You've misunderstood how it works. The number of given cells dictates the capacity, regardless of how they are configured. Cells in parallel have their Ah added together, while cells in series adds the voltage together.
Say you have 10,000 cells with 4V and 2.5 Ah. You can configure them in one long series, giving you a pack with 40 kV and 2.5 Ah. Or a capacity of 2.5 Ah * 40 kV = 100 kWh. Or you can wire all the cells in parallell, giving you a pack with 4 V and 25 kAh. Or a capacity of 4V * 25 kAh = 100 kWh.
The voltage is selected not because the capacity or power is different, but because some voltages are easier to work with. Lower voltage is better because it's less dangerous and you need less insulation on the wires, higher voltage is better because the resistive losses will be less, and you need less copper to conduct a given power. 400-ish Volts is a good compromise.
I would expect the Model 3 to have a voltage similar to the Model S.
--- Authors note: I screwed up on the math. Yggdrasill is correct. I have left my post for completeness. ----
Actually I understand quite well how batteries work and how packs are designed. My point is that given a small volume of space (Model 3 is smaller than a Model S) it makes sense to drop the voltage to make room for more parallel cells to keep the capacity up.
So for any given volume of space you can have X cells. Now you must balance voltage or wH (serial vs parallel cells) when you design your pack. So given a space constraint in the model 3 Tesla might choose to lower their voltage for more parallel cells in the pack (which gives greater range). This will drop the performance of the car since the amount of work that can be extracted from the pack is Voltage*Amperage.
WRT to supercharging: As long as the supercharger can match the pack voltage then it can be charged at 1,5C. We don't know what the lower voltage limit is on a supercharger but it could be lower than what we have seen to date. Granted that is pure speculation.
You are right about less resistance with a higher voltage which may also contribute to their final choice. It will be fun to see what rolls of the line.
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