400 volt pack needs twice the current, the current per cell is the same as the 800V pack. Bottom line: there is no reason an 800V pack can charge faster than a 400V pack - unless the limit is in the charging cable.
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The same logic applies to power delivery. If the pack has 200 cells, and is delivering 200 kW, that's 1 kW per cell, and that number is independent of whether the cells are arranged as 400 or 800V.
BTW, the number of cells may be much greater than 200, but the same logic applies.
Motors: The difference between and 800 and 400 volt motor is simply more turns of thinner wires in the windings, or fewer turns of thicker wires. The power per volume or per kg is nearly independent of the operating voltage.
Wires in the car: a clear advantage for 800V. They can be thinner and lighter. That saves a few kg. Not insignificant. But not something that would have a notable effect on any performance aspect of the car.
Control circuits: 400V semiconductors are more common, less expensive, and can conduct higher currents with lower losses than 800V semiconductors.
Given that current at 800V are half as much, this is mostly a wash for the two possible choices.
Because the limit is, in a real world practical sense, in the charging cable. An 800V fast charger can ramp to higher power levels much more cheaply than a 400V high current charger for all the same reasons the cars benefit from 800V. less metal or cooling/station = more stations (ceteris paribus)
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BTW, the number of cells may be much greater than 200, but the same logic applies.
Given that current at 800V are half as much, this is mostly a wash for the two possible choices.