I figure that if you have different-size cars with different battery capacities (and shapes of their battery cavities), then "Car X needs 10 battery modules, ca

I figure that if you have different-size cars with different battery capacities (and shapes of their battery cavities), then "Car X needs 10 battery modules, car Y needs 13" is simpler (in one sense) to manage than "I have car X, do you have any of the batteries made to fit in it? No, you only have Car-Y-shaped batteries? Whoops."