Those who TL;DRd - it's for the factory, not the cars!
Old EV batteries are great for energy storage. A worse weight-to-capacity ratio doesn't matter for batteries sitting on the ground. A battery that holds only 70% of its original capacity is considered worn-out for EVs (and even replaced under warranty), but grid storage isn't driving anywhere, so any capacity left is still useful.
Battery banks are worse than degraded raid arrays in some important respects. The bad cells tend to try to bring the rest of the pack with them. It’s one of the reasons people keep toying with partitioning cells and putting controllers onto individual cells or small groups of them.
Parting out two or three dead battery packs to cull the best of the survivors can improve things quite a bit. And as you say, on a stationary pack you can afford to overdo telemetry, cooling, and safety circuitry because it doesn’t have to move, let alone accelerate.
I don’t know what the half life is like for the reused cells though. Do the cells that lasted twice as long as their neighbors continue to outperform or do they revert to the mean over time? I could see either being true. The days when you accidentally produce cells that are several stddev better than your target quality should make cells that last longer, unless they’re sold to a leadfootted driver.
A degraded battery bank does not mean a bank with outright "bad" cells. The cells will probably be way more off than they used to be, but there can still be plenty of effective capacity in the bank. Heck, if space isn't an issue, it's productive as long as it isn't self-discharging too fast.
You can still have a working battery even with some "bad" (i.e., way out of spec) cells, depending on the BMS. All the thresholds are configurable, just that a regular EV setup would lean towards safety.
>The bad cells tend to try to bring the rest of the pack with them.
This is true (and in some cases potentially dangerous) when you have a several cells of varying voltages in parallel but it's fairly trivial (by EE standards at least) to overcome this with something similar to a charge pump.
> It’s one of the reasons people keep toying with partitioning cells and putting controllers onto individual cells or small groups of them.
I have been out of the battery tech game for a while now but decades ago we were balancing individual NiCd and NiMH cells for optimal performance, is this basically the same thing?
At the cell level they don’t degrade linearly. First it’s slow then it’s fast and then it’s abrupt collapse. You probably have noticed that yourself with old devices. Some do not hold charge even for a minute.
With battery packs probably you can do some smart things to make the degradation curve look more linear, but again there is only so much you can do.
There’s quite a lot you can do when you can isolate and deactivate individual cells, big battery packs like this really do not fall off a cliff in the same way you’re describing.
Sudden failure in a big battery like these is usually due to a single cell failing, which can usually be replaced and then the battery pack is back to the 70% capacity or whatever. Probably in this context of scale it's worth doing the work of replacing bad cells.
You do realize these batteries you're referring to resale at a decent price because, for the most part, they still function really well, just not in its existing capacity.
Balancing multiple battery packs at different wear levels is a huge nightmare. You have to run rebalancing operations all the time and on used packs it can be quite dangerous to trigger a thermal runaway.
If they do it with different types of batteries it is even more complicated, like you need to write some custom software to sync all that up. This is not a trivial project.
If you take car EV batteries and use them for stationary storage when past end-of-life, the fire risk becomes fairly substantial because EV batteries often have a little water ingress, physical damage etc.
It can be solved by isolating each battery in its own steel box, but that gets fairly expensive fairly fast.
I've seen a video on the youtube where a battery recycling company does this; they leave the car battery packs in their original housing, which I presume is water resistant enough. Each unit is also connected to a controller, which I also presume monitors battery health, temperature (assuming temp sensors on car battery packs), voltage, etc. If a unit is dying they can safely dispose of it, else, the units were out in the open and with several meters in between, meaning any fire would be unlikely to spread to something else and there's plenty of access opportunities.
Very space inefficient though, but there's more than enough of that in the US.
How much distance does one pack realistically need to not cascade? Honestly I can't imagine any more than half a meter since air is an extremely good insulator. Just make sure the fire can't crawl across though cable insulation?
