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Comment by colejohnson66 | Hacker News Reader

Assuming this image I found online is of 4680s, there’s quite a bit of wasted space: https://www.teslarati.com/wp-content/uploads/2021/01/tesla-4...

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The actual arrangement of cylindrical cells is staggered: https://cdn.motor1.com/images/mgl/XkV66/s1/tesla-model-y-468...

Exactly. Sandy Munro has an excellent video with multiple different battery packs torn down and compared: https://www.youtube.com/watch?v=4JiDZVO9NdM

You can see that the cooling channels in the Tesla pack are these hollow undulating aluminum shapes. Can imagine they're not very cheap. Also the Tesla packs are filled with some kind of bonding foam, making repairs near impossible.

> bonding foam

Is that maybe to reduce the spread of fire? I know Tesla put a lot of work into that. Others too, I hope.

Why not use two size tiers of cylindrical cells, smaller ones to fill in the gaps? With only one size the empty space ratio is the same.

It definitely isn’t the same. The square packing in the first picture uses pi/4 ~ 79% of the space; the second uses pi/(2*sqrt(3)) ~ 91% of the space.

They don’t bother with smaller secondary batteries because the extra manufacturing cost and operational complexity isn’t worth it for a 5%-10% bump in volumetric energy density.

I didn't mean to say the two packings were the same. 90% is better than I thought circle packing was though, I've mostly seen multi-scale packing used in thermal pastes and stuff and it is probably a bigger difference in 3d sphere packing.

Didn’t someone prove about 15 years ago that there’s an oval shape that packs tighter still? Given the hollow core that could probably be done, but cylinders are probably easier for the assembly line to handle. So then it comes down to whether that space is being utilized (for cooling, or weight per cubic inch of the support structure).

I think there are a couple companies with hollow designs for improving surface area for cooling, or at least patents around it.

I suspect the original picture is from either the end of the assembly line or a shipping facility. Your first clue that it’s not in a pack is the lack of connections between the batteries. Nobody makes an all parallel pack. 3V at how many amps would be too hot. They wire a bunch in series to triple digit voltages and then comment those packs together in parallel.

You could probably ship them staggered, but I’m betting they are at a weight limit anyway so making them more dense doesn’t save you anything.

Volumetric efficiency is not a big concern for EVs. They already have more storage than an equivalent ICE vehicle (frunk, no trans tunnel, etc). Giving up space for better weight and cooling is worth it.

Isn't some of that "wasted" space necessary for cooling the batteries?

offset every row and significant reduction in “wasted space” also cooling is a huge part of what makes these work. We have to stop looking at battery’s from purely a cell level. Add in the structural element, and the claim is they are now more volumetric and weight efficient than traditional packs.

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