EV charge rate explained

By Eric Cohen, WiTricity

When you drive an old-fashioned gasoline-powered vehicle, you know your tank’s gallon capacity, as well as your typical mpg in town or on the highway. It is therefore easy to calculate how much gas you will need for a particular trip. 

It is tempting to try to translate that mpg model directly to electric vehicles (EVs), but refueling your EV is a bit more complicated. You’re filling up with Watts (Watt-hrs to be precise!) instead of gas or diesel. We’ve all seen social media posts of EV owners wondering why the public charging station charged for more energy than the EV indicates it received.

Where do all those watts actually go?

First, it’s important to ground on a few facts about EV charging:

1. No EV charging is 100% efficient, not plug-in charging, not wireless charging, not DC Fast Charging (DCFC), not AC home charging. Regardless of how or where you charge, not all the electrical energy pulled from the grid—those watts—will make it into the “fuel tank” (battery). 

2. EV batteries store and produce DC electricity, not AC electricity. DC (direct current) and AC (alternating current) are different types of voltage, or current, used for the conduction and transmission of electrical energy. This is important because EV batteries are charged with DC power. Since Thomas Edison’s time, most electricity transmission to the point of use is AC power. That means somewhere in the process of moving watts from the electric grid to the EV battery, AC power must be converted into DC power. (More info)

3. Every EV has an on-board charger (OBC) that receives AC power from the Level 1 or Level 2 charging cord and converts it to DC power.

4. OBCs are not 100% efficient at converting AC to DC, as anywhere from 5-15% of the power from the grid ends up as heat during the AC to DC conversion process. 

5. DC fast chargers convert AC grid power to DC charging power in the charger itself, not in the vehicle. (That’s one reason the chargers are so expensive, and also why they can charge a battery at greater speed.) They lose approximately 5-10% of the power from the grid to heat during the AC to DC conversion process.

6. Every vehicle has an appetite for energy to monitor and control the charging process and to condition the battery. Not all electric power from the charger goes into the battery storage for driving.

7.  A warm battery performs with greater efficiency than a cold battery, impacting acceleration, driving range, regenerative braking, and the ability to safely accept DC fast charging. 

All of this matters because the speed of charging—regardless of whether the EV is charged wirelessly or through a plug—depends on where the AC to DC conversion occurs, how much power the car WANTS to draw, and WHERE the car wants to send that power. 

Why does battery temperature matter?

We’ve already explained that offloading the conversion of AC to DC to an off-board charger, rather than in the car, is one reason DCFC is faster. But beyond that, if the car is cold, energy may go toward warming the battery rather than filling it. In hot weather, or If the battery is already warm from driving, energy may go toward cooling the battery rather than filling it. (Yes, it is a little bit like Goldilocks.)

Generally, a warm battery provides more power than a cold one. What’s more, a warm battery is particularly important when using a DCFC because the battery temperature should be at least 50 degrees Fahrenheit (10˚ C) to avoid permanent battery damage. In fact, when connected to a DCFC cable, the vehicle will first check the battery temperature and will divert charge power to battery heaters to bring the battery to a suitable temperature to begin fast charging. The same will happen if your vehicle has been housed in a cold garage. 

With all this in mind, it is best to think about charge rate as what is left over for driving usage AFTER your car has charged. And this varies based on type of vehicle, temperature, and other factors. This is why auto companies suggest that you keep the car plugged in as much as possible so you are not using battery power for auxiliary operations such as battery conditioning or cabin preconditioning for comfort. 

As a result, one must remember to plug and unplug even if the battery is well charged. With wireless charging, you can park and walk away—and know that your vehicle will use grid power for its own energy appetite—so your battery is fully charged to satisfy your driving appetite!

As the car commercials used to say, “your mileage may vary.” EV charging rates vary, too.

EVA business member WiTricity, the global industry leader in wireless charging, is powering a sustainable future of mobility that is electric and autonomous. The company recently announced a limited beta of WiTricity Halo™ Charging, a complete end-to-end, and hassle-free charging experience. Just park and charge. All EVA members are encouraged to register interest.