How do you measure “efficiency” when talking about EV charging?
The concept of “efficiency” is often batted around when discussing EV charging, both with plug-in charging and wireless charging. In fact, it’s one of the most frequent questions we get. Search “efficiency” on the WiTricity website and you’ll find it mentioned in no fewer than 80 articles and videos! Part of the reason we keep telling people that wireless EV charging from WiTricity is comparable to the plug is the wealth of misinformation around efficiency and what is actually being measured.
For example, for AC chargers, is it just the efficiency of getting the energy to the vehicle, or does it include the efficiency of the on-board charger (OBC) that takes the AC energy and converts it to DC and feeds it to the battery? (The efficiency of the plug depends on both the plug, itself, and the vehicle.) For wireless charging, is it the coil-toil efficiency or grid-to-battery efficiency? Some may even be measuring DC to DC or AC to AC but how are we to know? The issue is that each of these measurements is very different as each is measuring efficiency of a specific path within the charging process. That’s why it’s important that, when talking about efficiency, we are comparing apples to apples. It’s also important to ensure that ambient temperature is the same when measuring one device against another since on-board charger efficiency decreases with higher power and cooling requests.
Measuring efficiency of the entire charging process
When talking about EV wireless charging, the only measurement that makes sense is grid-to-battery. Why? Because it’s measuring efficiency of the entire charging process. And that’s what matters in the real world. Anything less is only measuring one part of the entire process. It doesn’t make sense to tout efficiency of one piece of the charging pie when another piece may not be efficient at all.
Sometimes in R&D or lab settings, “efficiency” ratings greater than 95% are cited. One would do well to look a little deeper at what is being measured. Even with wireless charging, if the numbers seem too good to be true, it may be a measure of just a portion of the system. For example, the coil-to-coil efficiency may be what is being discussed. It is true that the coil-to-coil transfer of energy via magnetic resonance is generally the most efficient part of a wireless EV charging system.
When comparing wireless charging to plug-in charging, it’s important to measure from AC input to DC output going to the battery. For AC plug-in chargers this includes the efficiency of the OBC. It’s also important to remember that 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 and the vehicle you own.
Similarly, WiTricity’s wireless EV charging operates within a band of efficiency that is equivalent to a Level 2 plug-in charger. In addition, you get the added efficiency of not having to spend time plugging and unplugging the vehicle.
If you’d like a deeper dive into efficiency measurement, check out our blog post here.