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A manageable assembly size. A single module with 200 or 300 cells is pretty heavy and would need 2 hands to hold it, harder to move around and keep extra modules. At ~50g/cell, 200 cells weigh 10kg and 200 cells weigh 15kg. Smaller modules are easier to handle.
If a module fails, we need to replace the module. If we had only 4 modules in the pack, then we may only have the budget for a single spare module. Also, since there are more cells in a larger module, there is a greater chance that a cell within that module fails. Smaller modules means we can have more spare modules, and will statistically fail less often.
It is beneficial to make all modules exactly the same. Then we only have to keep 1 type of spare module, and any spare module can be swapped in for another. MSXII had 2 types of modules (and end module and a middle module) and we ended up running out of spare end modules, while still having a few spare middle modules unused.
More modules mean that we have more inter-module connections. This increases the number of failure point. Each inter-module connection also has a small resistance associated with it, so more connections means higher resistance in the pack’s current path, and more power loss (P = I^2 * / R).
If you compare the weight of the cells in a module to the weight of the rest of the items (cell holders, support brackets, cables, bolts, etc.) then the weight of the extra stuff in a larger module will be smaller per cell (LM = Large Module, SM = Small Module):
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