Can A Wireless EV Charger Outperform A Tesla Supercharger (With Regard to Current-Level & Cost-Effectiveness)?

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Originally published on EV Obsession.

What do you say to the idea of a commercial wireless electric vehicle (EV) charging system that can recharge a battery as fast as one of Tesla’s Supercharger stations can? Does that sound plausible to you?

While skepticism seems merited in this case, one of Momentum Dynamics’ researchers (John M Miller, PE, PhD, formerly the Director of Power Electronics and Electric Power Systems Center at Oak Ridge National Laboratory) did recently publish a technical paper in the IEEE journal Transactions on Transportation Electrification on this topic. The paper seemingly implies that the company’s resonant magnetic induction charging technology is “ready for prime time.”

wireless EV supercharger

So, what does this mean exactly? It means that, with the use of a 30-50 lbs receiver mounted on the bottom of an EV, charging levels as high as level 3 (Supercharger class) are possible — no cables needed. “Possible” means many things, though, of course — for instance, a system providing current at Supercharger levels would require a lot of infrastructure work (that possibly precludes widespread commercial use), even if the technology itself is quite sound.

As Miller himself notes:

Fast chargers (DCFC) typically require 480Vac, 3-phase supply. The Tesla pack (up to 90 kWh) can charge at 135kW off a commercial 480Vac, 3-phase utility connection that is rated for more than 162 Arms. So the service cabling must be rated for that level of current.

These DCFC’s or Supercharger are all part of the infrastructure (only in very special circumstances can 480Vac be plug and receptacle connected). Now, the Tesla pack is rated 400Vdc which at 90kWh means the cell pack is rated 225Ah (Amp-hour). That rating is 1C, which means that at 225A continuous discharge for 1 hour it will be fully depleted. That in turn implies a discharge (or charge) power of 90kW. So, for 135kW charging one requires only 1.5C-rate into the battery. Lithium-ion packs for EVs can easily absorb 2C-rate charging. Take the Nissan Leaf as an example. A DCFC for the Nissan Leaf (24kWh, 364V pack) has a 1C rate of 66A. The LEAF DCFC is rated 45 kW which implies 123Adc charging, or 1.9C-rate. So its actually the battery pack that determines how much power it can absorb. Lithium-ion packs rated 4C or higher are designed for power, such as those found in hybrid cars, and going higher still means packs for portable power tools. So the opportunity for high power wireless charging, just as for conductive charging, depends on the local infrastructure and the vehicle battery pack charge acceptance limits.

That said, efficiency could apparently be on par with cabled systems. As it stands, Momentum has demonstrated 91% efficiency, and is aiming for 93%.


Here’s more:

The system we saw was delimited to around 25 kilowatts – not Supercharger strength, but more than a Nissan Leaf’s 6.6 kilowatts, or the standard 10-20 kilowatt on-board charger of a Tesla Model S.

We’ve also seen an electric truck from one of the world’s largest shippers at Momentum’s laboratory warehouse, and seen it charged. The company may have more news on this and other behind-the-scenes work later this year.

The secrecy is at the client’s request, not Momentum’s, and its CEO Andy Daga, an engineer who’s done work for NASA, agreed with Miller the system can be scaled to Tesla levels.

“A more powerful 50 kilowatt or 100 kilowatt charger is certainly feasible and would almost certainly cost no more than a Supercharger, since many of the basic power electronics are similar,” said Daga who added beyond 135 kilowatt power is just as feasible. “The wireless charger would have lower lifecycle costs, however, because there would be no cord or plug to be damaged or vandalized.”

Hmm, interesting.

There’s actually quite a lot of further information in the original article that this is sourced from, so those interested are recommended to head here. It’s an interesting read, even if there is quite a lot of “sales talk.”

Image Credit: Momentum Dynamics

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James Ayre

James Ayre's background is predominantly in geopolitics and history, but he has an obsessive interest in pretty much everything. After an early life spent in the Imperial Free City of Dortmund, James followed the river Ruhr to Cofbuokheim, where he attended the University of Astnide. And where he also briefly considered entering the coal mining business. He currently writes for a living, on a broad variety of subjects, ranging from science, to politics, to military history, to renewable energy.

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32 thoughts on “Can A Wireless EV Charger Outperform A Tesla Supercharger (With Regard to Current-Level & Cost-Effectiveness)?

  • Have they approached Tesla about this or are they going the highest bidder route where we probably never hear about it again?

    90% efficiency is good. If he can’t improve it, 90% is acceptable. Hope we can use it soon.

