Giving EV batteries a second life

An MIT study shows that EV batteries could have a useful and profitable second life as backup storage for grid-scale solar PV installations, where they could perform for more than a decade in this less demanding role. This image shows a ‘cut-away’ view of a lithium-ion battery over a background of cars and solar panels. (Source: MIT News)

As the demand for electric vehicles (EV) increases and gains popularity, one challenge is becoming more prevalent: used batteries. Until now, batteries that have prematurely failed or surpassed their lifespan have had little use. However, a team of researchers at the Massachusetts Institute of Technology (MIT) conducted a study to reveal that batteries from any of these categories could still offer a profitable second life.

They’ve found that such batteries can be used as backup storage for grid-scale solar photovoltaic (PV) installations In such cases, the lifespan of these batteries can extend over a decade.

To this end a study, published in the journal Applied Energy, was conducted by a team of six current and former MIT researchers, including a postdoc Ian Mathews and Tonio Buonassisi — a mechanical engineering professor and head of the Photovoltaics Research Laboratory. A hypothetical grid-scale solar farm was used as a test case in California and they also considered the economics given several different scenarios.

As a test case, the researchers examined a hypothetical grid-scale solar farm in California. They studied the economics of several scenarios, which include building:

A 2.5-MW solar farm alone
Using the same array along with a new lithium-ion battery storage system
Using it with a battery array made of re-purposed 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).

The researchers found that the new battery installation would not provide a reasonable net return on investment, but that 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 challenges
The biggest challenge, according to the team, would be to implement this project on a large scale.

“There are many issues on a technical level,” explained Mathews. “How do you screen batteries when you take then out to the car to make sure they’re good enough to reuse? How do you pack together batteries from different cars in a way that you know that they’ll work well together, and you won’t have one battery that’s much poorer than the others and will drag the performance of the system down?”

Economics is a different matter altogether. For example, How can one be sure if these batteries are left with enough value to justify their costing? The test case under the typical conditions in California turned out to be a success for the team. They found that the batteries could achieve their maximum lifetime and values when used under gentle charging and discharging cycles. The batteries gave optimum performance between 15 to 65 percent charging.

This finding questioned the common assumption that running the batteries at maximum capacity initially provides the maximum 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. When we looked at that it just didn’t make sense at all. It was clear from the analysis that maximizing the lifetime of the batteries would provide the best returns,” said Mathews.

How long will they last?
How long the batteries will last in their second application is still unclear. A conservative assumption dictated that the batteries would be relieved from operations as they reached below 70 percent of their rated capacity. But Mathews says that it might be worthwhile to use these batteries even when their working capacity goes down to 60 percent or below of their original capacity. But longer-term pilot studies will be required to confirm that.