Tunable Power Management Could Get Wireless EV Charging on the Road

Drexel researchers helped to demonstrate at more controllable system for wireless charging of electric vehicles, that could one day help to enable in-road charging.

One of the primary issues slowing the adoption of electric vehicles is the fear of being stranded with a dead battery and no charging stations nearby. In addition to the Biden Administration’s recent push to install more charging stations across the nation, technology developers are also addressing this “range anxiety” with the vision of ubiquitous wireless charging built directly into roads. A breakthrough by an international team of researchers, led by engineers at Drexel University, could help make this sort of integrated wireless charging a reality.

Their discovery, reported in the journal IEEE Transactions on Industrial Electronics, offers a solution to one of the fundamental physical challenges facing all wireless charging technology: misalignment.

Though wireless charging has eliminated tangles of cables and frustration with incompatible or faulty plugs, anyone who has accidentally placed their phone on a charging pad a little off kilter — and returned to find it uncharged — can attest, it’s not yet foolproof.

Getting an electric vehicle to line up perfectly over a wireless charger can be an even more challenging task — with misalignment leading to a similarly unpleasant surprise. “In practice, the misalignment between receiver and transmitter is inevitable,” the team acknowledges in its report. But current wireless charging technology for EVs is unable to accommodate even the slightest offset without its function or efficiency suffering.

One of the primary issues slowing the adoption of electric vehicles is the fear of being stranded with a dead battery and no charging stations nearby. In addition to the Biden Administration’s recent push to install more charging stations across the nation, technology developers are also addressing this “range anxiety” with the vision of ubiquitous wireless charging building directly into roads. A breakthrough by an international team of researchers, led by engineers at Drexel University, could help make this sort of integrated wireless charging a reality.

Their discovery, reported in the journal IEEE Transactions on Industrial Electronics, offers a solution to one of the fundamental physical challenges facing all wireless charging technology: misalignment.

Though wireless charging has eliminated tangles of cables and frustration with incompatible or faulty plugs, anyone who has carelessly placed their phone on a charging pad can attest, it’s not yet foolproof.

Getting an electric vehicle to line up perfectly over a wireless charger can be an even more challenging task — with misalignment leading to a similarly unpleasant surprise. “In practice, the misalignment between receiver and transmitter is inevitable,” the team acknowledges in its report. But current wireless charging technology for EVs is unable to accommodate even the slightest offset without its function or efficiency suffering.

The researchers, from Drexel’s College of Engineering and Shanghai Jiao Tong University, Zhejiang University and Northwestern Polytechnical University, in China, developed and tested a charging system that is versatile enough to adjust the way it delivers an inductive charge, so that it can accommodate misalignment and various levels of battery charge.

“Creating a more forgiving system that can accommodate the realities of charging an electric vehicle is an important step toward widespread adoption of wireless charging,” said Fei Lu, PhD, an assistant professor at Drexel, who is one of the lead researchers. “There is great potential for wireless charging to support dynamic charging of vehicles while they are moving on the road, but it is crucial that we overcome the challenge of alignment first.”

Inductive charging works by using an electromagnetic field to transfer power, similar to the way wind or water might push a turbine to generate electricity. An induction coil receives the electromagnetic “push” from the charger and transforms that vibration into energy that is charges the battery of the device or vehicle.

But, like a misaligned turbine that misses the full force of the wind or water, inductive charging coils that are not properly coupled will not efficiently charge the battery and — with enough misalignment — they may fail to activate the charging process at all.

What the researchers have discovered, however, is that with inductive charging, it’s actually possible to wield a bit more control over the torrent transferring the energy.

The team’s strategy differs from current wireless chargers, which are only able to make adjustments to the voltage — the amount of charge — being pushed through the system. Their hybrid method shows that adjusting the frequency of the field – how quickly or slowly the voltage is being transferred – produces a greater range with which to adjust for misalignment or how charged the battery is.

“Think about our wireless charging challenge like trying to fill a small glass with water under a tap that’s going at full blast. It can be quite tricky, because if you don’t hold it directly under the tap a lot of water will miss the glass,” Lu said. “And once it is nearly full, the force of the stream will splash water out of the glass without it ever being filled to the brim. The key to our system is that it provides more control over the tap – or the charging power, as it were — so it’s easier to direct more of the flow into the glass and slow it down as the glass becomes full.”

To test their theory, the researchers produced a prototype of their system -- built to the standards for electrical vehicle wireless chargers set by the Society of Automobile Engineers. It included an adjustable resistor, in order to simulate various levels of battery charge, and a moveable platform that allowed the transmitter coil and charging coil to be pushed up to 100 millimeters out of alignment – which was about half the width of the charging surface.

They tested the system on both relatively charged and uncharged battery states, under well-aligned and misaligned conditions. By tuning both the frequency and voltage of the charging process, the prototype was able to provide a stable power input from 0 to 3.3 kilowatts, which is the standard charging range for plug-in EV charging. And under all four sets of conditions the team was able to achieve nearly 96% efficiency – on par with commercial wireless chargers and only slightly below that of plug-in chargers.

“Being able to vary the voltage yet maintain the power of the charging system means that this process could be used to efficiently charge both near-dead batteries and those that just need a top-off,” Lu said. “It also allows for adjustments at the charging station when multiple cars are using it at once, so that the charging power remains constant for all of them. We see this as a critical step in making inductive charging more resilient against with real-world conditions.”

The discovery could also enable electric vehicles to eliminate some of the power-conversion hardware currently necessary to manage direct-current wireless charging. This would reduce some of the weight and cost of the vehicle. The team plans to continue its efforts to improve wireless charging technology by adapting the system for the other types of wireless charging circuits, to ensure compatibility with all electric vehicles.

In addition to Lu, Hua Zhang, PhD, an assistant research professor at Drexel; Yao Wang and Amr Mostafa, doctoral students at Drexel; Chong Zhu, PhD, an assistant professor at Shanghai Jiao Tong University; Ningfei Jiao, PhD, an associate researcher at Northwestern Polytechnical University; and Ying Mei, a doctoral student at Zhejiang University.

This paper will be published in the Oct. 2022 edition of IEEE Transactions on Industrial Electronics, it is available online here: https://ieeexplore.ieee.org/document/9583844