    • Even 93% is very very bad efficiency, not acceptable! 98-99% is possible with this technology if they just have a mechanism for the plate under the car to automatically latch to the matching plate below or for the plate below to automatically latch to the plate under the car. YOU DON’T NEED TO BE A GENIUS to design a cheapo contraption that does this, and since it is plate to plate latching, accurate precision control is not needed unlike guiding a plug.

      • Is it a technology that can withstand public use and abuse. I totally agree with you about the waste at 93% and I’d find 98-99% acceptable. Making the moving parts robust enough may add too much weight/cost.

        • The technology of self-charging Roomba doesn’t cost that much, and this is way simpler than a Roomba.

        • As a compromise to 98% efficiency, the plate below could raise itself up to as near the matching plate of the car as possible. The smaller the gap, the more efficient it would be. This is not a complicated thing to design and could withstand use and abuse. Or simply install the base plate already raised a bit so that there is the smallest gap possible for a fully loaded car.

          • Good to see you in problem-solving mode.

            A raised sender with the receiver able to move up/down a bit might make for zero gap. There would need to be some standardization between models to make this work. Or at least cars that auto-positioned themselves.

  • Just curious, in comparison what is the efficiency of the currently advertised wireless charger for the Volt.
    Thank you.

  • Any company can bluff by taking about what is “feasible”.
    Especially a company looking to get recognition on the backs of Tesla’s credibility.

  • This is apparently good news for the development of electric roads, I would think.

  • No reason to think this is ever going to make it to market. It sounds too fishy to me; “possible” is the call of desperate technicians who badly need more funding (and often more lab work). If it were really “ready for prime time” parts would be on hundreds of cars and the other parts placed at charging stations all over the country.
    It’s not even as ready for prime time as H2 FCV’s. Sheesh.
    That said, I hope this works as well. But I’d need to know whether there was a lot of wasted electricity as with all the other forms of wireless induction charging. If there is I’m sort of leaning towards thinking that that form of charging should be banned.
    I just find most waste distasteful. For my phone and toothbrush inductive charging sounds great, sure, but not on millions of EV’s.

    • Had you read the comments prior to posting you would see that Plugless began selling wireless charging a year ago.

      Now let’s look at how much electricity would be wasted. The average daily drive is under 35 miles. At 0.3 kWh/mile that’s 10.5 kWh/day. A 7% loss would be 0.7 kWh per car.

      Seven-tenths of a kWh is energy lost, but not a huge amount. If the electricity comes from renewable sources there’s no great loss for humanity.

      If wireless charging gets more people into EVs and into them sooner that’s a great gain for humanity.

      • Yes Bob 7 % “waste” (inefficiency) is a lot less that the overall efficiency of FF, about 50 %!

        • Try 80% for cars and light trucks. Perhaps ~50% for the most efficient large diesel engines.

      • I’m not sure you are talking about the same thing. Is Momentum Dynamics the exact same company and technology as this Plugless you cite? I didn’t read one word about Plugless, you are right about that.

        • PLUGLESS L2 System manufactured by Evatran

          ” I do think 7% waste is unjustifiable”

          One needs to be careful not to let the perfect get in the way of the good enough.

          There’s a significant electricity input in extracting, refining and distributing fuel for ICEVs. If the extra convenience of not having to plug in moves some people off petroleum and to electricity then there is a significant net gain.

          • “One needs to be careful not to let the perfect get in the way of the good enough”
            Yes, but you are forgetting the unbridled joy of ideological purity.

          • Haha. That’s great.

          • I don’t believe it is good enough. People aren’t expecting perfection. The difficulty of plugging in falls by the wayside on the first or second day. After that everyone loves being full charge everyday.
            The issue now with supposedly flat interest and minor market share are simply due to a lack of affordable and long range EV’s. When affordable cars start selling in the 10,000’s in 2017 there will be enough to start the inevitable shift. I don’t see where people are going to be willing to deal with the issue surrounding parking properly to align with immoveable hardware just to get rid of the plug. Especially if it is going to throw away ~7% of electricity every mile they drive.
            I may be wrong of course but there isn’t a lot of reliable evidence that wireless charging is necessary to quicken the adoption of EV’s. I’d like to focus on improving things that ARE necessary to getting more folks into EV’s.
            Now, I’m not trying to say that you or others should stop. I’m saying that I don’t believe people will adopt a wireless charging system that wastes 7% much more than they would adopt EV’s that require plugs. Any absolutely independent studies?

