The shelf-life of electric vehicle (EV) batteries may be as much as 40 percent greater than previously assumed, a new study has found.

Stanford University scientists uncovered this possibility by changing the way they evaluate the life cycles of such batteries: Instead of conducting customary tests that involve a constant rate of discharge followed by recharge, they assessed the batteries under everyday, stop-and-go conditions.

When evaluating batteries that were subject to normal, real-world use — such as heavy traffic, long highway trips, short city travel and staying parked for a while — the researchers found the batteries could last much longer than previous forecasts indicated.

These findings suggest EV owners may not need to replace their expensive battery packs or buy new cars for several additional years, according to the study, published Monday in Nature Energy.

“We’ve not been testing EV batteries the right way,” said senior author Simona Onori, an associate professor of energy science and engineering at Stanford’s Doerr School of Sustainability, in a statement.

“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,” Onori added.

Battery scientists and engineers typically have tested the cycles of new batteries in laboratories, using a constant rate of discharge, followed by recharge, the authors explained. They then repeat this approach many times to learn if a new design could benefit the battery’s longevity.

Onori and her colleagues determined, however, this is not an ideal approach for predicting the life expectancy of EV batteries — a find of particular importance, since batteries still account for about a third of the price of a new EV.

To draw their conclusions, the researchers designed four kinds of EV discharge profiles, ranging from standard constant discharge to dynamic discharging that mimicked real-driving data.

They then evaluated 92 commercial lithium-ion batteries for more than two years across these profiles. The more realistic the profiles, the higher the EV life expectancy rose, according to the study.

Among the study’s key findings was a correlation between sharp, short EV accelerations and slower degradation — meaning, pressing hard on the pedal does not necessarily speed up aging.

The researchers also identified some key differences between battery aging due to repeat charge-discharge cycles, which occur in commercial fleet operations, and battery aging that simply occurs over time.

Lifecycle aging plays a bigger role in commercial EVs such as buses and delivery vans, as these vehicles “are almost always either in use or being recharged,” said co-lead author Alexis Geslin, a Stanford Ph.D. student in materials science and engineering, in a statement.

“For consumers using their EVs to get to work, pick up their kids, go to the grocery store, but mostly not using them or even charging them, time becomes the predominant cause of aging over cycling,” Geslin said.

Going forward, the researchers said that carmakers could update their EV management software to take advantage of the new findings — helping maximize battery life in real-world settings.

Evaluating new types of battery chemistries and designs that reflect realistic demands will also be important, added co-lead author Le Xu, an energy science and engineering postdoctoral scholar.

“Researchers can now revisit presumed aging mechanisms at the chemistry, materials and cell levels to deepen their understanding,” Xu added.