Researchers at the SLAC-Stanford Battery Center have found that the batteries of electric vehicles subjected to the typical use of real-world drivers, such as heavy traffic, long highway trips, short city trips, and mostly being parked, may last roughly a third longer than previously predicted. This implies that a typical EV owner may be able to buy a new vehicle or avoid replacing the costly battery pack for several years. The study was published in the journal Nature Energy.
From left, Simona Onori, Devi Ganapathi, Alexis Geslin, Le Xu, and William Chueh in the SLAC-Stanford Battery Center. | Jim Gensheimer/SLAC National Accelerator Laboratory
Battery engineers and scientists have almost always used a steady rate of discharge and recharging in lab settings to test the cycle lives of new battery designs. To quickly determine whether a new design is good for life expectancy, among other qualities, they repeat this cycle numerous times.
This is not a reliable method of predicting the life expectancy of EV batteries, particularly for those who own EVs for daily commuting. Batteries still make up nearly a third of the cost of a new EV despite a 90% price drop over the previous 15 years. Thus, present and future EV commuters may be pleased to hear that there are many more miles ahead.
We have not been testing EV batteries the right way. 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.
Simona Onori, Senior Author and Associate Professor, Stanford University
A Pleasant Surprise
Based on actual driving data, the researchers created four kinds of EV discharge profiles, ranging from dynamic to conventional constant discharge. The research team tested 92 commercial lithium-ion batteries across various discharge profiles for over two years.
Ultimately, EV life expectancy increased proportionately to how accurately the profiles represented driving behavior.
According to the study, some factors contribute to the surprising longevity. A machine learning algorithm trained on all of the team's data revealed the effects of dynamic discharge profiles on battery degradation.
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.
Simona Onori, Associate Professor, Stanford University
For instance, the study found a link between slower degradation and abrupt, brief EV accelerations. This went against the long-held beliefs of battery researchers, including the team conducting this study, that EV batteries suffer from acceleration peaks.
Forcefully pressing the pedal with the feet does not accelerate aging. According to Alexis Geslin, one of the study's three lead authors and a doctoral candidate in computer science and materials science and engineering at Stanford's School of Engineering, it actually slows it down.
Two Ways to Age
The research team also sought to distinguish between battery aging, which occurs naturally over time, and battery aging, brought on by frequent charge-discharge cycles. If the home batteries still function, they would not be as good as when first purchased because they have been sitting unused in a drawer for years.
We battery engineers have assumed that cycle aging is much more important than time-induced aging. That is mostly true for commercial EVs like buses and delivery vans that are almost always either in use or being recharged. 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.
Alexis Geslin, Study Co-Lead Author, Stanford University
At least for the commercial battery they tested, the study finds a sweet spot for the average discharge rate that balances time and cycle aging. Fortunately, that window falls within consumers' range of practical EV driving.
Automakers could update their EV battery management software to benefit from the new research and extend battery life in practical situations.
Looking Ahead
“Going forward, evaluating new battery chemistries and designs with realistic demand profiles will be really important. Researchers can now revisit presumed aging mechanisms at the chemistry, materials, and cell levels to deepen their understanding. This will facilitate the development of advanced control algorithms that optimize the use of existing commercial battery architectures,” said Energy Science and Engineering Postdoctoral Scholar Le Xu.
The study suggests that the implications go beyond batteries. The concepts could be used by researchers and engineers in other energy storage applications, as well as in other physical sciences devices and materials where aging is important, such as solar cells, plastics, glasses, and certain biomaterials used in implants.
“This work highlights the power of integrating multiple areas of expertise from materials science, control, and modeling to machine learning to advance innovation,” Onori said.
Journal Reference:
Geslin, A., et al. (2024) Dynamic cycling enhances battery lifetime. Nature Energy. doi.org/10.1038/s41560-024-01675-8.