EV Batteries May Last a Whole Lot Longer Than Consumers Think

WASHINGTON — The methods currently used to predict the lifecycles of electric vehicle batteries may be just fine for laboratory applications, but they’re apparently dramatically underestimating how long these batteries last when subject to normal use by real-world drivers, a Stanford University report suggests.

While the price of batteries has dramatically decreased over the past 15 years, batteries still account for almost a third of the price of a new electric vehicle.

But a new study from researchers at the SLAC-Stanford Battery Center, a joint center between Stanford University’s Precourt Institute for Energy and SLAC National Accelerator Laboratory, says EV drivers may still be getting more value than they think at the time of purchase.

In a study published in the Dec. 9 issue of the journal Nature Energy, the researchers say EV batteries could last as much as a third longer than is generally predicted.

The reason for the disparity is that the standard test for battery life simply isn’t a very good way of predicting battery life expectancy.

The norm in the industry, they say, is for scientists and engineers to test battery designs by using a constant rate of discharge followed by recharging.

They then repeat this cycle many times in rapid succession so they can quickly decide whether a new design is worth pursuing.

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

“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 said.

While battery prices have plummeted about 90% over the past 15 years, batteries still account for almost a third of the price of a new EV. So, current and future EV commuters may be happy to learn that many extra miles await them.

To prove their thesis, Onori and her team looked at four different kinds of EV discharge profiles ranging from the standard constant discharge test to “dynamic” discharging based on real-life conditions like being stuck in traffic, making quick trips to the grocery store and, mostly, being parked.

Over the course of the study, the researchers put 92 commercial lithium-ion batteries through their paces for more than two years across the discharge profiles. 

In the end, the more realistically the discharge profiles reflected actual driving behavior, the higher EV life expectancy climbed.

The researchers also found that a long held assumption among battery engineers — that cycle aging is more important than time-induced aging — is not always true.

When it comes to most consumers, who use their electric vehicles to get to work, pick up their kids, etc., but mostly are not driving or even charging them, time is the predominant cause of aging as opposed to cycling.

That outcome is dramatically different from batteries used in a commercial or public setting — like buses or delivery vans — which are almost always in use.

The researchers hope their findings will ultimately inspire vehicle manufacturers to update their EV battery management software to maximize battery longevity under real-world conditions.

Dan can be reached at [email protected] and @DanMcCue