An electric vehicle battery doesn’t stop working when it leaves the car. It simply stops being good enough for a car. That distinction explains second-life batteries. It also explains why the idea that they perform poorly is more myth than fact.
What is a second-life battery?
Car makers typically retire an EV battery once its capacity drops to around 70 to 80 percent of its original value. At that point, it can no longer deliver the range or power a car needs. But 70 to 80 percent of a battery’s original capacity still holds a lot of usable energy. A car simply can’t make good use of it anymore. That’s exactly why these batteries move on to a second job instead of a landfill.
Why a “retired” battery still has plenty left to give
Think about your own phone battery. After a couple of years, your phone doesn’t make it through a full day anymore, so you replace the battery. That old battery isn’t dead. It’s just no longer good enough for daily phone use. An EV battery follows the same logic, just at a much larger scale. A battery in a home, a building, or a section of the electricity grid doesn’t need sudden bursts of power for acceleration. It needs to store energy steadily and release it steadily. That’s a far gentler job than powering a car. It’s precisely the kind of work a battery at 70 to 80 percent capacity still handles well.
Where does the “poor performance” myth come from?
Partly from an understandable question: if a battery wasn’t good enough for a car, how good can it really be for anything else? The answer lies in how differently the two jobs treat a battery. A car battery endures extreme temperature swings, rapid charge and discharge cycles, and years of demanding daily use. A stationary battery faces a gentler, more predictable workload. It might store solar power during the day and release it in the evening, for example. Retiring from one demanding job doesn’t mean a battery has nothing left to offer in a calmer one.
Are second-life batteries actually safe?
This deserves an honest answer, not just a reassuring one. Used batteries need proper testing before reuse, because their history in a car isn’t always fully known. That’s exactly why testing and safety frameworks exist. UL 1974 is an internationally recognised standard for this. It sets out how repurposed batteries should be sorted, graded, and safety tested before they’re cleared for a second life. It sits alongside standards like IEC 62133-2, which covers safety testing for lithium battery cells more broadly. Researchers have also proposed methodologies for certifying second-life batteries under the EU’s Battery Regulation. That gives repurposing a clear, EU-recognised compliance framework instead of an informal one.
Not every expert agrees on every detail, and that’s worth saying plainly. A UK government-commissioned study found genuine disagreement among specialists. Some believe testing and known history can reliably assure a battery’s safety. Others argue safety can never be fully guaranteed for informally built home systems with unknown battery histories. That debate really centres on the edge cases, self-built systems using batteries of uncertain origin. It’s not about professionally tested and certified second-life systems generally. A battery tested, graded, and certified under a recognised standard is a very different proposition from one of unknown history repurposed without oversight.
Second-life batteries at work, right now
Professionally managed second-life systems already run in places you might not expect. That makes this more than a theoretical discussion. Take the Johan Cruijff Arena in Amsterdam, home of football club Ajax. Engineers built a 2.8 megawatt-hour energy storage system there using the equivalent of 148 Nissan LEAF battery packs. It stores solar energy from the stadium’s own roof panels. It also supplies backup power during an outage, helping the arena use its own renewable energy more intelligently while easing pressure on the local grid.
Rome’s Fiumicino Airport runs an even larger example. Engineers built a 10 megawatt-hour storage system there from 762 battery packs and modules sourced from several carmakers. It stores solar power from one of Europe’s largest airport solar installations and releases it when needed. The system supports the airport’s push toward net-zero emissions by 2030. Neither of these is a pilot experiment tucked away in a lab. They’re permanent installations doing real, measurable work.
The bigger picture: reuse and recycling work together
None of this means every retired battery is destined for a second life. Repurposing doesn’t come without challenges, either. Technicians need to test and sort batteries first. They often pair them with a properly engineered battery management system, too, before the batteries are safe and reliable for a new role. But the underlying premise doesn’t hold up: a battery no longer good enough for a car isn’t somehow no longer good for anything. With the right testing and standards in place, a battery’s story doesn’t end when it comes out of a car. It just changes direction.
Recycling and second-life reuse aren’t competitors, either. They mark two different points in a battery’s story. Reuse extends the working life of materials already in circulation. Recycling recovers those materials once a battery genuinely reaches the end of the road. Our article on whether battery recycling is actually safe explores that second half of the story in more depth. Understanding how reuse and recycling fit together is part of what a circular battery value chain looks like in practice. That’s exactly the challenge CIRCUBATT was set up to help address. Explore more of our work at circubatt-project.eu, or browse common questions on our FAQ page.
Frequently asked questions
Do second-life batteries perform worse than new batteries?
Not necessarily. They hold less total capacity than when new. But for applications like stationary energy storage, which don’t demand the same power output as driving a car, a battery at 70 to 80 percent capacity performs reliably well.
How long do second-life batteries last in their new role?
It depends on the application and how operators manage the battery. Stationary storage puts far less strain on a battery than automotive use does, which generally supports a longer service life the second time around.
Is it safe to use a repurposed EV battery at home?
Professionally tested and certified systems, built to standards like UL 1974, treat safety as the top priority. Experts remain more divided on informally built home systems that use batteries of unknown history. That’s a different situation entirely.
What’s the difference between second-life use and recycling?
Second-life use extends a battery’s working life in a new, less demanding application. Recycling recovers a battery’s raw materials once it genuinely reaches the end of its usable life. Both play a role in a functioning circular battery value chain.