Living in both marine and overland environments, ruggedness is important. But volume and mass are also very important. And price is important.
Agreed (I think we agree on most things actually)
I'm not at all convinced that plastic-shell cells are more rugged than the modern alternative. Do you have evidence of this?
Not personally, but I formed my opinion based on the evidence given by others who I have at least moderate trust in (Will, Dacian, folks in the Marine World) and just what I have observed (plastic cased cells are the default in the marine world, Victron uses Winston cells).
Some factors are pretty self-evident and don't need evidence: terminal size/depth, terminal screw diameter, busbar quality.
Then there are the pros/cons of a thicker Nylon shell vs an aluminum shell (which I have only a partial understanding of).
Then there are factors that I don't personally have evidence of (and would need to cut open a cell to verify) but I have heard from people I have moderate trust it (Will, Dacian), such as the terminals on many of the aluminum cells being spot welded as opposed to integrated into the shell of the nylon cased cells. And the nylon cased cells being more resilient to (1) puncture, (2) drops/denting/deformation (not a big issue once the pack is secured)..
If anything, the body of anecdotal evidence suggests that, historically, very large format plastic shell cells have struggled in mobile environments. You can read back and find people building batteries six or eight years ago advising to stay away from the larger cells (400Ah and larger, generally).
Are those issues rectified in today's designs? I don't know. But I don't think we have much data one way or another, and I'm inclined to believe that aluminum-shell cells are pretty rugged, too.
I would heed this advice and have no idea whether they are rectified in today's designs, I suspect not, I think it is just a design limitation of large form factor cells. But this has nothing to do with cell casing type, as you correctly noted it has to do with large form factor cells and internal volume/surface area ratio. I suspect (just speculation) that there is a reason we are just beginning to see aluminum cells push above 300Ah, and I suspect that has to do with casing strength, or maybe fatigue over time.
I understand the best practice in the marine world to still be (1) cell size of less than roughly ~200Ah, ~100Ah being better (2) robust cells and terminals (3) lightly compressed and well secured in a manner and location that minimizes shocks and vibration.
We've had this debate before, so I'll just re-summarize: I'm not convinced anyone knows what "A-grade" means, nor am I convinced that there is any "matching" going on when paying marked-up importer prices, nor am I convinced that "unmatched" cells are actually that far apart in practical terms. Finally, even if and when cells do vary a bit, I'm not convinced that a degree of "mismatch" matters, with care and proper provisioning. Outside of infant mortality, it simply is not an issue in a real ESS pack, which is what I think most of us are building in this forum.
You are entitled to this opinion. I don't agree with most of it, but I do agree somewhat with the last sentence or two. I would also point out that we are straying off course with this line of thought. My comparison compared new, grade-a, from official sales channels, because that is the only way to reliably compare apples-to-apples-ish, choosing resellers at random on Alibaba/Aliexpress just has too many unknowns to reliably compare.
One point I would make re: 'marked up importer prices' and matching. In the case of CALB (USA), Winston (EU), they are not 'importers' they are official distributors, and as far as I understand cells are binned, matched, warrantied, from the factory. For most of us the cost of this may not justify the benefit, for some of us it will. But that's a side issue, new-from-factory prices were chosen because its the most reliable way to compare like costs.
First, I can assure you that ECPC (your "src 1") is not matching, grading, or importing Frey cells that are any different than the ones I imported at half the price myself by spending the time to deal with a broker in Shenzhen.
You are probably correct, but they have a reputation for buying quality cells
through official channels (they are one of about a half dozen authorized CALB distributors in the US), as I understand it, the testing, binning, matching should happen at the factory, and is one of the reasons cells sold through official channels cost what they cost (they are also warrantied and supported).
On the other hand, you seem to be using alibaba prices ("src 2") for some of the other rows, which of course avoids the USA importer markup. That's apples and oranges.
If this were the case it would only strengthen the case of the plastic cells (since the alibaba links were for the aluminum cells) which based on your reasoning should make them appear cheaper than the US sourced plastic cells.
However, to nip this in the bud. US prices were only compared against US prices, and Alibaba prices were only compared with other alibaba prices (and
only from the manufacturer's official Alibaba page, not resellers or brokers where prices, quality, and honesty can differ substantially). The reason the alibaba listing was listed as a source was because I pulled density data for the 280 Al cell from there.
Second, your volumetric density calculations can't be right. Your mass densities look close enough for our purposes (though I still get 172Wh/kg for EVE 280Ah, not 160Wh/kg). How did you calculate volume?
Mass Density: [amp-hours x 3.2] / kg
Volume Density: [amp-hours x 3.2] / liters -- (calculated from L x W x H in mm)
Does this sound like the right math to you? Its not something I've calculated before, or thought much about.
As to your calculation of 172 Wh/kg for EVE that may be spot on, I don't know,
Definitely. As witnessed by the fact that almost everyone on this forum is using them, instead of plastic shell cell designs from ten years ago.
You keep saying this "from ten years ago" thing, most of these companies (aluminum and plastic cased) are roughly 10-15 years old, a few 20+ (EVE, Winston, Lishen). And all of the companies still in business are churning out 2020 cells, and at least one (CALB currently makes both plastic and aluminum cells for different use-cases). I believe the few people that have been able to verify date of manufacture for their EVE cells had cells from 2018, but I may well be wrong on this.
I'd apply a caveat: the plastic-shell cells we have access to were designed and implemented a long time ago, in lithium battery terms. All of them (CALB CA series, Winston, etc.) have been on the market, in those form factors and capacities, for many years now. It's possible that improved manufacturing process could yield a much denser plastic cell today. We don't know.
But I think it is super telling that we aren't seeing any new plastic shell cells, with better densities and perhaps in different form factors, coming onto the market.
Possibly, or possibly its just a different market segment. I believe that the plastic cells can't (and weren't designed to) compete in terms of energy density (from the tiny bit I know, LFP in general isn't really designed to either). But energy density isn't the end all and be all for every application. What the plastic cells have is robustness/durability, and the ability (apparently) to so far far exceed the aluminum cells in cell capacity (which as noted above may not be well suited for harsh environments).
If energy density was the only consideration, something like cylindrical NMC (or even cylyndrical LFP) cells would be the obvious choice right? But because we have a range of priorities, a range of products are available to meet different use-cases.
But I agree, it seems all of the plastic cased cells have been around in the current form factor for years. On the other hand, based on the minuscule amount I know about iterations in aluminum cells, it seems they haven't really iterated in form factor much either, the main change is pushing towards larger capacities (300Ah+), capacities which the plastic cells have exceeded for years (at the expense of energy density). If either cell casing type can deliver high capacity, equal strength, and less volume, that's a win, regardless of cell type. I think it would be great if Frey could produce a ~200Ah cell as robust as their 100Ah cell.
