Tesla could be using a “4416” battery cell in Model 3 and next-gen S, X fleet

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orcinus

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#41
44mm diameter Li-ion cells would be impossible to keep from overheating at such extreme currents (20C), due to the much lower surface to volume ratio. The smaller the cells are, the easier it is to evacuate the heat from them. That's the reason Tesla uses small cells, instead of pouch or prismatic cells.
It's not just the cooling that suffers.
Decreased surface to volume ratio reduces the efficiency of the battery too (the greater the surface area of the electrodes, the greater the ion exchange efficiency).
 

garsh

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#44
How about hollow-core cylindrical cells so cooling can run through the center of the batteries? Seems like a more efficient method of evenly cooling.
It may seem so, but...
  • The surface area between the cell & coolant interface would probably be smaller
  • The volume of coolant next to that surface area would be much smaller
  • Removing the outer-diameter coolant loops wouldn't allow you to pack the cells any tighter, because having them right next to each other would result in hot spots at those contact points. So your energy per volume for the pack would likely decrease.
If someone is feeling frisky, they can run through the math to see how it works out.
 

orcinus

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#45
Another thing to keep in mind (assuming we continue playing with the idea of a 44160 cell, even though it’s obviously either a fake, or a misprint/miscommunication):

The greater the diameter of the cell, the worse the packing density.
If it’s slightly wider by necessity, to accomodate an overall taller and bigger capacity cell, then it’s an acceptable tradeoff.
But if it’s as crazy as 44mm x 16mm, that’s a huge loss. And if you stack them two-cell, three-cell, four-cell bunch (come mister tally man, tally me battery), you lose on packing density yet again, because of the caps, contacts, busses etc. between the layers.
 
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#46
Perhaps 4416 is just the number of 2170 cells in the Model 3 pack.

If someone said, "The model 3 pack will use 4416 cells" you could easily misinterpret that statement.

Interpretation 1: It will use some new cell type called "4416" with different dimensions than a 2170 cell.
Interpretation 2: The total number of cells is 4416 (each cell is a "2170" cell).
 

Topher

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#47
If someone is feeling frisky, they can run through the math to see how it works out.
General principles agrees with you, not going to bother running the actual numbers. If you could cool through the center of the cylinder, you could instead cool in the interstitial space, which is inherent in a cylinder packing.

The greater the diameter of the cell, the worse the packing density.
Packing cylinders is pretty close regardless of the ratio of diameter to container length, especially once you get past the early bad cases, roughly 90% coverage.

Thank you kindly,
 

orcinus

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#48
Perhaps 4416 is just the number of 2170 cells in the Model 3 pack.

If someone said, "The model 3 pack will use 4416 cells" you could easily misinterpret that statement.

Interpretation 1: It will use some new cell type called "4416" with different dimensions than a 2170 cell.
Interpretation 2: The total number of cells is 4416 (each cell is a "2170" cell).
Napkin math time.

wheel base = 287cm
width = 188cm

battery diameter = 21 mm = 2.1 cm

2.1x = 188
2.1y = 287

x = 89
y = 136
x * y = 12104

Now, battery skate isn't *quite* the size of the wheel base * width of the car, but it's a good ballpark estimate.
Also, batteries aren't stacked with 0 distance between them, so again, ballpark estimate.

Even if you knock off 50% (which would be a HUGE margin of error), that's still ≈ 6000, which isn't quite 4416.
 

orcinus

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#49
Packing cylinders is pretty close regardless of the ratio of diameter to container length, especially once you get past the early bad cases, roughly 90% coverage.
No need to look at cylinders at all in this case, packing circles (cross-sections) is enough.
But you are correct, it's not dependent on the diameter at all, i was wrong (lack of sleep / not enough coffee).
 

garsh

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#50
Perhaps 4416 is just the number of 2170 cells in the Model 3 pack.
FYI, in garsh's Official Model 3 Battery Pack Capacity Calculation Thread, I came up with a value of 5120 cells in the whole pack.
But that was for a 90.5kWh pack. If you scale that down to 75kWh, then you end up with 4243 cells. Since an advertised 75kWh pack would actually have a slightly higher capacity, 4416 size-2170 cells seems completely reasonable. That would be a 78kWh pack.

In other words, I think you're onto something!
:)
 
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garsh

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#52
Oh, they're packed hexagonally after all?
Partially.
I believe it was JB Straubel who said that they were using battery pack technology similar to the 100 for the Model 3. Therefore, I assume the cells will be arranged similarly to that pack (it's the bottom pack in this picture - the top is an older 85 pack). It's closer to hexagonal packing than the older packs.
 
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#53
Here is my napkin math:

The Model S 85 has 7104 cells (18650). So about 85kWh/7104 = 12 Wh per cell.

A 2170 cell has about 1.465 times more volume than an 18650 cell (21*21*70 / 18 / 18 / 65 = 1.456)
So we might assume the 2170 cell has 12*1.465=17.6 Wh (assuming both have the same volumetric energy density).

17.6 Wh * 4416 cells per pack = 77 kWh

A total of 4416 cells per pack seems like the right ballpark for a Model 3 battery pack.
 

garsh

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#55

orcinus

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#56
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#57
It's definitely not 287cm. The 287cm wheelbase is axle-to-axle. The battery is smaller than 287cm x 188cm.

It's pretty safe to say that each cell is somewhere in the 17 Wh range, so they definitely don't need much more than 4416 total cells to get 75kWh. I'm guessing the overhead for the battery packaging is more than you think it is and the dimensions of the battery is a fair bit less than 287cm x 188cm.
 

orcinus

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#58
Oh, it's less than that for sure. But like i've said, that was just a ballpark - i assumed max. error of 50%, and even that was > than 4416 cells. And that was assuming square cell packing, not hexagonal.

(Motors inside the wheelbase explain it completely, though.)
 
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MichelT3

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#59
Well done guys! Battery rumour explained and dismissed. If all last minute noise could be silenced this easily...
I expect that Friday will give some answers, but will lead to even more questions. It's still a long way to go before we know it all. (Even though there are people who now already think they ...)
Anyway, just a few more days.