PALO ALTO, CA—Electric vehicle batteries may last up to 40 percent longer than expected, according to a new study conducted by the SLAC-Stanford Battery Center. Real-world stop-and-go driving actually benefits EV batteries more than the steady use simulated in many laboratory tests of new battery designs. In fact, batteries subjected to heavy traffic, long highway trips and short local trips could last about one-third longer than researchers have generally forecast.

Traditionally, engineers have tested the cycle lives of new battery designs in labs using a constant rate of discharge followed by recharging. They repeat this cycle rapidly many times to learn quickly if a new design is good or not for life expectancy, among other qualities.

“We have not been testing EV batteries the right way,” says Simona Onori, Ph.D., an associate professor of energy science and engineering at Stanford University. “To our surprise, real driving with frequent acceleration, braking that charges the batteries a bit, stopping to pop into a store and letting the batteries rest for hours at a time, helps batteries last longer than we had thought based on industry standard lab tests.”

Onori and her colleagues designed four types of EV discharge profiles, from standard constant discharge to dynamic discharging based on real driving data. They tested 92 commercial lithium-ion batteries for more than two years across various discharge profiles. In the end, the more realistically the profiles reflected actual driving behavior, the higher EV life expectancy climbed.

According to Onori, several factors contribute to this unexpected longevity. For example, there’s a correlation between sharp, short EV accelerations and slower degradation. This is contrary to long-held assumptions of battery engineers that acceleration peaks are bad for EV batteries.

“Pressing the pedal with your foot hard does not speed up aging,” explains Onori. “If anything, it slows it down.”