I've personally set RC lipo on fire with the wood-nail-hammer technique and while the fire out of the pack is intense I can't imagine it igniting another pack.
Except that the cigarrette is only 30% smoked and still perfectly fine to smoke for a while longer (if you insist on an analogy).
Car battery packs are really good; even the oldest Teslas are only now getting to less than 80% capacity. They are designed not to swell/fail if they're worn, else there would be a lot more car fires.
Car power packs are batteries in the other sense of the word. They can be disassembled and culled. So what matters is the health of the best 1/kth cells in the array not the overall array health.
If you’re ever in Hiroshima I can recommend a trip to the Mazda museum (Cosmo! 787B!), which includes a factory visit across a raised gantry. It’s free, but you need to book in advance: https://www.mazda.com/en/experience/museum/
Neat idea to mix batteries of different age and chemistries. I've wondered why EVs couldn't do that too with some power electronics and SW. If an EV battery could have multiple such modules, it'd:
1) Make it easier to carry a cheaper lighter less-natural-resources-consuming battery most of the time. Go to some "gas station" to rent and add more modules when taking a road trip
2) Make it cheaper to replace the 1 module used a lot at its EOL, thereby making EVs last longer and be viable as cheap used cars even past 10 years like ICE cars are
3) Allow easier upgrades as chemistry improves: solid-state, sodium ion, etc.
Modules could be electrically tested for fit. I'd think the fit range would be quite wide (e.g. if one supported lower max discharge rates than another) given the headroom we have with EVs' power these days: they have far-more-than-needed power (which mostly comes for free with EV range).
The tradeoff is that they'd need to be built to be modular with some standardization on module dimensions (maybe we'll have "ZZ" size like we have AA, C, etc today), and would take a tad more volume in the vehicle (though the limiting factor is weight rather than volume). Easily worthwhile over the current model with a huge monolithic pack.
It's quite likely that lower discharge rate requirements are a large part of what makes this system function. Batteries with different internal resistance can work reasonably well even in a naive series system at low discharge rates but absolutely will not work at high discharge rates.
I suppose this has a better chance to happen in city trucks than in cars. Delivery trucks are used more heavily, owners care more about efficiency, they are bigger and taller, so they can offer easier structural options to house swappable batteries. Also, swappable batteries could make charging very fast, if a sufficient stock of batteries is kept on a charging station.
Cars could follow, but it's significantly more involved in them. In most cases, the batteries are a relatively thin layer covering the entire floor space, or similar.
EV battery packs operate at voltages that are seriously hazardous. Consumers coming anywhere near those plugs is a non-starter, so even more bulk, weight, and complexity would need to be added to make the installation process foolproof.
Waterproofing is critical, the mechanism has to work flawlessly over insertion/removal cycles to keep a watertight seal.
Great points. Re safety, I wasn't imagining the consumer doing it themselves but some robot like ones imagined for whole-battery swapping (e.g. https://youtu.be/Oj6LaYFall4); I think any battery swapping only makes sense for batteries you rent rather than own.
Also, you lose the ability to mold your battery along the bottom of the car, using dead space and keeping the center of gravity low. Gas tanks aren't even boxes, they are molded to fit around other parts with no dead space. So, add that space efficiency loss to all the additional space needed for access panels, installation foolproofing, additional waterproofing, etc.
I have to imagine that even accounting for the variance in internal resistance over a large set of batteries in a demanding application like a car is probably a host of problems makes this unattractive from the outset.
GM claims that this is exactly how their Ultium battery pack architecture works. It is made up of multiple modules each with their own BMS, and supposedly one module can be replaced without having to be a match in chemistry and degradation to the other modules.
I'm unsure if that will actually work so well in practice, where you still need to charge all the cells simultaneously when doing DC fast charging etc.
Also all of that extra architecture adds cost and complexity to each vehicle that rolls out the door, compared to a pack that just packs in a bunch of cells together with the necessary cooling etc. as one contiguous unit.
