EVs Explained: Charging Losses

John Voelcker
·4 min read
Photo credit: Car and Driver
Photo credit: Car and Driver
  • The amount of energy you add to your EVs battery is not equal to the amount of energy available to move your vehicle down the road.

  • Generally speaking, your EV may use 12 to 15 percent more energy than what you add to your battery. That number could be higher depending on charging conditions.

  • There are a number of reasons for this. Some energy is converted to heat, some is necessary to keep the battery at the right temperature during charging, and some is written off to what's known as "transmission loss."

It's easy to think recharging an EV is like filling your car with gas. If it takes 13.0 gallons of fuel to fill an empty 13.0-gallon tank, then why wouldn't an empty battery with a capacity of 60.0 kilowatt-hours require 60.0 kWh to reach full charge?

Because the battery is more complicated than a simple container. A rough expectation is that your EV may use as much as 12 to 15 percent more energy than what you add back to the battery. Some energy is written off to what's known as “transmission loss,” some is converted to heat, and some is used to keep the battery at the right temperature during charging.

Using the 2021 Tesla Model Y as an example, Tesla’s own data—buried deep in 49 pages of certification documents filed with the EPA—shows it took 87.868 kWh to add 77.702 kWh to the battery of the Long Range version. That’s a 13 percent overage. For the Model Y Performance version, adding 81.052 kWh to the battery required 92.213 kWh, or 14 percent more.

Worth noting: those sessions used 240-volt Level 2 charging equipment, but took batteries from 0 percent to a 100-percent state of charge, which is more challenging for a battery than typical use. Especially as the battery reaches it's maximum capacity, heat can increase, reducing the efficiency of the charge. On the other hand, the tests were performed in a climate-controlled chamber, which, as you'll see, is more efficient than a 115-degree afternoon in a parking in Phoenix, or a sub-freezing winter's night in Michigan.

How can the charging losses be minimized? Higher voltage charging equipment is one way. Our long-term 2019 Tesla Model 3 Long Range Dual Motor test car is currently averaging 95 percent efficiency from a Level 2 Tesla 240-volt wall connector. Staffers charging at home using a typical 120-volt wall outlet saw efficiency from, at best, 85 percent, and it dropped to as little as 60 percent in very cold weather, when charging the battery requires expending significant energy to keep it warm.

Where does the extra energy go? If you’re charging at 120 or 240 volts, the car has to convert the alternating current (AC) provided by the circuit to direct current (DC) that can be used to charge the battery. That conversion produces heat, which is why the power electronics in an EV are usually liquid-cooled. Our experience has been that residential 120-volt current is inherently more “lossy” when charging EVs.

DC fast charging cuts out the AC-to-DC conversion losses and is more efficient still. Our Model 3 is averaging 99-percent efficiency at our most frequented local Supercharger, which operates at 400 volts.

Extreme temperatures are also a drag on charging efficiency. Most high-volume EVs on the market today, the Nissan Leaf aside, have thermally controlled battery packs. That means they actively try to keep their battery temperature within a set range. (A good rule of thumb for batteries is that they’re like humans: happiest around 70 degrees.)

If it’s cold outside, some vehicles use wall or battery current to warm the pack so it can accept charge more quickly. Even so, more energy is used for less recharging in cold weather than in more temperate climes. On the other end of the scale, most EVs will run a cooling circuit to keep battery temperature down when charging in hot weather. Some use fans to circulate air, but most pump liquid coolant through the pack itself.

In either case, EV drivers may hear the noise of fans or pumps deep within the car as it is charging. There are also slight losses from the car staying “awake” and monitoring its own charging process while plugged in.

The good news, though, is that the additional energy doesn’t cost much. If you use 15 percent more energy on top of adding 60 kilowatt-hours to your EV, that’s 9 kWh. At the average U.S. cost for residential electricity ($0.127/kWh), that’s only about $1.14—or the price of one-third to one-half a gallon of gasoline these days. Even extrapolated out to 15,000 miles driven annually, it's less than $100 extra per year.

You Might Also Like