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Why do they make EV batteries round instead of flat sheets?
Beamin:
I understand using 18650;s intially they are already being made cheaply but now they are going to a new battery 45% bigger but still round. The obvious fact is that you have alot of empty space. Why not make huge 1 sq meter X 1 cm flat sheets with metal plates in between where you can run coolant to cool or heat the batteries. No wasted space, minimal bus wires to link them together and could be cut into weird shapes to fit things like drive shafts and other things . Same with house power banks flat sheets win. :-// :-//
wraper:
You would need to wind internal layers with extremely sharp angles to keep the square shape, and it's a big no-no. Also I guess it becomes way more difficult to produce airtight shell of such shape. IMHO you would end up with a lot of empty internal volume instead of empty external volume.
EDIT: did not notice 1m square. That will not fly. You simply will not get required voltage as you need like 100 cells in series. And such large cell will be extremely unsafe besides other issues.
SVFeingold:
A few reasons spring to mind...
Round, steel can cells still have superior energy density to pouch-style cells. Even accounting for packing inefficiency.
These batteries get tested many, many times before making their way into a car. As individual cells after formation, then during incoming quality control at the OEM, then again if they've been sitting for awhile, then again after they're assembled into modules, then again after they're assembled into packs. Sometimes multiple times at each stage. Round cells are far, far easier to handle in mass automation/testing compared to big, thin, fragile 1x1m sheets.
If one of your 1x1m cells fails a test, that is a whole lot of material that now has to be scrapped and recycled. And the bigger you make it, the harder it will be to achieve the same yield.
I imagine that with cells so large you might run into weird issues given that some areas of the cell will charge/discharge faster than others. Some areas may be significantly hotter or cooler than others. One such area getting overdischarged or overcharged due to chemistry imbalances, temperature imbalances, etc. is not really much different to an individual cell in a big parallel pack suffering the same. Only now this virtual "cell" is buried in some arbitrary portion of the huge single-cell pouch and there is no easy way to monitor or correct for such things. And this is a runaway process. Hotspot forms -> supplies/draws more current -> gets hotter -> repeat. "Thermal runaway" is industry-speak for "big metal fire."
On the same note, current density variations throughout the cell can become a problem for the same reason.
If you look at a teardown of a Tesla pack, you'll notice that all of the cells are connected with wire bonds. These serve as handy little fusable links in the event that any particular cell is subject to excessive current draw. With just a few giant cells? This sort of "last line of defense" safety doesn't work so well.
It's much easier to deal with a high pack voltage (~200-400V) for such applications. With giant flat cells you either need to connect a lot of them to get this kind of voltage (again, very thin, fragile cells to make such a stack), or you need to step up a very low voltage which is much less efficient. P=VI being what it is, for a given power output your current draw goes up in inverse proportion to your voltage. Packs in cars like Teslas can easily source a couple kiloAmps in short bursts. The more you drop the voltage the more current you need to draw to achieve the same power output. More current = more heat = more cooling required + much larger harnesses, bus bars, more demand on power conversion components, etc... It's a compounding problem. There's a reason some EV makers are going to 800V packs.
Definitely not saying something like this could never work. But like most things, it's more complicated then it initially seems. :)
S. Petrukhin:
They just went and bought standard items from a nearby store for the first car. And then everyone else said, " Hey, guys, this is working! Let's all make battery-powered cars now." :)
amyk:
--- Quote from: Beamin on January 06, 2021, 02:32:17 am ---but still round. The obvious fact is that you have alot of empty space.
--- End quote ---
--- Quote ---Why not make huge 1 sq meter X 1 cm flat sheets with metal plates in between where you can run coolant to cool or heat the batteries.
--- End quote ---
What do you think the empty spaces between 18650s are being used for...? ::)
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