Friday, December 13, 2013
As winter weather arrives, electric car owners are worrying about what the cold will do to the range of their vehicles. Message threads with titles like “Winter driving warning” and “Another way to stay toasty on long trips without running heat” are showing up on online customer forums run by Tesla Motors, which sells many of its cars in particularly cold places such as Norway.
Cold weather presents two main challenges for electric vehicles: cold air limits battery performance, and running the heater drains the battery. As temperatures go below freezing, some drivers accustomed to traveling 250 miles on a single charge have seen their car’s range drop to 180 miles. Drivers in extreme climates might see the range decrease even more. That might force drivers to choose cars with bigger batteries than they would need in the summer, adding $10,000 or more to the cost of the cars.
There are some measures drivers can take to improve an EV’s range. But with existing batteries and heaters, some loss of range is inevitable. Researchers are working on technological fixes that won’t be ready for years.
Cold temperatures primarily affect how quickly the energy can be taken out of the battery or put back in—that is, how much power it can deliver for acceleration, and how quickly it can be recharged. To compensate for this, automakers sometimes use an electric heater to warm up the battery. If that heater is powered by the battery itself, it uses energy that otherwise would go to propelling the car. Nissan’s Leaf is one such car.
Tesla takes a different approach. Once you start driving, heat generated by the motor is used to heat up the battery. This approach is more efficient, since it uses waste heat rather than electricity. But it takes a while to work because the motor doesn’t produce much heat. As a result, it might take several minutes before the battery is warm enough to provide full acceleration.
A cold battery also limits regenerative braking, which uses the momentum of the car to charge the battery and help increase its driving range. Regenerative braking pumps large amounts of energy into a battery; since this can damage the battery at cold temperatures, the Model S doesn’t use regenerative braking until the battery warms up. (To avoid these limits on power and regenerative braking, Tesla’s chief technical officer, JB Straubel, recommends setting the timer on the car’s battery charger so that it finishes charging right before you start driving, since the act of charging the battery also warms it up.)
One way researchers might eventually improve cold weather performance is to increase the conductivity of the electrolyte in the battery, says Jeff Dahn, a professor of physics and chemistry at Dalhousie University. The challenge is that highly conductive electrolytes available now might not last the lifetime of a car. Decreasing the size of the particles that make up the electrode materials can also help, by reducing the distance that ions have to travel through solid material. But smaller particles can also be more chemically reactive, so battery makers need to take steps to ensure that such batteries are safe.
The biggest reduction in cold weather driving range comes from using the heater.
Gasoline engines are inefficient, so they produce large amounts of waste heat that is used to heat the passenger cabin. But electric motors don’t produce much heat because they are very efficient. That’s why electric vehicles typically use electric heaters to keep passengers warm.
Preheating the car while it’s plugged in (which can be done via a smartphone app) can reduce the drain on the battery. But that might not be an option on road trips when someone might need to park overnight at a hotel. Cars like the Leaf and Model S have electric seat heaters, which can make passengers feel warm without spending a lot of energy heating the air in the cabin. Nissan is starting to offer a heater that uses 20 percent less energy, improving cold weather range.
For larger energy savings, the U.S. Department of Energy is funding the development of thermal storage materials that can be heated while the car is plugged in, then deliver heat for the duration of a drive (see “Novel Heating System Could Improve Electric Car’s Range”). But Straubel says those aren’t close to being ready for commercial use. More promising in the near term is better insulation and the use of coatings on the windows that help trap heat. “There isn’t much space inside a car. In principle, it shouldn’t take too much energy to heat it,” he says.
For now, drivers are looking for creative ways to cope with less heat, especially on long trips. On the Tesla forum, one Model S owner recommends buying heated jackets and gloves designed for wearing on motorcycles. Dahn says the solution is “snowmobile suits.”
In the long term, for electric vehicles to become mainstream, the best solution may be to increase the number of places where they can be quickly recharged (see “How Tesla Is Driving Electric Car Innovation” and “Forget Battery Swapping: Tesla Aims to Charge in Five Minutes”). That way, no one would have to worry about being stranded in a cold snap.