By Amy Barzdukas
When we talk to people about wireless charging for EVs, one of the first things we have to convince them about is that wireless charging is real. With EVs now shipping across Asia with wireless charging installed – and lots of videos demonstrating them (see the Hyundai Genesis GV60, or the FAW HongQi E-HS9 for example) – and with our own announcement about the WiTricity Halo™ wireless charging upgrades to come, “realness” isn’t really in question. The myth we do have to address, however, is the deep-seated suspicion about the efficiency of wireless charging. How does it compare to plug-in charging?
Let’s start by establishing the efficiency of plug-in charging. (For more about charging, take a look at our blog here.)
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 for increased efficiency and decreased system size. You’ll also see that the inverter stage involves a change in the frequency, which allows for better controllability of the current.
In the plug-in AC EV charging scenario, much of the heavy lifting is done on the vehicle through the On-Board Charger (OBC) that includes the Rectifier PFC, Inverter, Transformer, and Rectifier – all of which are needed to make 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.)
But with wireless charging, the vehicle doesn’t need an On-Board Charger (OBC), so this helps to reduce the charging complexity on the vehicle. Some of those same switch-mode power operations happen in the wall box as opposed to in the vehicle:
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.