Given that EV battery packs in the real world are trending to last longer than the cars they come in, going with a simpler pack design and swapping in a refurbished pack if you experience a premature failure might be the more economical route.
You shouldn’t need to charge them all at the same rate. Put in some cells that charge slower, fast charge the rest and continue charging the slower cells until unplugged. Consider for instance fast charging the array to 70% instead of 80%, where 1/3 of the cells are charged to 50% and the rest to 80%.
I believe we have a new generation of supercapacitors in R&D stage. There were some experiments done in the last year that showed that some assumptions about what makes supercaps work so well turned out to be wrong. I can’t recall the details but it turns out the foamy structure of charcoal is not the optimal structure. So that should result in higher energy density per unit volume.
Re: 1, ignoring the complexities, is really interesting but depending on the effort to change our battery banks quickly makes renting a car more feasible.
And this highlights American traffic and sparseness.
- plug-in hybrids have 10-13 mile range which is great for running a few errands (this is only slightly more feasible than in a golf cart or ebikes) - also great for last mile connectivity for mass transit n users;
- the Nissan leaf 2012 had an 80 mile range - perfect for most daily commutes in a metro area
- modern electric vehicles have 200-300+ mile range, good for weekend getaways; esp with a charge at the destination
PHEVs were quoting 20+ miles on electric last decade, I think 25-35 is common now?
Actual distance depends on elevation changes and speed/driving, but 15-20 is quite acheivable, as long as you don't make it to highway speeds. And if you go a bit farther and use a splash of gas, no big deal, that's why it has a tank.
I’ve done the math a couple of times and IIRC if we can get the charge density per kilo to about twice what lithium ion can do, you hit a point where a deposit battery that’s around 20 kilos has enough range extension to start being worth doing. That’s the weight of a large bag of kitty litter or a commercial bag or rice. Put a good ergo handle on it and most people should be able to lift a few of them consecutively.
But until one unit is worth about 8 miles of extended range, there would be no point. 3@25 or 30 miles might make it worth the trouble for a road trip, or camping.
actually if you can get a late-1990-ish 90-100cc 2T japanese scooter like Honda Lead 90 or Suzuki Address 100, or even later Yamaha Neos / MBK Ovetto 100cc of 2005-ish vintage this whole discussion about ranges and fuel consumptions becomes pointless.
because those have had fuel consumption of like 2-3L per 100km. with fuel tanks of about 6L you had all the range for errands you could possibly need.
and they were capable of moving two persons around _and_ moving a ton of grocery, or something like an ironing board.
I hadn’t thought about different sizes/weights but this does remind me of Nio’s battery swap network. Which I’ve always been fond of in principle. I think at some point in the future, when range isn’t such a competitive advantage in the EV space we’ll see a push towards standardization and something like this will likely occur. I’d guess something around the 1000 mile mark for an EV. Absurd, yes, but at that point no one will complain about range and that sort of implied density/efficiency also allows for a better towing experience (at least here in the states). If someone can get a 1000 miles of range, but only drives 100 miles at a time TCO drops immensely because tires/brakes last much longer at lower weights.
It feels like Neo is the opposite of where things should go. The great thing about EV is that every outlet is a refueling station. More EV charging for apartment buildings and parking lots feels like the answer. Those battery swap stations are insanely complex and expensive, and the complexity and added weight on the car are very significant. Unlike a gas station that just needs two nozzle types, or an EV charging port standard, having to standardize battery packs for everything from compacts to trucks would be a nightmare. Most people are not taking hundreds of mile road trips most of the time. Slow charging your car while it sits at home or at work or at the shop will work for 90% of cases. I could see the case for industrial or work vehicles, where keeping them running constantly is a legitimate need, but for personal vehicles that sit in driveways, parking lots, or garages 80% of every day, battery swaps feel like massive overkill.
