Fast Wireless EV Charging

Wireless EV charging offers convenience, but is that enough to justify the added expense? Maybe there's more...

Wireless charging is becoming popular for cell phones, tablets, and other personal electronics. No need to find the adapter and plug it in – just set the product down on a charging pad and let electromagnetic induction do the work. Soon, electric vehicles will have a wireless charging option*. As long as the car is parked above the charging pad, it can charge its batteries without plugging in. It’s safe and convenient – what more could you ask for?

How about fast and efficient?

Engineers at the Oak Ridge National Laboratory (ORNL) recently demonstrated a wireless fast-charging system that’s about as efficient as a plug-in charger. The 20 kW charger delivers energy three times faster than many standard plug-in units while achieving over 90% efficiency from the wall outlet to the EV battery. But at half the power of the BMW i3’s fast charger and only one-sixth the power of a Tesla Supercharger, convenience comes at a price: longer charge times. Of course, this is just a stepping stone. ORNL is already looking toward its next target: a 50 kW wireless charger that’s comparable to many of today’s plug-in fast chargers.

Wireless Quick Charger

According to ORNL, the new wireless quick charger is the first 20 kW model designed for passenger vehicles. (The same principle is already being used in some electric buses.) The charger was tested on a Toyota RAV4 EV, as shown here:

In the block diagram below, efficiencies are shown in red. The overall system efficiency from the AC outlet to the battery is 92%. “It’s actually very close to the conventional [wired] charging efficiency,” says Dr. Omer Onar, ORNL Research Fellow.

Is It Safe?

Sending 20 kW of power through the air generates an enormous magnetic field, so safety is clearly a concern. According to Madhu Chinthavali, ORNL Power Electronics Team leader,”The high-frequency magnetic fields employed in power transfer across a large air gap are focused and shielded. This means that magnetic fringe fields decrease rapidly to levels well below limits set by international standards, including inside the vehicle, to ensure personal safety.”

How Fast Is It?

A typical EV has a battery bank in the 22 kWh to 60 kWh range, so a 20 kW charger can fully charge a battery in roughly one to three hours. Since the charging curve is exponential, a car can charge from near-zero to 80% in 20 to 60 minutes, depending on the size of its battery bank. Assuming they can make a 50 kW unit, it would take 8 to 24 minutes to bring the car’s battery to 80% capacity.

The Real Goal: Charging On-the-Fly

So if I’m the owner of an EV and I have a plug-in charging unit in my garage, the only inconvenience in charging is the plug itself. (#FWP) If that’s the only issue, then wireless EV charging is a solution in search of a problem. But the long-term goal isn’t just to eliminate the plug, it’s to allow cars to charge as they’re moving. Granted, electrified highways are pretty far down the road, if you’ll excuse the pun. But the technology would reduce the need for a large battery bank, allowing cars to travel farther without the extra weight and expense.

Is This Really Necessary?

I have to admit, I have mixed feelings about this technology. A wireless charger will cost more than a plug-in unit, while delivering the same charging speeds at best. For buses, which make frequent stops at pre-defined locations, wireless charging makes perfect sense. But for stationary use in private cars, I don’t see the convenience being worth the additional cost. What about electrified roads? Frankly, I think we’d be better served by a high-speed maglev train system.

What do you think? Please comment below.

Images (except where noted) courtesy of Oak Ridge National Laboratory

*To my knowledge, no electric car manufacturers are currently offering wireless charging units, although there is a third-party aftermarket option available for several models. According to the company, its 3.3 kW wireless chargers are 7% to 12% less efficient than plug-in chargers.


Follow Dr. Tom Lombardo on Twitter,  LinkedIn, Google+, and Facebook.