Arun Vinayak
Pranav Srivilasan
Batteries wear out as they’re used over time. For an EV using lithium-ion cells, once the battery drops below a certain percentage of its original capacity, you replace it. The exact threshold varies depending on the vehicle and the application, but 70% state of health is a common recommendation.
To many people, it feels like a waste to junk a battery that still has 70% of its capacity though.
One idea that has a lot of takers is that you can reuse these degraded batteries, putting them into applications that don’t need as much performance as a vehicle. This is usually called a “second life” application for the battery, usually as stationary energy storage – i.e. in a battery bank connected to a solar farm or somewhere on the grid.
It’s an attractive idea, but we think it completely falls apart in the real world:
1. Combining packs of different sizes, different chemistries and varying degradation is a nightmare. It’s technically possible, but it requires a huge effort for a sub-par solution.
2. Battery reuse is also increasingly unnecessary, since battery tech is constantly improving (so batteries are way cheaper and last much longer than before) and Li-ion packs are highly recyclable.
We have to look back and understand that the concept of giving retired EV batteries a “second life” came about when batteries were very expensive with a limited lifespan. Extending the usage of these expensive assets in energy storage seemed like a clever financial trick at the time.
None of this is true today. We can have battery packs that last thousands of cycles. And for personal vehicles, which don’t run up a lot of kilometers, a battery should easily last the entire life of the vehicle, possibly over multiple owners.
To be blunt, battery reuse is an idea that makes sense only in Excel sheets. Not in the real world.
In our opinion, the industry and policymakers alike need to focus on building out an ecosystem of battery recycling. Creating a circular economy is going to be extremely important as EV adoption shoots up and eventually battery packs have to be retired.
An unnecessary struggle
The best way to build a battery is to build a symmetric battery.
Large battery packs – whether for vehicles or for storage – have multiple modules or sub-packs, and each module has multiple cells. For grid storage, multiple packs or racks of batteries are combined into big shipping-container-like boxes.
When we say “symmetric”, what we mean is that all the sub-packs and cells should be matching. Ideally the same type of cell, at the same level of health and with the same performance characteristics.
The first and biggest problem with “second life” applications becomes obvious: Different vehicles have different pack sizes, they use different cell chemistries, and they come to a recycling center with different levels of health. One pack might be at 80% health, but another might have degraded to 70%. One pack uses LFP cells, another uses NMC cells, a third uses NCA.
The logistics of taking these widely varying batteries and combining them into a single energy storage system are painful. It can be done, but it becomes highly labour- and process-intensive:
Sort and test every used pack, for safety, electrical performance, thermals.
Tear down the packs to get individual modules.
Test each module to assess the state of health or wear level.
Separate modules based on the chemistry of the cells used.
Combine the right modules together.
Integrate a master battery management system that can coordinate across modules and account for differences in age and wear levels.
And this isn’t even accounting for the fact that cells used in EV batteries and in energy storage batteries have very different requirements for energy density, cost, cycle times, calendar ageing, charge and discharge rates.
Using cells and modules optimized for energy storage is a much cleaner option than force-fitting automotive cells into grid storage.
The real second life
When EVs were still new, the expectation was that batteries might need to be replaced every few years. But with newer chemistries and battery management technology, over time, what we’ve seen is that battery life just keeps increasing.
(As an aside: At Exponent, we’re known best for our 15-minute charging tech, but the way our battery management systems work is that we’re also able to guarantee much longer life for the battery. We’ve had it independently tested, and our batteries retain more than 80% of capacity even after 3,000 cycles of only rapid charging.)
For personal vehicles like cars, it increasingly looks like we will comfortably see battery lifetimes of 12-15 years for normal use. Surveys have shown that 10-year-old Tesla cars on average retain something like 80-85% of their original battery capacity. Those are EVs with decade-old battery tech and shorter-lifespan chemistries like NMC and NCA.
Modern LFP cells, with the right battery management, can easily last much longer. Which means that, in the near future, we’ll see a much better secondary market for used EVs. The second life of the battery then is actually the second life of the entire vehicle, as it passes from the first owner to a second owner to a third owner.
People will buy used EVs instead of buying retired batteries. Just like ICE vehicles are resold with the same engine. Batteries now outlast cylinder heads.
The spreadsheet math of second-life use falls apart. Which leads us to the better solution.
Recycling is great
If we throw away the second-life storage idea, once EV batteries have worn out after many years in the field, what are we supposed to do with them?
The answer is to recycle them, which is what you would eventually have to do even if you reuse them first.
EV batteries aren’t going to go into landfills, which would be extremely dangerous. Governments around the world – including here in India – have set clear regulations on how they need to be managed and recycled. Recyclers are certified and empanelled, and manufacturers have a responsibility to help retire used batteries safely.
And the good news is that battery recycling is highly efficient at extracting the key metals and salts from used cells.
Image reference: Li-ion recycling diagram (ResearchGate)
Used lithium-ion batteries are safely discharged, the electrolytes and gases extracted, then crushed up and turned into a black mass or powder, from which materials like lithium, nickel, iron, cobalt, aluminium and copper are extracted.
There are a bunch of different processes used for recycling, which we won’t go into in detail (at least not today), but depending on the process used, ~85-95% of materials can be recovered. Those materials then go into new cells and packs, reducing the need for new mining and creating what is called a circular economy.
The future is better batteries, longer life, a stronger second-hand EV market and more efficient recycling.