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Discussion in 'TSLA Investor Discussions' started by Fact Checking, Jul 8, 2019.
I remember the Tesla battery chemistry being more volatile than other common batteries as well. The aluminum cylindrical shells help slow the propagation of fire between cells. Pouches would make a fire really interesting. Tesla would have to change their chemistry to move to pouches.
This is by far the biggest advantage. The electrodes want to expand as you shove lithium ions into them. Left unconstrained, this causes tiny cracks which ruins the electrodes. Silicon anodes are especially bad at this, which is why we still don't have them despite their other wonderful traits.
Cylindrical cells efficiently constrain against expansion. Prismatic cases do so much less efficiently, pouch cells not at all and generally must be externally constrained.
As you point out, cylindrical is not great thermally. More layers = worse thermal performance and the inner part of the cell has a lot of layers for very little active material.
What I see and takeaway from this picture of the Tesla battery pack,
The cylindrical batteries and the serpentine nature of the cooling channels they are packed in,
is that every single battery is and can be temperature controlled with maximum surface contact of the cylinders for maximum control
note: the cooling channels touch completely vertically or almost completely the sides of the individual cylinder and partially wrap around them for maximum contact
Yeah, but small cells are much better thermally than large cells. (Assuming each cell is surrounded with coolant, as it is in Tesla's system.)
AFAICT Tesla is the only manufacturer to have gone with small cells rather than large cells.
I suppose someone sometime might go with small prismatics or small pouches, but I frankly don't see the advantage. Maybe there is one? But most manufacturers are doing prismatics or pouches so that they can use large cells.
OK. So I have this vision of a very small prismatic in the shape of a square cylinder But I don't think the benefits over the *round* cylinder are significant enough to be worthwhile, and the manufacturing process would probably be a lot more finicky (lots of chopping edges instead of rolling).
Ahhhhh. The spiral roll process can be made faster more easily than the "stacked sheets" process. Same reason lots of bulk products come in rolls instead of stacks.
Yeah, I think this is the bottom line -- for the same physical size of cell, the cylindricals can be manufactured faster. Other companies get around the speed problem by using larger cells, but we already know the cooling and swelling problems are both worse with larger cells.
Rimac switched to cylindrical and Rivian may use cylindrical as well.
Battery Packs | Rimac Automobili
That first video is lab scale equipment. Pouch cell mass production equipment would look more like a newspaper printing press, with multiple layers all joined together before cutting, as at the start of this video. Newspaper presses run at ridiculous speeds.
(suggest 2x speed watch if you do)
a pecision roll in an elongated oval, than a "smoosh flat" with a fair amount of empty space in the middle and the 4 corners
and cylindrical with slicing open
Geez, so the fastest way to mass-produce prismatics is to produce cylinders and then squash them.
(1) Obviously cylindrical is better than *this* method of making prismatics
(2) No wonder there's more swelling problems on prismatics and pouches