          • No. I’m just looking at what I know about human behavior. And I’m pretty sure that there are some who will avoid moving to an EV because they have to deal with wires and plugs and stuff.

            They’re the kind who routinely pay extra to have someone put gas in their car rather than pump it themself and save some bucks. Or pay someone to hose off their solar panels.

          • I hear you, there are those folks out there. Maybe I’m too optimistic and project how I enjoy the ease of plugging in/unplugging on to the larger population. That’s one reason I’d like to know if there’s been any studies done yet on the subject. (And it’s why I like the Tesla snake-robot charger that plugs itself in.) I don’t have an issue with the technology in and of itself as I do with H2 FCV’s for example.
            Still, on a personal level I am not sure I’d accept a 7% loss on my refueling at home.

      • I’m highly sceptical that there are a lot of people who would buy an electric car if only they could spend an additional $1500+ so they wouldn’t have to plug it in.

        I’m highly confident that there are a lot more people willing to buy an EV whether or not they have to plug it in, and also willing to pay $1500+ for even greater convenience.

        Wireless cell phone chargers are growing quickly in numbers. Yet not having wireless charging certainly didn’t stop almost everyone from owning cell phones.

        So in the end, we’ll get a flood of people who would have bought an EV either way buying a wireless charger and wasting 7-11% of the electricity. If we were 100% renewable powered, that’s no problem. But we’re only at 14.3% renewable power in the first half of 2014 (which includes biomass which is not always sustainably harvested). We can’t afford a 7-11% waste of power when we’re already moving too slowly at getting off fossil fuel.

        And why are we considering wasting this power? FOR CONVENIENCE?! When will humanity wake up and make the tiniest of sacrifices to save their planet? Plug the damn car in.

        • Everyone is entitled to an opinion.

  • I can see construction of the charging bay being no more difficult than something like the rail system in an existing car wash. What would be the price for this system?

  • I’m not clear on the large scale application of this wireless charging. Home community power feeds are typically one phase at 240 volts so this could not be used in your garage and besides you do not need such fast charging at home. This leaves the application to businesses with the required utility power supply. I can envision a truck of some type such as a garbage truck with a predictable route and a center of operation or a city bus where this technology would be useful.

  • We achieved 94.6% @ 120kW, with a 200mm air gap. You don’t want plate to plate latching as any mechanical component brings risk of malfunction and injury (people or pets getting caught ).

    Cannot completely avoid thermal losses – cabling, connectors, even the most advanced rectifiers.

    • Who’s “we”? More to share?

      I’m interested in hearing about practical use for curbside charging. How would one deal with snow clearing, repaving?

      • I’m working for a Swiss startup company, Mobisystems, which brings to market technology developed by the Swiss EPFL (our MIT).

        There are two parts in the ground station, one roadside cabinet with the power electronics, and a transfer coil buried in the asphalt or concrete (could be placed on top of the asphalt if in a hurry, but not so good for snowploughs). Coil is fairly robust, in a fiberglass and resin compound.

        A typical electric public transport bus needs about 3kWh per mile, not counting climate control. The bus recovers a large part of the kinetic energy through regenerative braking. The goal is to compensate friction losses and climate control. Path and weather analysis determines how many charging points, more often on uphill sections, less on downhill, summer and winter temperatures, etc.

        If repaving the road was expected to damage the coil, this component can be replaced.

        • Thanks. I don’t find anything on the web about your company. (I found a page link but the page is missing.)

          Can you tell us how far along you might be? Running any test routes with buses or getting close to doing so?

          • We are about to integrate with a bus manufacturer in a few weeks.
            We have a smaller deployment at Geneva Airport, Tarmac-side on a light electric passenger vehicle which has been running for about a year, so we have some customer feedback and reliability data. Same control card, different power modules of course.

          • Let us know when you’ve got some performance data and we can do an article.

            Best of luck…

          • Thanks!

            I will create the website when I have pictures and videos worth showing. Although our prototypes on a wood table work fine, they aren’t really presentable yet. For the time being we focus on function more than form.

          • In your “spare” time you might be thinking about a wireless system that could be installed for a full block of curbside parking spaces.

            Perhaps the central controller sits in a box near the corner and services two (right angle) streets of parking.

            Card insert operation at the corner? Wireless communication between car and charger system?

            A system that could be quickly installed in/under a city street to create lots of EV charging opportunities.

            And a question. Do you get lower efficiencies when the sender is located under pavement? Able to reveal any numbers?

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