Published on August 24th, 2015 | by James Ayre0
Wireless EV Charging As Fast & Efficient As A Tesla Supercharger?
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.”
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%.
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.”
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