SO, does anyone know FOR SURE whether range and percent have the same accuracy? In my mind, they SHOULD be the same accuracy, but my mind didn't design the hardware/firmware/software that leads to the 2 numbers. (Presumably one is a simple calculation from the other ... but who knows for sure?) Thanks.
I can say for sure they are not
equivalent. A % is just that - it is the formula given above by
@hugh_jassol, so it's simply a measure of your usable energy remaining (that is your energy not including the buffer), relative to what the maximum energy you could have remaining (not including the buffer), according to the BMS. Meanwhile, the rated miles are equivalent to available energy (above the buffer) at any time, and not related to the maximum energy remaining. They are not equivalent.
Accuracy? Technically accuracy is the difference between the measure and the actual value...in that respect,
once you know your maximum energy you can have, the two (% and miles) are basically the same accuracy (the rated miles have a bit better resolution though). However, your maximum energy available according to the BMS can and does change over time.
Summary: So 1% when your car is new will not necessarily represent the same amount of energy as 1% a year later will represent. However (unless the constant is changed, which is rare), one rated mile will always represent the same amount of energy for your specific car, no matter how old your car is.
Here is a later version of the linked post above, which may be helpful:
2020, 2019, 2018 Model 3 Battery Capacities & Charging Constants
I'd actually recommend sticking to the indicated "Trip Meter" constant in this spreadsheet, since that most directly represents how the trip meter behaves relative to displayed rated miles, and the energy content of each rated mile. You can really only directly relate the
charging constant to energy remaining
easily at SoC of 100% (Max Energy Including Buffer = Rated Miles@100% * Charging Constant), due to the buffer, which screws up the math and makes the formula non-intuitive when trying to use the charging constant at other SoCs...
Available Trip Meter Energy Above Buffer = Rated Miles Remaining * Trip Meter Constant
I'm not going to delve into existential questions about how accurate the BMS is or whether it really represents a good measure - mostly because it's fairly hard to measure against anything, and I really don't know if it gets out of whack over time. To some extent you have to draw these conclusions on your own with your own observations. All I can say is that I generally see that the trip meter seems to read fairly consistently, for a given journey under a given set of conditions. That is to say, the trip meter seems to measure energy used precisely. Furthermore, no matter how widely I cycle my battery (5% to 100%), my rated miles at 100% have always been amazingly consistent (and on a downward trend which has now leveled off).
As far as the rated miles/BMS goes: One way you can approach this, if you have an energy measuring charge meter, is to monitor your consumption during a single controlled
large charging event, at a particular charging wattage, and log the number of miles added and the energy from the wall. It needs to be a well controlled charging event (no opening the door to get the heat going, etc.) to get consistent charging efficiency. And then track the behavior over time. My feeling is that if it takes 84kWh from the wall to charge your battery from 0->100% when new, and then when your rated range shows 300 miles rather than 322 rated miles at 100% (or whatever) a year later, and it then takes 78.5kWh from the wall to charge from 0 ->100%, that's an indication that the BMS is actually pretty good at measuring available energy and represents a real change in capability (capacity) if it shows that behavior (it of course could indicate a software change improving charging efficiency too...at that point you'd have to compare to someone else's vehicle with a different number of rated miles available)! I've got some historical data on this from Chargepoint & Stats, and efficiency is amazingly consistent for controlled charging events over a wide range of durations, where I did not use preheat.