This is not recycling. Recycling implies that you can produce the same product again many times; it's a sustainable practice. This is repurposing or upcycling. It's cool they're getting a second life, but they won't get a 3rd, 4th, Nth life unless the batteries are actually recycled into their component materials at end-of-life. It's kind of like the plastic brick companies: cool that plastic is being turned into a construction material, but it doesn't mean we can stop mining for the primary source material any time soon.
Toyota owns a de facto controlling stake in Subaru (~20%).
Due to typical Japanese corporation by-laws, it only takes 33% share ownership for uncontested control of a corporation, and >50% of 33% means they'll never lose a vote for simple majority matters, which is basically everything except selling or dissolving the company.
The 20% threshold is for a guaranteed seat on the board, which lets them put issues up for a vote.
Related: Ford once had a 33.4% stake in Mazda and there was a bunch of cross-pollination between the two companies at that time. My 2002 Mazda Protege was loaded with various parts bearing the FoMoCo logo. And Mazda engines/chassis powered a bunch of Ford cars world-wide.
Ownership changes over time. At one time ford was in control. Which is why I checked - what toyota is doing makes no sense until you see current situations, now there is an obvious expected return on investment (if it works out of course- if it does they will do this to their own factories, if not they will write it off and not lose much.)
Looks like this is a PoC for Toyota's "sweep storage system" using "low voltage MOS", which seem to be a fancy invented term for a charge/discharge current limiter. [2] has photos of previous PoC from 2023.
What's interesting is that if the batteries are being sourced from JDM cars the batteries are probably relatively young due to the average age of Japanese cars being relatively low (8.7 years) and the amount of yearly mileage is also half for JDM cars when compared to the US. So if you tried the same in the US it may not be as viable.
Readit News
Old EV batteries are great for energy storage. A worse weight-to-capacity ratio doesn't matter for batteries sitting on the ground. A battery that holds only 70% of its original capacity is considered worn-out for EVs (and even replaced under warranty), but grid storage isn't driving anywhere, so any capacity left is still useful.
Parting out two or three dead battery packs to cull the best of the survivors can improve things quite a bit. And as you say, on a stationary pack you can afford to overdo telemetry, cooling, and safety circuitry because it doesn’t have to move, let alone accelerate.
I don’t know what the half life is like for the reused cells though. Do the cells that lasted twice as long as their neighbors continue to outperform or do they revert to the mean over time? I could see either being true. The days when you accidentally produce cells that are several stddev better than your target quality should make cells that last longer, unless they’re sold to a leadfootted driver.
You can still have a working battery even with some "bad" (i.e., way out of spec) cells, depending on the BMS. All the thresholds are configurable, just that a regular EV setup would lean towards safety.
This is true (and in some cases potentially dangerous) when you have a several cells of varying voltages in parallel but it's fairly trivial (by EE standards at least) to overcome this with something similar to a charge pump.
I have been out of the battery tech game for a while now but decades ago we were balancing individual NiCd and NiMH cells for optimal performance, is this basically the same thing?
https://electronics.stackexchange.com/questions/463591/nicke...
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With battery packs probably you can do some smart things to make the degradation curve look more linear, but again there is only so much you can do.
If they do it with different types of batteries it is even more complicated, like you need to write some custom software to sync all that up. This is not a trivial project.
Making your own cells is fun.
For Toyota, this is trivial and the energy storage these “left over” batteries provide, given a tinkering, is sufficient.
It can be solved by isolating each battery in its own steel box, but that gets fairly expensive fairly fast.
Very space inefficient though, but there's more than enough of that in the US.
I've personally set RC lipo on fire with the wood-nail-hammer technique and while the fire out of the pack is intense I can't imagine it igniting another pack.
worn-out batteries can swell and fail spectacularly, with fireworks
Car battery packs are really good; even the oldest Teslas are only now getting to less than 80% capacity. They are designed not to swell/fail if they're worn, else there would be a lot more car fires.
