There are basically two things that depleted EV batteries can be used for. They can be broken down so their raw materials can be used to make new battery cells, or they can be pressed into service for grid-scale energy storage. An EV battery has two critical parameters — energy and power. Energy is what keeps a car rolling along at 65 mph on the highway. Power is what allows it to sprint ahead of the guy in the Lamborghini in the next lane when then light turns green. Older batteries may still store plenty of energy but can’t deliver enough power for rapid acceleration.
New research from MIT finds that re-purposing EV batteries for grid-scale energy storage duties can be profitable for all parties, provided some basic assumptions are met. First, the batteries should have a minimum of 80% capacity remaining. Using a hypothetical 2.5 megawatt solar farm located in California as a model, the researchers found adding a new lithium-ion storage battery would cost more than using re-purposed EV batteries, provided the used batteries cost 60% or less of their original value.
Local regulations can play a crucial role in value calculations. Some local rules allow the price of storage systems to be included in the overall cost of a new renewable energy supply for rate-setting purposes, while others do not. “A lot of states are really starting to see the benefit that storage can provide,” says post-doctoral candidate Ian Mathews. “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.”
“There are many issues on a technical level,” Matthews adds. “How do you screen batteries when you take them out of 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?”
The study also examined how to get the maximum benefit from used batteries. “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,” Mathews says. “When we looked at that, it just didn’t make sense at all.” Instead, maximizing the battery lifetime provides the best economic returns. The researchers examined scenarios that kept the batteries in energy storage service until they had only 60% of their original capacity remaining and found they could be expected to last for 10 years or more in such use.
Predicting battery longevity is the subject of much research, Matthews 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. We think this could be a great application for machine learning methods, trying to figure out the kind of intelligent methods and predictive analytics that adjust those control policies over the life of the project.” Mathews says some EV companies are already designing their battery packs specifically to make this end-of-life repurposing as easy as possible. Rivian, founded by an MIT alumnus, is one company that is doing so.
The economics of second-use batteries are sound, Matthews says, but for the idea to get traction, a lot of people in a lot of different fields need to start talking to each other. “There’s a lot of stakeholders who would need to be involved in this. You need to have your EV manufacturer, your lithium ion battery manufacturer, your solar project developer, the power electronics guys.” The intent of the study was to get the conversation started.
There will be many more second use batteries available in coming years. Now is the time to start planning how to put them to use in the most efficient and cost effective way.