The secret to long-lasting rechargeable batteries may lie in embracing the differences. New modeling of the degradation of lithium-ion cells in a pack shows a way to match charging to each cell’s capacity, allowing EV batteries to handle more charge cycles and prevent failures.
By Adam Hadhazy
Stanford University researchers have developed a new way to make lithium-ion battery packs last longer and suffer less wear and tear from fast charging.
The study, published Nov. 5 in IEEE Transactions on Control Systems Technology, shows how actively controlling the amount of electrical current flowing to each cell in a pack, rather than delivering the charge evenly, can minimize wear. The approach effectively allows each cell to live its best – and longest – life.
According to Stanford professor and senior study author Simona Onori, initial simulations suggest that batteries managed with the new technology could handle at least 20% more charge-discharge cycles, even with frequent rapid charging, which puts additional strain on the battery.
Most previous efforts to extend battery life in electric cars have focused on improving the design, materials and manufacture of individual cells, based on the premise that, like links in a chain, a battery pack is only as good as its weakest cell. The new study begins by understanding that while weak links are inevitable – due to manufacturing flaws and because some cells degrade faster than others when subjected to stresses such as heat – they don’t have to bring down the whole pack. The key is to match charge rates to each cell’s unique capacity to prevent failure.
“If not properly addressed, cell-to-cell heterogeneities can affect a battery pack’s longevity, health and safety and lead to early battery pack malfunction,” said Onori, the assistant professor of energy science at Stanford’s Doerr School of Sustainability. “Our approach balances the energy in each cell of the pack, bringing all cells to the final target state of charge in a balanced manner and improving the longevity of the pack.”
Inspired to build a million mile battery
Part of the impetus for the new research comes from an announcement in 2020 by Tesla, the electric carmaker, that it was working on a “million-mile battery.” This would be a battery that can power a car for 1 million miles or more (with regular charging) before it reaches the point where the electric vehicle battery, like the lithium-ion battery in an old phone or laptop, contains too little charge to be functional .
Such a battery would exceed the typical eight-year or 100,000-mile warranty offered by automakers for electric vehicle batteries. Although battery packs routinely outlast their warranties, consumer confidence in EVs could be boosted if expensive replacement battery packs become even more scarce. A battery that can still hold a charge after thousands of charges could also pave the way for the electrification of long-distance trucks and the introduction of so-called vehicle-to-grid systems, in which EV batteries store and distribute renewable energy would the power grid.
“It was later explained that the million-mile battery concept wasn’t really new chemistry, just a way to run the battery by not using the full charge range,” Onori said. Related research has focused on single lithium-ion cells, which generally do not lose charge capacity as quickly as full battery packs.
Intrigued, Onori and two researchers in her lab – postdoctoral fellow Vahid Azimi and PhD student Anirudh Allam – decided to investigate how inventive management of existing battery types can improve the performance and lifespan of a complete battery pack, which may contain hundreds or thousands of cells.
A high fidelity battery model
As a first step, researchers created a high-fidelity computer model of battery behavior that accurately represents the physical and chemical changes that take place within a battery over its lifetime. Some of these changes unfold in seconds or minutes – others over months or even years.
“To the best of our knowledge, no previous study has used the type of high-fidelity, multi-timescale battery model that we have created,” said Onori, who is director of the Stanford Energy Control Lab.
Ongoing simulations with the model suggested that a modern battery pack can be optimized and controlled by accounting for differences between its individual cells. Onori and colleagues envision their model being used to guide the development of battery management systems in the years to come that can be easily inserted into existing vehicle designs.
Not only electric vehicles benefit from this. Virtually any application that “takes a heavy toll on battery” could be a good candidate for better management informed by the new results, Onori said. An example? Drone-like aircraft with electric vertical takeoff and landing, sometimes called eVTOL, which some entrepreneurs expect to operate as air taxis and others to offer urban air mobility services over the next decade. However, other applications for rechargeable lithium-ion batteries are enticing, including general aviation and large-scale renewable energy storage.
“Lithium-ion batteries have already changed the world in many ways,” Onori said. “It’s important that we get the most out of this transformative technology and its future successors.”
Azimi is now a Research Associate at Gatik, a B2B short-haul logistics company in Mountain View, California. Allam is now a battery researcher at Archer Aviation, an eVOTL aircraft company based in San Jose, California.
This research was supported by LG Chem (now LG Energy Solution).