Why you might want a heat pump in your electric car

Source: By Shannon Osaka, Washington Post • Posted: Monday, January 9, 2023

Electric cars function well in subzero temperatures — but some do better than others

An electric car is charged in Montreal. (Marc Bruxelle/Shutterstock)

Last January, just days after the start of the new year, a sudden snowstorm left hundreds of drivers stranded on a 40-mile stretch of Interstate 95 in Virginia. The commuters — which included Sen. Tim Kaine (D-Va.) — were stuck for more than 24 hours, turning their engines on and off to keep warm, layering the clothes they had, and even searching for sustenance from a nearby bread truck.

Eventually, after an ordeal that stretched over a chilling night, emergency crews cleared the accidents and freed cars that had frozen to the roadway. Everyone went home. But the incident provoked anxiety among those worried about an all-electric future. What if, they wondered, all the cars had been battery-powered? Would their drivers have made it through with enough battery life to drive away?

As frigid temperatures sweep across the country once again — and motorists from Buffalo to Seattle have contended with snow and ice — electric cars are facing fresh scrutiny. Lithium-ion batteries perform more sluggishly in cold temperatures, cutting into an electric car’s range when temperatures drop close to freezing. (For what it’s worth, gas-powered cars also don’t perform perfectlyin the cold.)

But there’s a fix for electric cars with dwindling range in winter: the humble heat pump.

Two factors account for why electric vehicles have a slightly harder time in cold weather than gas-powered cars. One stems from the simple reality that the massive lithium-ion batteries in electric cars perform best around 70 degrees Fahrenheit. “Cars are like humans,” said Anna Stefanopoulou, a professor of mechanical engineering at the University of Michigan. “They like room temperature.”

At cold temperatures, lithium ions — which flow from the anode of the battery to the cathode of the battery to create an electrical current — move more slowly through the battery and face greater resistance. That causes the battery to function less efficiently.

But the second reason has to do with the human sitting inside. No one wants to sit in a frigid car during the winter. When it’s cold, gas-powered cars can redirect waste heat from the engine into the cabin to warm the driver and passengers. (Gas-powered cars aren’t particularly efficient — when the engine is on, only about 20 percent of the energy produced is actually going to turn the wheels.)

Gas vehicles “produce so much wasted heat,” said Scott Case, the CEO of Recurrent, a data science company that focuses on electric vehicles. “It’s a smart idea from an engineering perspective to make lemonade out of lemons.”

Electric cars, however, are super efficient. According to the Department of Energy, EVs use over 75 percent of their energy to propel the car. That means that there isn’t much waste heat available. Instead, many EVs warm their passengers through electric resistance heating (essentially heating a wire and blowing air over it).

Cabin heating accounts for the lion’s share of what drains the EV battery in cold temperatures. According to a 2019 study from AAA that tested five different EVs in 20-degree-Fahrenheit temperatures, on average the cars lost about 41 percent of their range with the cabin heater on. With the cabin heater off, however, they lost only about 12 percent of their range.

But not all EVs are created equal. According to a study from Recurrent that looked at real-world data from thousands of electric vehicles, EVs can lose anywhere from only 3 percent (the Jaguar I-PACE) to 32 percent (the Chevy Bolt) of their range in subzero temperatures, depending on how the car manages cold weather.

Part of that, according to Case, is whether the car uses inefficient electric resistance heating or a much more efficient electric heat pump. Heat pumps work like air conditioners, moving heat instead of creating it — so they can be three to four times more efficient than other forms of heating.

Recurrent’s data compares cars like the Tesla Model Y — which comes standard with a heat pump — to cars like the Ford Mustang Mach-E, where no models offer a heat pump. According to the study, the Mustang loses almost 30 percent of its range at 20 to 30 degrees Fahrenheit, while the Model Y loses only about 15 percent.

“A car with a heat pump does much better when it drops down to freezing than one without,” Case said.

There are other means to minimize range loss when it’s cold. Many EVs come with seat heaters or even heated steering wheels, which can warm passengers without heating the car’s entire interior. Some cars use the minimal waste heat from the engine to warm the battery and make the charge last longer. EV drivers can also direct their cars to warm up the battery while the car is still plugged in, again extending the range of the vehicle while driving.

Ultimately, experts say, cold weather is not a reason to avoid electric vehicles entirely. Jay Friedland, a policy adviser at the advocacy group Plug in America, points out that the country with the highest number of EVs per capita is Norway, where temperatures frequently hover in the 20- to 30-degree Fahrenheit range.

“If you’re going to be driving in cold weather, you just need to know you’re going to have some range loss, and plan accordingly,” Friedland said.

One thing is certain, however — a stopped EV can keep its occupants warm for long periods of time in a snowstorm pileup. After the Virginia highway debacle, many EV owners, YouTubers and journalists tested their electric cars in freezing temperatures to see what would happen. One journalist for Car and Driver found that, at subfreezing temperatures, his Tesla Model 3 with a resistance heater could keep the cabin warmed to 65 degrees Fahrenheit for a maximum of 45 hours. That number could rise to 50 or 60 hours with a heat pump.

“You don’t have to worry about being stuck in long winter traffic jams,” Case said.