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.
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.
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.
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.
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.
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.
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
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.
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.
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.
I don't really know or care what any of this is meant to mean, but you can't comment like this on HN, regardless of the topic. We have to ban accounts that do it repeatedly.
This seems very bizarre given Mazda is probably the least (of all "major" manufacturers) focused on EV and electric initiatives.
Mazda only had one EV, the MX-30 EV. Less than 600 of the MX-30 EV were sold in the US during its production. It was a complete flop right out of production. Mazda leadership has been notorious for pushing rotary engines and shifting further away from EV initiatives.
Mazda as a company has a very good track record of adopting green production initiatives. For example, they were one of the first to switch to water based paint to reduce VOC emissions, and specifically formulating the paint to not require heat-drying to lower energy use.
Their current stance seems to be that PHEVs are better than EVs for the environment because it better matches the driving patterns of the typical customer and charging availability, and minimizes the weight of the vehicle and production of batteries, both of which are still contribute significantly to pollution.
it's unrelated to the manufacturing of EVs. If any factory reaches a significant energy generation (usually this means from solar) it makes sense to look into a battery solution.
It just happens to be Mazda's manufacturing plant.
pornel|6 months ago
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.
hinkley|6 months ago
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.
whatever1|6 months ago
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.
DanielHB|6 months ago
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.
reactordev|6 months ago
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.
londons_explore|6 months ago
It can be solved by isolating each battery in its own steel box, but that gets fairly expensive fairly fast.
slt2021|6 months ago
worn-out batteries can swell and fail spectacularly, with fireworks
robin_reala|6 months ago
jama211|6 months ago
zhyder|6 months ago
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.
bri3d|6 months ago
nine_k|6 months ago
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.
omgwtfbyobbq|6 months ago
https://technode.com/2025/04/22/catl-says-its-next-gen-dual-...
TGower|6 months ago
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.
rubyn00bie|6 months ago
momoschili|6 months ago
stetrain|6 months ago
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.
hinkley|6 months ago
smileysteve|6 months ago
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
bluGill|6 months ago
redwall_hp|6 months ago
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.
burkaman|6 months ago
lupusreal|6 months ago
throwawaymaths|6 months ago
robofunk|6 months ago
wiredpancake|6 months ago
numpad0|6 months ago
1: https://global.toyota/jp/newsroom/corporate/43207750.html
2: https://www.power-academy.jp/sp/electronics/report/rep03200....
mNovak|6 months ago
willlma|6 months ago
wiredpancake|6 months ago
[deleted]
tomhow|6 months ago
https://news.ycombinator.com/newsguidelines.html
We detached this comment from https://news.ycombinator.com/item?id=45043113 and marked it off topic.
skhameneh|6 months ago
Mazda only had one EV, the MX-30 EV. Less than 600 of the MX-30 EV were sold in the US during its production. It was a complete flop right out of production. Mazda leadership has been notorious for pushing rotary engines and shifting further away from EV initiatives.
kenhwang|6 months ago
Their current stance seems to be that PHEVs are better than EVs for the environment because it better matches the driving patterns of the typical customer and charging availability, and minimizes the weight of the vehicle and production of batteries, both of which are still contribute significantly to pollution.
wallaBBB|6 months ago
momoschili|6 months ago