So Randy Carlson has posted another article on Seeking Alpha:
Tesla: Model 3 Update - What To Look For
In this article, he wonders if Tesla can get away with arranging the cells horizontally instead of vertically. In particular, he wonders if chemistry improvements might allow Tesla to get away with having only a single layer of horizontal cells in the packs. Given my calculations of module size above, we could then fit at most (940mm/70mm=) 13 by (330mm/21mm=) 15, or 195 cells/module (Randy estimates 220 cells/module).
This gives (195*8=) 1560 cells for the whole vehicle. In order to get even a 70kWh pack out of this arrangement (since we've heard rumors that this pack size does exist), then each cell must be capable of holding (70000/1560=) 44.9 watt-hours. This would be a HUGE jump from the 12.1 watt-hours contained in current 18650 cells.
Even a double-layer of cells would require 22.5 watt-hours to be contained within each cell. That
would allow the battery pack to be 28.5mm thinner (assuming a 0.5mm overlap in rows), which is something, but not a huge win. And this is at least within striking distance of the 18 watt-hours per cell that I calculated above, which assumes that the cell chemistry hasn't changed from the 18650 so that only the volume difference is important. But that would also dash my dreams of owning a P85D Model 3. We shall see.