Electric buses struggle in the cold, researchers find

The study is the first to assess and analyze electric buses' performance in the northeastern U.S., with an unprecedented dataset that covers significant distance -- nearly 50,000 miles- at cold temperatures.

Tompkins Consolidated Area Transit (TCAT) in Ithaca faced issues with the manufacturers of the buses, in addition to the buses struggling in Ithaca's hilly terrain and being unreliable, with reduced range, in cold weather.

For the study, researchers analyzed two years of data and quantified the increased energy consumption of the pilot fleet, finding that the batteries on the electric buses consumed 48% more energy in cold weather (between 25 to 32 degrees Fahrenheit) and nearly 27% more in a broader temperature range (10 to 50 degrees Fahrenheit).

"One of the lessons we've learned is that these buses should be designed for the whole country, including states with colder climates," said senior author Max Zhang, professor of engineering. "We've also found that they're different from conventional diesel buses, with different behaviors, which require different strategies to take advantage of this."

The researchers found that half of the increased consumption in cold weather comes from the batteries' need to heat themselves. Batteries in electric vehicles operate at an optimal temperature of around 75 degrees Fahrenheit, and the colder the battery is when the bus starts, the more energy it takes to warm it. The other main culprit is the heating of the bus's cabin. With frequent stops, especially on urban routes in which the doors are opened and closed every few minutes, the batteries must work harder to heat the cabins.

The researchers also found that regenerative braking, whereby the battery recharges by capturing energy during braking, was less efficient in cold weather. They said this is likely because the battery, which is about eight times the size of a standard electric vehicle battery, struggles to maintain an even temperature across its cells.

Short-term strategies to improve the batteries' function include storing the buses indoors when not in use, so the ambient temperature is warmer; charging the batteries when they're still warm; and limiting the length of time the bus doors are open at stops.

On a larger scale, first author and doctoral student Jintao Gu said the research points to the need for greater adjustments in, or assessments of, infrastructure to accommodate electric buses.

"You have to try to optimize the schedule of all of the buses and to consider the capability of your infrastructure -- how many charging stations you have, and if you have your own garage," he said. "You have to train the drivers, the dispatchers and the service workers. I think from an operational and infrastructure perspective, there are a lot of messages here for future transit system planning."