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Cylindrical vs Pouch cells

Buckminster

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Aug 29, 2018
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image001.jpg
 
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animorph

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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.
 
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Doggydogworld

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It might be that other advantages of cylindrical cells (such as their mechanical stability against internal swelling) dominate.
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.
 

winfield100

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Note how wasteful cylindrical cell packing is, compared to flat cells. Plus the cooling channels only touch part of the surface of the cylindrical cells:

Tesla-Model-S-battery-pack-Ricardo-photo-2_1280.jpg


Note how in that tear-down image the cooling channels are touching maybe 30% of the circumference of each cell. Cylindrical cells also complicate pack design due to their very granular and not easy to pack nature.
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
 
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neroden

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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.
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).
 
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neroden

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Apr 25, 2011
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The inner of core does not have to be the tap for the positive and the outer does not have to be the tap for the negative. The current collectors lay opposite each other the same as the prismatic cell and the taps can be anywhere as designed along the sandwich. There are many youtube videos on how cells can be made.

You can get the idea of which one is faster to create and why Tesla (IMO) is using cylindrical.
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.

I don't see prismatic cells ever compressing tight enough or being created fast enough. Second video. Sure you can speed up that second video's machine but you can also speed up the cylindrical one just the same.

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.
 

Doggydogworld

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The inner of core does not have to be the tap for the positive and the outer does not have to be the tap for the negative. The current collectors lay opposite each other the same as the prismatic cell and the taps can be anywhere as designed along the sandwich. There are many youtube videos on how cells can be made.

You can get the idea of which one is faster to create and why Tesla (IMO) is using cylindrical.


As others have said. Heating is not an issue for any style if the right amount of cooling is supplied to any cell. Obvious to me because the Model 3 charges faster than anything on the market.

Still there may be an argument for a hybrid type cylindrical cell that has heat sink(s) inserted in the roll so the finished product looks more "eye" shaped or flattened roll. This would give the cell more surface area contact with the cooling plates and suck some heat out of the center through the heat sink(s). (A bit hard to describe the creation method)
There is also no reason a cylindrical cell can't have a heat sink in the middle.
also.... what if there was a way to make the separator a type of heat sink?

I don't see prismatic cells ever compressing tight enough or being created fast enough. Second video. Sure you can speed up that second video's machine but you can also speed up the cylindrical one just the same.
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.
 
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winfield100

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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.
BMW prismatics
(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
 
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neroden

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Apr 25, 2011
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BMW prismatics
(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
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
 
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mongo

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May 3, 2017
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And in the podcast, Elon states Tesla moving to batteries without modules, whatever that means


Right now they assemble the cells into plastic modules which are then linked together in the pack with coolant and electrical interconnections. He is speaking of removing the intermediate module unit so you would have a pack with cells only. No extra divisions.

Or put another way, a pack sized module.
 
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