Why Wireless EV Charging is Just as Efficient as Plug-in EV Charging
By Amy Barzdukas, CMO, WiTricity
With anything new, there are always doubters. We love those who question wireless EV charging because they help make all of us smarter. By learning what they question about wireless charging, we learn to better inform and educate consumers on how the technology works and how it compares to plug-in charging.
If you’ve followed WiTricity, you know we love to bust myths. One of those myths is efficiency. Is wireless charging as efficient as plug-in charging? Can you charge as quickly wirelessly as you can using a cord? Is the amount of energy delivered wirelessly equivalent to the amount of energy delivered via a cord? Yes. Yes. And yes.
With real-world experience charging a variety of vehicles wirelessly – Hyundai Genesis GV60, Tesla Model 3, Ford Mach-E, Ford eTransit, VW ID.4 and ID.5, KG Mobility Torres, and others – we know wireless charging is just as efficient as plug-in charging. But why?
Let’s Start at the Very Beginning (a Very Good Place to Start)
To understand the efficiency of wireless charging, it’s important to first establish the efficiency of plug-in charging. (For more about EV charging, take a look at our blog here.)
First, plug-in charging is not 100% efficient. Energy loss, primarily in the form of heat, occurs every step of the way from grid to battery. What’s more, regardless of the brand, a plug-in EV charger is made of many components, any one of which may be more, or less, efficient than similar components in another charger. So, the “efficiency” of the transfer of energy from the grid all the way to battery encompasses a range; a typical Level 2 home charger operates in the range of about 83-94% efficiency grid-to-battery depending on which one you buy.
Below is a diagram that charts the path from grid to the battery using a Level 2 plug-in charger. You will note that there are various places in the path where the current changes from AC to DC and back; this is the concept that switch-mode power supplies use to increase efficiency and decrease system size. Note the inverter changes the signal from DC to AC and a rectifier does the reverse.
In the plug-in AC EV charging scenario, much of the heavy lifting is done on the vehicle through the On-Board Charger (OBC), which includes the Rectifier PFC, Inverter, Transformer, and Rectifier – all of which are needed to transform the grid power into power that the EV battery can consume. (The unit hanging on the wall is relatively uncomplicated as this video very well explains.)
AC Charging
Wireless Charging
Mind the Gap
But why doesn’t the “gap” between the ground pad and the vehicle create loss? It seems counterintuitive that space wouldn’t introduce inefficiency.
The ground pad and vehicle pad convert the alternating current into the magnetic field that transfers power over the air gap. And, because we use magnetic resonance with specially designed low-loss resonators to transfer power, the loss is very small. In fact, the air gap between the ground and vehicle serves the same safety function as the isolation that occurs for plug-in charging through the isolation transformer (in the OBC between the grid connection and the vehicle). With the highly resonant design of the wireless charger, it’s nearly as efficient as the isolation transformer used for plug-in charging.
Wireless charging operates within a narrow band of efficiency (88-93%) that is equivalent to Level 2 plug-in charging, plus you get the added efficiency of not having to spend time plugging and unplugging the vehicle.
Last but not least … every time you park and charge wirelessly, you’re more likely to be operating in the 20-80% state of charge (SOC) range that the battery likes – and is the most efficient. With plug-in charging, it’s less likely that the 20-80% SOC range will be maintained since drivers tend to forget to plug-in, or don’t bother when they know they have enough battery left for their next journey. In fact, many people plug in once a week, drive all week, and then plug in on the weekend. Not only is this less efficient but it’s harder on the battery.
If you’d like to take a deeper technical dive into magnetic resonance, check out our whitepaper here. And if you want to receive our monthly newsletter, click here.
About WiTricity
WiTricity is the pioneer in wireless charging for electric vehicles, leading the development and implementation of magnetic resonance technology across passenger and commercial vehicles alike. The company’s products are backed by an extensive patent portfolio critical to ratified global EV wireless charging standards including SAE, ISO, and GB. Automakers and Tier 1 suppliers rely on WiTricity to help accelerate the adoption of EVs by eliminating the hassle of plug-in charging and setting the stage for future autonomy. Beyond EVs, WiTricity technology is indispensable to the wireless charging of all products, from consumer electronics to micro-mobility to robotics.