It's a familiar event for drivers from Massena, New York, to Manitowoc, Wisconisin: cranking a dead car battery on a frigid winter morning.

So it may come as no surprise that researchers at the Energy Department's Idaho National Laboratory have found that cold temperatures impact the electrochemical reactions within lithium-ion battery cells and onboard battery management systems in electric vehicles.

Their study was published online by Energy Policy journal. In it, researchers looked at data from a fleet of EV taxis in New York City and found that charging times increased as temperatures dropped.

The researchers acknowledge that the drop in charging efficiency when temperatures fall has been known for a long time. But most of the current knowledge comes from experiments with smaller batteries in the lab, not data from large, electric vehicle batteries in real-world conditions. Further, EV manufacturers often provide consumers with only rough estimates of charging times, and they typically do not specify the range of conditions for which those estimates apply.

The Idaho researchers analyzed data from a fleet taxis over roughly 500 Direct Current Fast Charge events.The Idaho researchers analyzed data from a fleet of Nissan Leafs operated as taxis over roughly 500 Direct Current Fast Charge (DCFC) events. Temperatures for the charging events ranged from 15 to 103 degrees Fahrenheit.

They found that charging times increased significantly when the weather got cold. When an EV battery was charged at 77 degrees, a DCFC charger might charge a battery to 80 percent capacity in 30 minutes. But at 32 degrees, the battery’s state of charge was 36 percent less after the same amount of time.

The more the temperature dropped, the longer it took to charge the battery. Under the coldest conditions, the rate of charging was roughly three times slower than at warmer temperatures.

The researchers used the data to create a map showing regions of the country where EV owners might experience longer charging times due to cold temperatures. As expected, high-elevation regions and the northernmost parts of the country were more likely to be identified as places where cold temperatures could most affect charging times.

The predicted state of charge of a battery after 30-minute charge on median-temperature day across the United States. The map assumes that the battery starts at 20 percent capacity and is charged with a 50 kW DCFC system. Source: INLPeople who charge their EVs in a warm garage and who use their EVs for commuting within the range of their battery might not experience much inconvenience. Decreased fuel economy in cold weather is also a well-known phenomenon with gasoline and diesel-powered vehicles.

The impacts of cold on EV battery charging times could be especially important for commercial vehicles that run on schedules, such as buses. In those instances, variable EV charging times could throw off the schedule or require heated spaces for charging.

Likewise, that extra charging time could inconvenience EV drivers on long trips or people who live in apartments and don’t have access to chargers. More affordable EVs with smaller batteries that need to be charged more often might also pose more challenges in the cold.

The findings pose questions not only for EV customers, but also for utilities and charging infrastructure providers. For instance, the location or abundance of charging infrastructure may need to be different in colder climates, and electric utilities might see electricity use vary as the seasons change.

The current study relies on a single type of electric vehicle, the Nissan Leaf, and a single type of charging system, 50 kW DCFC. Further, the data only included charging events less than 60 minutes long.

Funding was provided by the U.S. Department of Energy Vehicle Technologies Office.