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...
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....
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.
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.
* https://en.wikipedia.org/wiki/Mazda_Wankel_engine
* https://en.wikipedia.org/wiki/Miller_cycle
* https://en.wikipedia.org/wiki/Skyactiv#Skyactiv-X
* https://en.wikipedia.org/wiki/Homogeneous_charge_compression...
There are so many questions this (the battery storage) raises regarding ROI and alternatives. I think it's great they're trying something, but I can't help but wonder if this will be another failed attempt on their track record.