1) Make it easier to carry a cheaper lighter less-natural-resources-consuming battery most of the time. Go to some "gas station" to rent and add more modules when taking a road trip
2) Make it cheaper to replace the 1 module used a lot at its EOL, thereby making EVs last longer and be viable as cheap used cars even past 10 years like ICE cars are
3) Allow easier upgrades as chemistry improves: solid-state, sodium ion, etc.
Modules could be electrically tested for fit. I'd think the fit range would be quite wide (e.g. if one supported lower max discharge rates than another) given the headroom we have with EVs' power these days: they have far-more-than-needed power (which mostly comes for free with EV range).
The tradeoff is that they'd need to be built to be modular with some standardization on module dimensions (maybe we'll have "ZZ" size like we have AA, C, etc today), and would take a tad more volume in the vehicle (though the limiting factor is weight rather than volume). Easily worthwhile over the current model with a huge monolithic pack.
state-of-charge / depth-of-discharge vs lifetime accumulated "discharge stress" so to say also matters a lot.
batteries aren't simple, even lifepo4 ones.
Cars could follow, but it's significantly more involved in them. In most cases, the batteries are a relatively thin layer covering the entire floor space, or similar.
https://technode.com/2025/04/22/catl-says-its-next-gen-dual-...
EV battery packs operate at voltages that are seriously hazardous. Consumers coming anywhere near those plugs is a non-starter, so even more bulk, weight, and complexity would need to be added to make the installation process foolproof.
Waterproofing is critical, the mechanism has to work flawlessly over insertion/removal cycles to keep a watertight seal.
I'm unsure if that will actually work so well in practice, where you still need to charge all the cells simultaneously when doing DC fast charging etc.
Also all of that extra architecture adds cost and complexity to each vehicle that rolls out the door, compared to a pack that just packs in a bunch of cells together with the necessary cooling etc. as one contiguous unit.
Given that EV battery packs in the real world are trending to last longer than the cars they come in, going with a simpler pack design and swapping in a refurbished pack if you experience a premature failure might be the more economical route.
And this highlights American traffic and sparseness.
- plug-in hybrids have 10-13 mile range which is great for running a few errands (this is only slightly more feasible than in a golf cart or ebikes) - also great for last mile connectivity for mass transit n users;
- the Nissan leaf 2012 had an 80 mile range - perfect for most daily commutes in a metro area
- modern electric vehicles have 200-300+ mile range, good for weekend getaways; esp with a charge at the destination
Actual distance depends on elevation changes and speed/driving, but 15-20 is quite acheivable, as long as you don't make it to highway speeds. And if you go a bit farther and use a splash of gas, no big deal, that's why it has a tank.
But until one unit is worth about 8 miles of extended range, there would be no point. 3@25 or 30 miles might make it worth the trouble for a road trip, or camping.
because those have had fuel consumption of like 2-3L per 100km. with fuel tanks of about 6L you had all the range for errands you could possibly need.
and they were capable of moving two persons around _and_ moving a ton of grocery, or something like an ironing board.
hell, in 2000s we were doing 700km trips on them.
They also own Denso, which is the second largest auto parts company.
And they partner with Subaru on some things, such as the Subaru BRZ and Toyota GR86, which are basically the same car with different badging.
Due to typical Japanese corporation by-laws, it only takes 33% share ownership for uncontested control of a corporation, and >50% of 33% means they'll never lose a vote for simple majority matters, which is basically everything except selling or dissolving the company.
The 20% threshold is for a guaranteed seat on the board, which lets them put issues up for a vote.
Perhaps more relevant, the Subaru Solterra and Toyota bZ4X (renamed bZ for 2026) are on a shared EV platform.
So for all intents and purposes, Toyota is the largest singular voting shareholder.
https://en.wikipedia.org/wiki/Skyactiv
https://thedetroitbureau.com/2019/07/toyota-teaming-up-with-...
1: https://global.toyota/jp/newsroom/corporate/43207750.html
2: https://www.power-academy.jp/sp/electronics/report/rep03200....
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