(Moved from the
2017 Investor Roundtable Discussion thread, where it was rather off topic)
So there's been some discussion (confusion?) from several folks regarding what the new 2170 cells bode in terms of additional capacity within the Model 3 pack footprint. Specifically,
@CanadaEV stated:
CandaEV said:
I did see Buzz's post. I also saw the same calculations/assumptions other places. He, and others, have stated, 'The increase in diameter will be exactly offset by the reduced number of cells that fit in that area.'
I'm not sure who first put that idea out there, but it seems to suggest that there is no gain in energy density because the increased cell dimensions of the 2170 mean fewer cells, exactly cancelling out its increase in energy. It's an idea that has been repeated, apparently on the assumption it has merit.
Well, this is extremely unlikely to be the case. The 2170 has 47% greater volume, and between 30% and 50% greater energy density, depending on your source. Back in 2016, JB Straubel predicted 30% increase in energy density, so let's go with that, although it could well be higher now. Therefore the 2170 delivers a compounded (1.3 X 1.47) = 1.91 greater energy per cell when you consider both added volume and better chemistry.
Yes, there may be fewer cells, but not 91% fewer 2170 cells. The problem is that Buzz's assertion equates the increase in the area of the of the 2170 in the pack footprint with the increase in volume. He doesn't realize that volume increases as a cube whereas area increases as a square. Additionally, 2170 cells are 7.6% higher.
If you look at the actual cell positioning in a pack, they are not touching. They have cooling space between them. It is unknown how much that space would need to be increased, if at all.
It is doubtful you will need to reduce the number of 2170 cells in this pictured module by 91%. The actual area increase of the diameter of the 2170 compared to the 18650 is 36%. It is unlikely the number of cells would be reduced by that percentage. Even if it were the case, the increase in energy per area footprint would still be over 50%.
Let's settle this "area occupied by varying circles" confusion.
Here's a model approximating the 444 cells of a Tesla module. I made it 14 rows in one axis as a constant (as per the pic above). The module doesn't have a constant number of cells in the other axis in the pic, so I made the containing rectangle bounds such that they would hold the number of cells closest to the 444 count.
This results in a rectangle 243mm X 533mm that can hold
442 cells
18mm in diameter in a staggered pattern:
That same rectangle can hold
319 cells
21mm in diamter:
The total circular area of a 21mm cell is
1.36x more that of an 18mm cell.
The total # of 21mm cells that fit in the same module is
1.38x less than that for 18mm cells. Essentially the same relative percentage.
The total area represented by 442 of the 18mm cells:
112,418 mm^2
The total area represented by 319 of the 21mm cells:
110,433 mm^2
The difference is less than
1.8%. Again, essentially the same total area. The larger cells actually represent the slightly lesser volume... but by the time you figure the amount of casing material as opposed to actual cell chemistry, overhead for cooling, other pack logistics, etc... it ends up a wash.
So the
ONLY real volumetric difference for the new cells within a given pack footprint is the ~7.6% due to height. Any other gains with that same size pack will be due to cell-level energy density gains, or pack layout changes (which has already happened once with the 90 & 100 packs... but the above relative calculations hold for the same overall area occupied within the pack)
