As electric vehicles rapidly grow in popularity, there soon will be a wave of used batteries whose performance is no longer sufficient for vehicles that need reliable acceleration and range. New research shows that these batteries could still have a useful and profitable second life as backup storage for grid-scale solar photovoltaic installations, where they could perform for more than a decade in this less demanding role.

As a test case, researchers examined a hypothetical grid-scale solar farm in California. They studied the economics of several scenarios: building a 2.5-megawatt solar farm alone, building the same array along with a new lithium-ion battery storage system, and building it with a battery array made of repurposed EV batteries that had declined to 80 percent of their original capacity — the point at which they would be considered too weak for continued vehicle use. They found that the new battery installation would not provide a reasonable net return on investment but a properly managed system of used EV batteries could be a good, profitable investment as long as the batteries cost less than 60 percent of their original price.

The study used a semiempirical model of battery degradation, trained using measured data, to predict capacity fade in the lithium-ion batteries under different operating conditions and found that the batteries could achieve maximum lifetimes and value by operating under relatively gentle charging and discharging cycles — never going above 65 percent of full charge or below 15 percent. This finding challenges some earlier assumptions that running the batteries at maximum capacity initially would provide the most value.

The study made a conservative assumption that the batteries would be retired from their solar-farm backup service after they had declined down to 70 percent of their rated capacity, from their initial 80 percent (the point when they were retired from EV use). Continuing to operate down to 60 percent of capacity or even lower might prove to be safe and worthwhile.

The actual economics of such a project could vary widely depending on the local regulatory and rate-setting structure; for example, some local rules allow the cost of storage systems to be included in the overall cost of a new renewable energy supply for rate-setting purposes, while others do not.

For more information, contact Karl-Lydie Jean-Baptiste at This email address is being protected from spambots. You need JavaScript enabled to view it.; 617-253-1682.