If one were to set out to create a single large battery out of massive array of extremely heterogenous individual battery packs, what would that look like? I guess you could pair each battery pack with its own individual charger and inverter so they can all feed into some common bus at a given voltage and frequency -- sort of like the "microinverter" setup for solar panels.
You could cut down on heterogeneity by only using one type of battery in one installation, and trade batteries between installations or even with external companies so everyone can get matching sets. Sort of like the card game Pit. You could also bin the batteries by quality, like how manufacturers of vacuum tubes have a hard time replicated electrical characteristics exactly from one tube to the next, so they sort them into bins after manufacture and ship them out as matched sets that came from the same bin.
As it so happens, there's already a lot of agreement. Many many cells are either 18 mm, 21mm or 28 mm diameter cylinders & 650-700 mm long. "18650" cell is very common. 28650 used to be a "big" cell but it's not around as much I feel like. "2170" is a "21700" that is increasingly popular. Most of these cells have similar charge behavior, of applying a fixed amps up to a point, perhaps with temperature sensors to detect problems/slow down, and then a slower finishing charge, that ends around 4.2V per cell.
Using solar grid-tie micro-inverters would probably work fine to join together a bunch of different packs.
No, they won't. The voltage range is all wrong for a full pack (microinverters work on one or two panels, so a normal working range of 30-80V, usually 50-60V per panel open circuit), and they tend to operate at their maximum power point.
You'd have far better luck with string inverters that can operate in that range (a typical residential string inverter will work from 200V to 600VDC which is just perfect for EV packs), and with a few modifications, you could get one that allows control over the output power (I expect some have that already, I'm just not sure which ones might).
Actually, that sounds just about right, if you're working with modules rather than full packs. Tesla has various sizes of battery module, but 57 volts is one common size. (I think a typical model S has somewhere around 14 of them.) So, one microinverter per module would be fine. LG Chem also makes a module that's about 60 volts. (I think it's made up of around 16 pouch cells in series.)
Of course, if you're doing this on a large scale, you could just have any kind of inverter made that handles whatever voltage you want.
On the other hand, if you're using full packs you have the possibility of recycling motor controllers (also sometimes called inverters) from cars, which are designed to send 3-phase power to an AC motor, as a regular electrical system inverter running at fixed frequency. (Maybe fake the motor position sensor data so it always thinks the motor is spinning at a constant 3600 RPM, or 60HZ.)
I don't know if modern motor controllers output a real sine wave or if they just output a square wave. I'm guessing it's probably the former, otherwise I'd expect you'd get a lot of extra noise and vibration.
You could also re-use the car's original charger (the onboard part that converts 110V or 220V AC from a level1/level2 charger to DC at pack voltage). On the other hand, if the input power for charging is DC at the appropriate voltage to begin with, that part isn't needed.
but there's no general market atm for such inverters. so building lots & lots of lower voltage packs & using low voltage solar targeting modules, which are everywhere, works out.
but it's for sure not as good & makes less sense.
repurposing motor controllers is an interesting notion. but I'd probably rather diy than try to circuit bend that. big awesome mosfets &c are cheap. motor controllers have to deal with inductive loads (big coils) so they tend to have slow slopes. which matches up ok for low 60Hz. but it's still be an effort to repurpose & reuse. just redoing the inverter seems like the win. don't even try to find out what design parameters don't line up between motor drive & inverter drive, don't spend the time, for something that won't work as well as having the right thing.
You're probably right that just engineering a specific solution or ordering them from a supplier that makes such things is probably less hassle and possibly cheaper in the long run.
I'm currently doing an EV conversion. I'm amazed that you can buy a box that's about ten or fifteen pounds or so for a couple thousand dollars that can handle about 90 kilowatts of power (in short bursts), converting around 100-200 volts to AC power to drive a motor. That's probably enough to power a whole residential block by itself, and yet it's not even a very powerful controller compared to what you find in a high-end OEM EV. I'd expect anyone building a large scale grid backup battery storage system to have access to inverter parts at least that good and at least that cheap, and if they don't they could afford to design it themselves.
Sell a PowerWall type thing made with a single 60/70/85KwH pack pulled from a vehicle after its downgraded to ~70% capacity. design your vehicle packs from day one for a second life as stationary energy storage, where size and weight don’t matter.
But, how do you solve the materials problem that closes the loop: somehow recycling old EV raw materials into new EV materials? AFAIK there still aren't great SOTA solutions to this. At some point, it's hard to have a nice closed cycle of production and waste-recyling for ecological and cost optimization purposes.
There's actually not as much variability as you might be inclined to think - in terms of pack voltages. China is their own set of problems I'm not familiar with, but in the rest of the world, most modern EVs are using a 3.7V nominal chemistry (2.5V/cell fully discharged, 4.2V/cell fully charged), and almost always in a 96S pack of some capacity or another (so about 400V fully charged, again, give or take a bit).
