It's interesting that they're also not using the 4680 cells that they have been shouting about so much.Interesting they are using 18650 rather than 2170 format.
Since everything about the construction is based on the Model 3 design, rather than the original Model S/X, the longer 2170 cells should have fit in height - interleaving both bus plates on the top of the module instead of bus plates alternating top and bottom saves the height needed for the longer cells.
22S per module multiplied by 5 modules is 110S overall - a significant increase from the 96S of all 16-module Model S/X packs, but not a big deal. While 96S is by far the most common choice for modern EVs, the Jaguar I-Pace uses 108S, for instance.The Tesla Plaid pack uses 7,920 18650 cells arranged in 5 modules that have an 72P22S arrangement each. (total of 72P110S)
85 kWh Model S/X packs have used 74 cell groups (so 96S74P for 7104 cells total) and 100 kWh used 86 cell groups (so 96S86P for 8,256 cells total); this is down to 72P so even with more groups in series the total cell count is down from the previous 100 kWh pack. All 7,920 cells fit in the space previously occupied by 14 of the old modules (7,224 cells) because there is less space lost to gaps around modules.
The original Model S pack used seven pairs of modules, each pair spanning the width of the pack. The Model 3 uses only 4 modules, each spanning the length of the pack. This is in-between, with each module spanning the width (by itself, instead of a pair).Each module is 15 3/8" (390mm) long X 55 1/4" (1404mm) wide X 3" (76mm) high. If you include the coolant manifolds and lines, the width is 57 3/8" (1458mm).
An interesting feature is that it doesn't have the modules stuck up in the front where the original 16-module Model S/X stuck the extra two modules that didn't fit in the main under-floor area. Instead, that front area is used for ancillary equipment which was separate from the original pack, taken from the "penthouse" of the Model 3 pack.