Used EV batteries may provide cheaper options for grid-scale solar storage

A US study examining the potential for used electric vehicle batteries to have a valuable and value-giving second life as grid-scale solar energy storage systems has returned promising results, including extending the ex-car batteries’ lives by a further decade.

The MIT study, published in the Applied Energy journal, compares the economics of building a 2.5MW solar farm in California without batteries; building the same solar farm with new lithium-ion battery storage; and building it with a battery system made of repurposed EV batteries.

The researchers found that the example using new batteries would not deliver a reasonable net return on investment, but that a properly managed system of used EV batteries – working at roughly 80% of their original capacity – would offer a good, profitable investment.

But it’s not as straight-forward as all that. First, the cost of the used batteries would have to fall to less than 60% of their original price.

Second, there are a number of technical questions to answer, such as how to screen the used batteries and determine they are good enough to re-use, and how to combine batteries from different cars in a way that they will work well together and won’t drag each other down.

Finally, there would need to be enough value left in the used EV batteries to justify the cost of sourcing them, collecting them, checking them over, and repackaging them into a new application?

As far as the research team’s modeled case has found, the answer on the value proposition has so far been “a solid yes” – but only by using the storage system in a way that would maximise the batteries’ lifetime.

The study 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.

MIT post-doc researcher Ian Mathews said these findings challenged some earlier assumptions that running batteries at maximum capacity initially would provide the most value.

“I’ve talked to people who’ve said the best thing to do is just work your battery really hard, and front load all your revenue,” he said. “When we looked at that, it just didn’t make sense at all.”

Mathews also noted that the study had initially made the conservative assumption that the used batteries would be retired from their solar-farm backup role once they had declined down to 70% of their rated capacity (from the initial 80%, when they were retired from EV use).

But the research appears to have shown that continuing to operate down to 60% of capacity, and even lower, could also prove to be safe and worthwhile – although Mathews notes longer-term pilot studies will be required to determine that.

“That’s a whole area of research in itself,” he says, “because the typical battery has multiple degradation pathways. Trying to figure out what happens when you move into this more rapid degradation phase, it’s an active area of research.

“So, you might actually adapt your control algorithms over the lifetime of the project, to just really push that out as far as possible,” he said.

The study also noted that the economics of used battery applications could vary widely depending on local regulatory and rate-setting structures.

“A lot of (US) states are really starting to see the benefit that storage can provide,” Mathews said. “And this just shows that they should have an allowance that somehow incorporates second-life batteries in those regulations. That could be favorable for them.”

Mathews also notes that some EV companies, including the MIT-alumnus-founded Rivian, are already designing their battery packs to make this sort of end-of-life repurposing as easy as possible.

“The point that I made in the paper was that technically, economically, … this could work.”

From this point, however, a lot of stakeholders would need to get involved up and down the EV and battery chain to really make it work.

The intent, he says, “was to say, ‘Hey, you guys should actually sit down and really look at this, because we think it could really work.’”

The study was carried out by six current and former MIT researchers, including Mathews and professor of mechanical engineering Tonio Buonassisi, who is head of the Photovoltaics Research Laboratory.

The study team included postdocs Bolum Xu and Wei He, MBA student Vanessa Barreto, and research scientist Ian Marius Peters. The work was supported by the European Union’s Horizon 2020 research program, the DoE-NSF ERF for Quantum Sustainable Solar Technologies (QESST) and the Singapore National Research Foundation through the Singapore-MIT Alliance for Research and Technology (SMART).

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