Putting Wireless EV Charging Efficiency & Speed to the Test

By Amy Barzdukas

The #1 misconception that people have about wireless EV charging is that it somehow must be less efficient and slower than plugging in – I mean, there is an AIR GAP. At WiTricity we talk until we are blue in the face, trying to explain that our technology is not the same as the old-fashioned inductive charging your electric toothbrush or smartphone wireless charging uses.  Pushing power across the air gap is very efficient because of the unique design of the coils. The losses in any charging system, wireless or wired, are mostly from the power electronics that convert AC power from the grid to DC power the battery needs. Remember that plug-in charging isn’t 100% efficient— the vehicle’s on-board charger (OBC) has losses that are about the same as the wireless charging system!

But the conversation about “percentage points of efficiency” is mostly of interest to engineers. People who own and drive EVs tell us they think of efficiency as “how long does it take to charge my car?” The answer, of course, is that “it depends.” It depends on a variety of factors including how depleted the battery is, the max power rating of the wireless charger or OBC, the amperage rating of the circuit that the wall box is connected to, and/or the temperature of the vehicle.

There are three general categories of charger:

  1. The “Level 1” portable charger that comes with most EVs and plugs into an ordinary 100V AC wall outlet.
  2. A “Level 2” charger, a faster charger that many EV owners choose to have installed at their home and connects to a 220V AC circuit, is the level of charger most widely available at public charging facilities.
  3. A “DC Fast Charger,” which is the fastest charge of all at anywhere from 50 kW to 350 kW, (and which is much more expensive than the vehicle itself!)

The WiTricity Halo™ wireless charger launching this year is an 11kW charger that we frequently tell people is just as fast at the plug – and in this case, we mean the Level 2 plug. But that’s where the “Level 2” designation gets REALLY complicated—Level 2 chargers can be rated for anywhere from 6.6 kW-19 kW. The vast majority of Level 2 chargers in use today aren’t anywhere close to 11kW. Speaking from personal experience, my own home EV charger operates at about 5kW (and happily charges while we sleep and we never worry). Most public Level 2 chargers operate at around 7 kW.

Enough with the watts. What does that mean in practical terms?

We’ll show you.

We put our Ford Mustang Mach-E (upgraded to support wireless charging in addition to the plug) to the test. We charged for 30 minutes each on our WiTricity Halo wireless charger and on a ChargePoint Level 2 charger publicly available in the same garage that houses the Detroit Smart Parking Lab.

The results?

In 30 minutes, the ChargePoint plug moved the Mustang Mach-E from 50% State of Charge (SOC) to 51%, adding roughly 2 miles of range to the car. The WiTricity Halo wireless charger moved the Mustang Mach-E from 50% SOC to 57%, adding roughly 12 miles of range to the car. To the EV owner, that’s a real difference. Not only is WiTricity Halo charging as efficient as the plug, in many cases it may be measurably faster than the Level 2 plug.

Now, doubters will want to know the details.

  • All measurements of the battery’s state of charge (SOC) and miles available were taken from the Mustang Mach-E display
  • For both charging scenarios, we started with a 50% SOC on the battery
  • Both scenarios were charging at roughly 43-46 degrees Fahrenheit (about 5-6 degrees Celsius), in a parking garage exposed to the elements. (Yes, this was chilly.)
  • The first charging was the wireless scenario, where the battery was cold: the vehicle had sat overnight in the garage.
  • The second charging, with the L2 plug, was done after the wireless session and after we had burned off charge to make sure the battery was at the same SOC. The battery, itself, may actually have been a little bit warmer than in the wireless charging scenario, but not by much.
  • In the video where you see a yellow box flash up on the display in both instances, that’s when we had to open the car door to make the display we were filming stay lit. There was no interruption or change to the charging process which continued unaffected.

To the consumer looking at “how many miles can I get in how much time,” and not particularly knowledgeable about amps or watts or currents, that’s a real difference.

Related Asset:

Wired vs Wireless Charging Efficiency for EVs: A Comparison