Stanford researchers successfully test wireless, in-motion electric charging

McAllen, TX, has been testing an electric bus that is being charged by a wireless system built by Wireless Advanced Vehicle Electrification. That system requires the bus to park over a charging pad. Stanford researchers are working on a wireless system that could charge a vehicle while it is in motion.

McAllen, TX, has been testing an electric bus that is being charged by a wireless system built by Wireless Advanced Vehicle Electrification. That system requires the bus to park over a charging pad. Stanford researchers are working on a wireless system that could charge a vehicle while it is in motion.

Test could signal the breakthrough needed to keep electric vehicles continuously charged

A major drawback of electric vehicles is their range, so-called range anxiety. That is true of cars and especially true of commercial trucks, which take longer to charge, and require more power to move heavier loads. The search for a solution to this problem has been ongoing and until it is found, many experts believe electric vehicles will remain a niche market in the transportation world.

However, a breakthrough in the ability to recharge vehicles may have occurred recently at Stanford. Researchers there published an article in the June 15 issue of Nature in which they describe “Robust wireless power transfer using a nonlinear parity–time-symmetric circuit.” Put more simply, researchers have developed a way to wireless charge a moving object.

“In addition to advancing the wireless charging of vehicles and personal devices like cellphones, our new technology may untether robotics in manufacturing, which also are on the move,” said Shanhui Fan, a professor of electrical engineering and senior author of the study, in a statement. “We still need to significantly increase the amount of electricity being transferred to charge electric cars, but we may not need to push the distance too much more.”

The breakthrough came using a moving LED lightbulb – ironically enough – and involved transmitting electricity. The demonstration builds upon work done at MIT in 2007 which transmitted electricity over a few feet to a stationary object. The researchers note that a car requires tens of kilowatts of power to operate whereas the LED lightbulb required just 1 milliwatt, but it remains confident that the amount of electricity transferred can be scaled and transfer distance can be extended.

“In theory, one could drive for an unlimited amount of time without having to stop to recharge,” Fan said. “The hope is that you’ll be able to charge your electric car while you’re driving down the highway. A coil in the bottom of the vehicle could receive electricity from a series of coils connected to an electric current embedded in the road.”


The hope is that you’ll be able to charge your electric car while you’re driving down the highway. A coil in the bottom of the vehicle could receive electricity from a series of coils connected to an electric current embedded in the road.
— Shanhui Fan, professor of electrical engineering, Stanford

If successful, driverless vehicles could become commonplace as embedded roadways are charged by solar power or other sources and automatically charge vehicles as they drive.

The system features a mid-range wireless power transfer based on magnetic resonance coupling. An oscillating magnetic field is generated as electricity moves through coils, which also causes electrons in a nearby coil of wires to oscillate and therefore transfer power, the researchers explained. However, once movement is added to the equation, the continuous flow of electricity is disrupted unless aspects of the circuits, such as the frequency, as manually tuned.

To solve this problem the researchers added an off-the-shelf voltage amplifier and feedback resistor to eliminate the radio-frequency source in the transmitter. The system automatically determines the correct frequency for various distances without the need for human interference.

“Adding the amplifier and resistor allows power to be very efficiently transferred across most of the three-foot range and despite the changing orientation of the receiving coil,” said graduate student Sid Assawaworrarit, the study’s lead author. “This eliminates the need for automatic and continuous tuning of any aspect of the circuits.”



The general-purpose amplifier the team used had a relatively low efficiency of about 10%m whereas a custom-made amplifier could boost that efficiency to more than 90%.

“We can rethink how to deliver electricity not only to our cars, but to smaller devices on or in our bodies,” Fan said. “For anything that could benefit from dynamic, wireless charging, this is potentially very important.”

Wireless charging of a vehicle is not a new concept. In fact, last year, McAllen, TX, took delivery of two buses from Wireless Advanced Vehicle Electrification (WAVE) and Complete Coach Works (CCW) that charge automatically.

The city says the wireless charging systems, which include pads at designated locations that the buses stop above, will double the daily range of the buses.

“We are excited to have reached this stage of the project, as it will provide great benefits to our Transit System. Along with the anticipated cost savings, and the environmental impact this project will have, the inductive charging while on route makes it convenient for our operations,” McAllen Transit Director Mario Delgado said at the unveiling. “We will be able to complete one full day as a result of the inductive charging, so it makes sense operationally. We will give our bus the range it needs during a whole day without disturbing our service. The WAVE in-route charging roughly doubles the range of the bus on a given day.”

An embedded charging pad has been placed in the roadway at one of the McAllen stops. The embedded pads measure about three square feet. The pad is flat and seamlessly blends in with the asphalt, allowing any vehicle to drive over it.

“The bus will arrive over the charging pad every hour, during the standard layover of about 10-15 minutes so that the bus can charge, causing no disruption to the route,” said Delgado.

If the Stanford researchers have truly found a breakthrough to wireless charging, the McAllen bus project may seem like light years ago.