Um. That's a little odd.
ADC's come in various "bit" sizes: 8 bit, 12 bit, 16 bit, etc. They'll typically have a precision reference voltage (band-gap diode reference or something) hooked up to the top of a resistor chain and ground or something at the bottom of said chain. An 8-bit ADC can measure 256 possible values, plus or minus the usual inaccuracy. But 8-bit ADCs are relatively uncommon; the cost difference between an 8-bit, 12-bit, or 16-bit ADC is minimal, so people usually go for the 12-bit (4096 values) or 16-bit (65536 values). The bigger bits are slower, typically, than the smaller bit types, but we're talking typical sample rates of 1 MSa/s and up for the cheapies, and I'm pretty sure that a BMS doesn't need to know state of charge on a microsecond-by-microsecond basis. (Now, if you want to sample along at 10 GSa/s, then you might be restricted to an 8-bit ADC, but that's a different story.)
Further, I suspect you're not going to find very many people willing to sell you an ADC that takes in 800V directly; that's a good way to end up with silicon vapor. Typically, one uses a resistor divider so the maximum input voltage to the ADC is 80% of the maximum for the ADC. (And the accuracy of the measurement is directly related to the accuracy of the resistors; in an application like this, one would need to use matched resistors, built on the same substrate, around .1% accuracy, typically, without spending too much money.)
So, to measure 800 V, we'd divide this down to 80% of the maximum of the ADC, then measure. So, using a 12-bit ADC, we'd have 800/(4096*0.8) = 244 mV/step; with a 16-bit ADC, that'd be 800/(65536*0.8) = 15 mV/step.
First off: in my extremely uneducated BMS opinion, 244 mV or 122 mV (for a 400V system) seems too large to keep track of the voltage on a battery chain; for sure, if one was monitoring the overall battery voltage, 16-bit and 15mV (or 7.5 mV for a 400V system) seems to be the way to go. Now, if I can figure this out without an envelope, so can anybody else.
Second: I dunno if Porsche (or Tesla, or whomever) is running every single battery cell in series (unlikely in the extreme) or all in parallel (impossible if one wants high voltage), or some combination of series and parallel (a lot more likely), the individual voltages that the BMS would be monitoring would be individual cell voltages or smaller groups of cells, then do some math to add it up to the overall battery pack voltage (if desired). Remember: They're trying for the amount of charge on cells, I would think. But that means that the voltages they're measuring would be, for the Porsche, 800/N, where N is the number of series groups.
Which means that my step sizes up above would also be divided by N. So, for example, there's 10 bunches of cells in series, with 80V across each bunch, then a 12-bit ADC on that bunch would have 244/10 = 24.4 mV/step. Which is probably getting down to the point where a 12-bit ADC would be doable.
Conclusion: This ain't rocket science, at least not for a practicing EE. So, while it sounds like a nifty idea (ADC mistake!), it's probably not that. Sorry.
