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Real-world EV data reveal how uneven cell aging shortens battery lifespan
Electric vehicles (EVs), including cars, buses and trains, could significantly reduce the substantial greenhouse gas emissions produced by the transportation sector. These vehicles' performance and reliability over time, however, depend largely on the batteries used to power them.
Most current EVs are powered by packs of lithium-ion battery cells. While battery manufacturers try to ensure that these cells are as similar as possible, they sometimes differ in capacity (i.e., how much electric charge they can hold) and resistance (i.e., the extent to which they oppose the flow of electric current).
These differences between cells in the same pack, also referred to as cell-to-cell inconsistencies, can increase as batteries are repeatedly charged and discharged. This phenomenon can gradually hinder the overall performance of batteries, reducing the distance that EVs can drive before their batteries need to be recharged.
Researchers at Chalmers University of Technology, the Chinese Academy of Sciences, the Beijing Institute of Technology and Zeekr Technology Europe recently analyzed battery data recorded from electric vehicles in real-world operating conditions to further investigate how cell-to-cell inconsistencies affect the performance of EVs over time.
Their findings, published in a paper in Nature Energy, suggest that differences between cells do impair the overall capacity and useful life of the battery packs powering both cars and buses.
"An electric vehicle battery pack contains many cells that must work together, but these cells do not remain identical as they age," Changfu Zou, co-author of the paper, told Tech Xplore. "In a series-connected pack, its performance can ultimately be limited by its weakest cell, much like a chain is limited by its weakest link. This question grew naturally from our group's long-term work on battery management and reconfigurable batteries."
Cell-to-cell inconsistencies have been widely investigated in the past, primarily in laboratory settings. Some energy engineers have also developed strategies designed to balance the performance of different cells in a pack and reconfigurable architectures that allow engineers to alter the electrical connections between cells while they are operating.
While some of these strategies have achieved promising results, the long-term impact of cell-to-cell inconsistencies on the performance of EVs has rarely been explored using data collected from actual vehicles.
"For me, knowing that the problem exists was not enough," Zou said. "We wanted to understand how much battery health, lifetime, power capability and lifetime energy utilization are actually lost over years of driving."
Tracking EV battery data under real-world conditions...Zou and his colleagues analyzed operational data collected from two real-world EV fleets. These included 116 passenger cars powered by NMC batteries and 17 buses powered by LFP batteries.
NMC and LFP are two of the leading lithium-ion battery chemistries on the market today. NMC batteries have cathodes made from nickel, manganese and cobalt, while LFP batteries have cathodes made from lithium iron phosphate.
The operational data they analyzed were collected over more than three years of regular vehicle operation. Some of the vehicles had traveled up to 300,000 kilometers (186,000 miles).
"We selected suitable charging periods to estimate the capacity and internal resistance of individual cells," Zou said. "Capacity tells us how much energy a cell can still store, while resistance is closely related to its ability to deliver power. One of the main challenges was separating actual aging from apparent changes caused by operating conditions such as temperature, current and state of charge."
To compare the cells under the same reference conditions, the researchers also analyzed the data using machine learning models. They introduced six key measures that allowed them to quantify how cell inconsistencies affected the overall performance of a battery pack.
"What surprised me most was not that cell inconsistency matters, but how large its accumulated effect becomes over the life of a vehicle," Zou said. "Across the two fleets, it shortened battery pack lifetime by approximately 18–23% and reduced power capability by around 13–15%. By the time the packs reached retirement, around one-fifth of their potential lifetime energy resources remained unused in the passenger cars, and more than one-quarter remained unused in the buses."
The results of the team's analyses do not mean that 19% or 27% of the batteries' energy was lost on every car and bus journey, respectively. Instead, they suggest that healthier cells still have usable capacity and a substantial lifetime when the weakest cells start limiting the performance of a whole pack.
"We also found that state-of-charge imbalance generally reduced usable charge capacity by less than 2%," Zou said. "The larger problem was that cells aged at different rates. For me, the clearest message is that battery management should not only keep cells equally charged but also try to keep them aging more uniformly."
Guiding the advancement of EV batteries...The results of this study confirm that cell-to-cell inconsistencies can limit the performance and lifetime of two types of lithium-ion batteries used in current EVs. In the future, they could inspire further efforts aimed at developing more uniform and durable rechargeable batteries for EVs.
"Our findings provide quantitative support for improving manufacturing consistency, cell selection and grouping, thermal management, balancing and cell-level monitoring," Zou said. "They could also help determine when more advanced solutions, such as reconfigurable battery systems, are worth their additional cost and complexity."
The researchers hope that their efforts will inspire other research teams to assess the performance of EV batteries in real-world settings. As part of their next studies, they plan to assess the potential of various strategies for mitigating the effects of cell-to-cell inconsistencies.
"For our research group at Chalmers University of Technology, the next step will be to move from measuring the problem to actively mitigating it," Zou added. "One direction I find particularly promising is reconfigurable battery systems. These systems can adjust how individual cells or cell groups are used according to their states and predicted lifetime. We are already working along this path."
Zou and his colleagues recently published another paper in Nature Communications, in which they quantified the extent to which dynamic reconfiguration could extend the life of batteries and reduce their lifetime costs. In addition, they published a perspective article in Joule outlining how AC-native battery architectures could improve cell-level monitoring, energy routing and fault isolation.
---Written for you by our author Ingrid Fadelli, edited by Sadie Harley, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work---
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