Stanford Researchers Demonstrate A Method for Recharging Electric Vehicles While Driving

May 13, 2020 by Luke James

The engineers claim that their method, which uses magnetism, represents a big step towards making it practical for electric vehicles to recharge as they trundle along roads and highways.

Although wireless charging pads already exist for consumer electronics, they only work if the device is stationary. Now, Stanford engineers have demonstrated a technology that could be used to provide electricity to robots and drones as they are operating, too. The team says that their technology and method will need to be scaled up before it is able to be applied to technology on the move, however.

The work builds upon previous efforts carried out by the same engineers three years ago. Back then, electrical engineers Shanhui Fan and Sid Assawaworrarit built a system that could wirelessly recharge moving objects. However, it was not efficient enough for applications outside of the lab. 


Transmitting Electricity at Varying Distances

Wireless power transfer technology typically suffers from an inherent sensitivity to the relative movement of the device with respect to the power source. Although nonlinear parity-time symmetric circuits could be used to deliver robust wireless power while a device is moving rapidly, previous implementations have relied on an inefficient gain element based on an operation-amplifier circuit. These exhibit inherent loss and therefore lead to poor system efficiency. 

In their first breakthrough in 2017, the research team developed a wireless charger that was able to transmit electricity even as the distance to the receiver changes.

This was achieved with an amplifier and feedback resistor that let the system adjust its operating frequency as this distance changed. And although it worked in the lab, it was too inefficient for practical real-world applications—the amplifier used too much electricity to produce the required amplification. 


An electric vehicle charging.

Today's electric cars are restricted by wired charging technology


Wireless Transfer Efficiency of 92%

In their recent work, however, the duo shows that “robust and efficient wireless power transfer can be achieved by using a power-efficient switch-mode amplifier with current-sensing feedback in a parity–time-symmetric circuit,” where in the circuit, the parity-time symmetry guarantees that the effective load impedance on the switch-mode amplifier stays constant. This allows the amplifier to maintain high efficiency when transfer distance varies. 

“This is a significant step toward a practical and efficient system for wirelessly re-charging automobiles and robots, even when they are moving at high speeds,” Fan said. “We would have to scale up the power to recharge a moving car, but I don’t think that’s a serious roadblock. For re-charging robots, we’re already within the range of practical usefulness.”


Scaling the Technology

By replacing the original amplifier with a more efficient switch mode amplifier, the researchers were able to boost the system’s wireless transmission efficiency to 92%. This new prototype is reportedly able to wirelessly transmit 10 watts of electricity over a distance of two-to-three feet. And although 10 watts is nowhere near the amount of power that an electric vehicle would draw, Fan claims that there “aren’t any fundamental obstacles” to scaling this technology in a system that can transmit the tens or hundreds of kilowatts that a car would need. 

Although it may be many years before this technology is being used in power solutions for cars, Fan claims that for recharging robots, “we’re already within the range of practical usefulness.”