If you're dealing with 3.7V cells in 96S packs, the rest of the pack details simply don't matter beyond individual pack management and balancing. The voltages of the storage rail are compatible across all of them. Some may be 50Ah packs off a PHEV, some may be 250Ah packs off a long range BEV, but they all operate at the same general range of voltages, and so can be put on the same voltage rail once they match.
You'd want per pack fusing (probably a contactor assembly that can report current, and disconnect a misbehaving pack if the current goes too high), but at that point, if you can talk to the BMS on each of the packs, they'll just work.
The only real gotcha with this is getting the pack voltages lined up with the main voltage rail in the first place. If the rail is sitting up at 390V (almost fully charged) and you put a 250V pack (fully discharged and then some) on it, the internal resistance isn't enough to prevent you from pushing an awful lot of amps into the pack and likely damaging something. So you'd have to have a separate set of chargers to get the packs close to the main rail voltage, but once it's within 5-10V, hook it up and it will equalize out safely.
If the facility is somewhat climate controlled, you likely don't need to hook up the cooling loops for the packs (if they have them) - but you might want to, because the difference between a failed cell and a runaway pack is typically going to be the cooling loop. If a cell fails and runs away, a pack with good coolant flow probably won't propagate. You might not want to be near it, but... it probably won't burn the facility down. Without cooling flow, welp. Whole pack will just run away, one cell at a time, and that's the sort of thing you want to watch from a very safe distance.
The practical limit on pack life is going to be how evenly matched the cell groups are. If the pack is worn evenly, all the cell groups will generaly decrease in voltage equally. But as the pack wears out, if one cell group loses capacity more than others (it may be a hot spot in the pack), you start having to cut off the discharge as that group hits the low voltage cutoff (while the bulk pack voltage is still acceptable). If this starts tripping the pack off too early, it's not going to be useful on the rail, and it's probably time to recycle it.
Really, though, it comes back to how much risk you're willing to tolerate. An individual car battery pack is likely to be perfectly fine. A warehouse of 5000 of them, out of used/wrecked/who-knows-what cars... that's a little bit scary. If one of them runs away and isn't contained, you have a warehouse full of packs running away, venting violently toxic chemicals where there's no good option. If the vent plumes are not on fire, you stand a good chance of wonderful things like HF (there are less friendly things to humans, but not many). If the vent plumes are on fire, the resulting gasses may be mildly less toxic, but now you have what amounts to a giant hazmat warehouse fire.
I'm really not sure what the right balance is, but with the challenges and risks of second life EV batteries, I honestly don't expect them to be particularly popular with any of the big energy companies.
I could imagine a facility where each pack is separate by a non-flammable barrier (concrete, firebrick, or whatever works in this case) to stop one out-of-control pack from igniting its neighbors. And then larger groups of packs being segregated from each other with thicker fire barriers, and so on. And coolant constantly pumped through the facility: the batteries, and maybe even the fireproof barriers between packs. The whole facility can be located away from cities or towns, so fires don't cause (as much) of a health hazard. Maybe a ventilation and filtration system that can capture the smoke and filter the worst pollutants in the event of a fire.
This whole thing sound kind of messy to build, but then again it's just a centralized version of the related idea of using people's cars connected to smart bi-directional chargers as a backup power source for the grid. If it can (theoretically) be done with a bunch of geographically distributed individual cars, then it ought to be possible to do something similar in a centralized way with whatever safety precautions are needed.
You can hook a Tesla or Leaf module to a BMS and charge controller that supports lithium like Victron, and have a massive battery for off grid solar and RV's. Not many are doing this, but enough that it keeps used module prices up.
Rumors you don't even need to do balance charging on these packs because they're designed for hundreds of kilowatts output and nobody runs them anywhere near that.
Lithium batteries get unbalanced due from varying internal resistance between cells. But when you're not drawing anywhere near their rated output, the differences don't matter much.
Its a great idea. Given them to commercial establishments where energy costs are 3 times domestic ones and you have a pretty decent deal.
Alternatively, with modern LED streetlighting, a 50 kWh pack can probably light up a small street throughout the night. Local govts can store excess energy during day and use that at night. Its a fairly standard charge and discharge cycle, perhaps giving 4 to 5 years of lifecycle additions.
For my leaf, I have a simple 12v pure sine wave AC inverter. In a pinch I can turn on my car and hook up the inverter to the lead acid battery, which is charged from the leafs main battery.
If you want to plug an EV in to charge, put a circuit in and have fun.
If you want to backfeed any power to the grid, well, you need an application with the power company (which usually has an application fee), plans that often have to be reviewed and approved (which is a giant pain in some areas, mine included), then the new rate schedules, inspections of the generating facilities for initial operation and continuing operation, and it's a giant pain in the rear. Plus, there's really no strong incentives to actually do this sort of thing, financially - if you've got a solar install, OK, but just a battery... eh. A backup generator is an awful lot cheaper in most cases.
Something tells me that it will be a while before we have EVs with built-in 240V split-phase inverters.
IMO, a good V2L setup needs to have split phase 240V in the US. But that would add significantly to the manufacturing cost of the car, so I won't hold my breath.
It's a lot if you needed the car to go 80% of it's maximum range when new.