 6 feet apart, even when humans and other obstacles are placed in between. “In the MIT experiment, the magnetic field appeared to have no impact on people who stood between the coils,” Fan says. “That’s very important in terms of safety.”

Wireless charging

The MIT researchers have created a spinoff company that’s developing a stationary charging system capable of wirelessly transferring about 3 kilowatts of electric power to a vehicle parked in a garage or on the street.

Fan and his colleagues wondered if the MIT system could be modified to transfer 10 kilowatts of electric power over a distance of 6.5 feet—enough to charge a car moving at highway speeds. The car battery would provide an additional boost for acceleration or uphill driving.

Here’s how the system would work: A series of coils connected to an electric current would be embedded in the highway. Receiving coils attached to the bottom of the car would resonate as the vehicle speeds along, creating magnetic fields that continuously transfer electricity to charge the battery.

To determine the most efficient way to transmit 10 kilowatts of power to a real car, the Stanford team created computer models of systems with metal plates added to the basic coil design. “Asphalt in the road would probably have little effect, but metallic elements in the body of the car can drastically disturb electromagnetic fields,” Fan explains. “That’s why we did the APL study—to figure out the optimum transfer scheme if large metal objects are present.”

Using mathematical simulations, postdoctoral scholars Xiaofang Yu and Sunil Sandhu found the answer: A coil bent at a 90-degree angle and attached to a metal plate can transfer 10 kilowatts of electrical energy to an identical coil 6.5 feet away.

“That’s fast enough to maintain a constant speed,” Fan says. “To actually charge the car battery would require arrays of coils embedded in the road. This wireless transfer scheme has an efficiency of 97 percent.”

Wireless future

Fan and his colleagues recently filed a patent application for their wireless system. The next step is to test it in the laboratory and eventually try it out in real driving conditions. “You can very reliably use these computer simulations to predict how a real device would behave,” Fan says.

The researchers also want to make sure that the system won’t affect drivers, passengers, or the dozens of microcomputers that control steering, navigation, air conditioning, and other vehicle operations.

“We need to determine very early on that no harm is done to people, animals, the electronics of the car or to credit cards in your wallet,” says Sven Beiker, executive director of the Center for Automotive Research at Stanford (CARS). Although a power transfer efficiency of 97 percent is extremely high, Beiker and his colleagues want to be sure that the remaining 3 percent is lost as heat and not as potentially harmful radiation.

Some transportation experts envision an automated highway system where driverless electric vehicles are wirelessly charged by solar power or other renewable energy sources. The goal would be to reduce accidents and dramatically improve the flow of traffic while lowering greenhouse gas emissions.

Beiker, who co-authored the APL study, says that wireless technology might one day assist GPS navigation of driverless cars.

“GPS has a basic accuracy of 30 to 40 feet,” he says. “It tells you where you are on the planet, but for safety, you want to make sure that your car is in the center of the lane.” In the proposed system, the magnetic fields could also be used to control steering, he explains. Since the coils would be in the center of the lane, they could provide very precise positioning at no extra cost.

The researchers also have begun discussions with Michael Lepech, assistant professor of civil and environmental engineering, to study the optimal layout of roadbed transmitters and determine if rebar and other metals in the pavement will reduce efficiency.

“We have the opportunity to rethink how electric power is delivered to our cars, homes and work,” Fan said. “We’re used to thinking about power delivery in terms of wires and plugging things into the wall. Imagine that instead of wires and plugs, you could transfer power through a vacuum. Our work is a step in that direction.”

More news from Stanford University: http://news.stanford.edu/

chat9 Comments


  1. Tony P

    The only issue with this is how do you charge for the charge? I suppose you could use an RFID system like EZPass. But this is a good idea. Use the highways, and you could even embed in city roads.

  2. David

    You’d have to consider that will slow the car down – moving resistive conductors through powerful magnetic fields DOES tend to have a drag effect, induced current magnetism countering the energy delivered to the vehicles, and warming the material too.

    There’s also the potential health effects of being exposed to powerful dynamic magnetic fields while driving down the freeway. I don’t think I’m sensitive to it, but given millions of people, you’re going to find effects.

    It might be worth considering that we have established technology for transmitting power to moving vehicles . Dropping a steel trailing wheel onto a flush rail in the road and raising a pantograph to overhead wire is not impossible. Also, electric busses often run on dual wire overhead trolley systems. There’s no reason an electric vehicle couldn’t use the same.

    But if you really want to save mass quantities of energy, make the energy source strong enough for large trucks to utilize. They’d be happy to draw power off a fixed grid instead of paying for expensive fuel.

  3. Roberto DePaschoal

    Similar system has been tested for years by http://www.Bombardier.com not for just charging, but to also energize heavy streetcars car convoys. I am also involved in something similar mentioned on http://www.ev-motion.com with a novel pat.pending affordable concept EV with unlimited range of operation.

  4. Mark Gray

    It would be a real game changer to have major highways that can automatically fuel electric vehicles. My vision of the highway of the future certainly includes this feature.
    When they talk about 6+ foot range, they must be thinking of either very large vehicles or of interacting with the same coil for some length of time – a passenger car is much closer to the pavement than that. With an automatic suspension on the charging coil, it could be kept within a few inches of the road surface, potentially improving greatly on efficiency.
    It sounds like these researchers are a little overconfident about untested safety issues and over-enamored of simulation. Computer modeling is a fine first step, but please start digging some holes in an actual road and driving an actual car across. Results of real-world experiments are what will convince people this could actually work.

  5. Roberto DePaschoal

    Well, technology exists even for hands free driving, like the experiment of 1997 http://www.youtube.com/watch?v=C9G6JRUmg_A I just wonder what is preventing us from having clean and unclogged highways.

  6. Steve

    Guys this is not a new concept and I know someone who already may have a copyright and a possible patent pending on the design and use. The idea is 4 years old its called inductive pulse charging and it was discussed over 10 years ago with Edison International, Flour and Duracell. I have 25 years of DC motive and charging industry experience not much you see today is really “high” tech.

    Ever heard of a Tesla Coil? Tesla used to transmit electricity wirelessly remember….. Most inventions are generally a rehash or improvement on an idea someone else invented years ago or just reapplied to another industry. To bad Tesla is dead he would love this…. Even the mechanics and electronics of the Toyota Prius is not new. Regenerative breaking to recharge the batteries is old school as is the engine generator battery concept. Its all been used in the industrial markets on electric forklifts for years.

    If i recall there were a couple of material handling companies that used an in the floor coil concept to charge and guide AGV’s back in the manufacturing plants we used to have in America…

    This should be interesting

  7. Roberto DePaschoal

    Hi guys, take a look on the video mentioned on my previous comment. Bombardier is also working on a project to equip EVs. with similar system. http://www.youtube.com/watch?v=1JVtvQe30wk

  8. Jasmine

    What about the high electromagnetic field charges as a result of this process on highways and other places ?
    Is it harmful for life, including humans ?
    What are the other consequences ?
    If it is safe for life in general, it is an excellent technology.

  9. coach

    More than the wireless thing, how long does it take to recover completely the battery ?
    And as Jasmine said, what about electromagnetic consequences on human exposition for a long time ?

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