Sorry, this is going to be long and a bit detailed.
This is not unusual. It APPEARS they just have not adjusted the mileage to be correct for your car (wrong constant). It looks like you just have the 390@100% display (should be visible on the slider in the app if you do that, assuming it works that way for Plaid) - this is appropriate for the 19" version. Would be great to confirm this though. This happens ALL the time with Tesla at the beginning of a new model introduction. If you have 21" wheels (from your Monroney), you should expect a range (constant) adjustment at some point. But they may also unlock capacity or make efficiency changes so who knows what your final rated miles at 100% will be after things settle out (it could easily be more than 348 miles, since that is not an official EPA result yet).
From Tessie and your data, your constant is like all others captured, at 35.2kWh/143rmi = 246 Wh/rmi. (The 143mi is not from your original post, it is from your follow-up graphs, which are more accurate.) If you capture a picture while supercharging, you'll get the same constant of 246Wh/rmi, as described earlier (kW / mi/hr) - notwithstanding intervening software updates, which could change the display constant.
So the calculation I referred to (which has always worked, to my knowledge before) gives: 390rmi*246Wh/rmi = 96kWh (this is including the buffer; the way the calculation works). This gives the value of the degradation threshold, or the nominal full pack, whichever is less. In this case I definitely think it is the degradation threshold!
My current understanding is that the “kWh added” values displayed by the car during a charge session are not correct (for any Tesla). When the car nominal full pack capacity is below the degradation threshold, they’re always off from the actual kWh added to the battery by the buffer %. (This can be readily confirmed via readbacks from SMT; for example for Model 3 below the degradation threshold (showing rated mile loss), you'll find the SMT delta in nominal remaining will be 4.5% less than the car says is added during the session. You can also (probably) confirm it by carefully integrating the kW from the supercharger over time and compare to what it says the kWh added is - but that's harder since you have to do trapezoidal integration frame by frame, lol.)
Anyway, these kWh added values are simply a calculation from rated miles added * charging constant. That's what they're always equal to. It's not a kWh meter measuring actual energy added to the pack. (You really need SMT or similar to access that value easily.)
Ok, so what is (slightly) surprising to me:
1) The 96kWh degradation threshold (not your full pack capacity in this case, nearly certainly, since the car is new) is considerably lower than what the Monroney sticker implies (103kWh as previously discussed).
In and of itself, it's not unusual for this calculation to come up with a slightly lower value than the actual "design" pack capacity for a new car - but normally it is just by a 1-2kWh at most - not 7kWh (for example 2021 Model 3 Performance has a pack at about 81.5kWh, with a threshold of 80.6kWh). In any case, Tesla can pick the constant however they want - but to the extent it results in a degradation threshold much lower than the actual nominal full pack, it just indicates that the rated miles will contain more ACTUAL energy than that constant actually indicates, until the ACTUAL capacity of the pack drops below that threshold. (This fact has been verified on the 2021 Model 3 Performance recently, and it's long been speculated.)
Going back to the above discussion about "added kWh", this means that each rated mile (not to be confused with the displayed rated miles; the displayed miles are lower energy content than the "charging" rated miles, by the buffer %) on your vehicle right now may well contain more than the 246Wh implied by the constant. So that 35.2kWh shown on the instrument panel above may well have been more like 103kWh/390rmi * 143rmi * buffer adjustment (say 0.97) = 36.6kWh truly added to the vehicle (what you would see in SMT as the delta, if it were connected).
2) The constant is very low right now (246Wh/rmi). For perspective, the constant on the new 2021 Model 3 Performance (with heat pump!) is 255Wh/rmi. So I do not believe that 246Wh/rmi is a reasonable value for Plaid in spite of all of the improvements (it has a larger frontal area and probably higher drag in spite of its lower Cd, and it's a heavier vehicle - though it will be very efficient I think). I expect a value (for the 390-mile vehicle) to be closer to 103kWh/390mi = 264Wh/rmi (and similarly 296Wh/rmi for the 21" version with 348 rated miles - though this may end up a bit lower if the EPA test does better than 348 miles, which it very well might). These values are pretty excellent for such a large vehicle, though!
Anyway these constants are kind of arbitrary in the end, and they really don't matter for pack capacity, or anything, really - constant goes up, range goes down, and vice versa, for a given pack size. They're just a unit of measurement.
Here's what you could do to help continue to answer this question about actual pack capacity:
The ACTUAL energy content per displayed rated mile can be measured:
1) Switch to Energy Display -> Distance Mode (Not Energy - so miles (or preferably km actually - 1.6x better - can switch back to miles for the actual drive) shows in the instrument cluster, not %).
2) Take a decent length trip (using over 100 rated miles would be great, but even 30-50 miles (or 100rkm!) would be ok in this case), without stopping. Log the rated miles at the beginning of the trip, before starting moving, exactly. (Picture #1)
3) At the end of the trip, IMMEDIATELY after putting the car in park, log the rated miles. (Picture #2) Simultaneously, again immediately after putting in park (forces update to the trip meter), log the trip meter "Since x:xx" details, exactly. If it's a shorter trip, try not to end the trip with too much regen (drive some surface streets and come to a stop on a slight uphill). (Picture #3)
You post pictures and do the calculation: (Trip Distance * Trip Wh/mi) / (Rated Miles used)
I would expect for your vehicle, right now, for this calculation to come up with:
264Wh/rmi * 0.96 = 253Wh/rmi (approximately). (Again, with just 30-50 rated miles used for a shorter trip, we'll only have 2 sig figs, so we can only say it will be between 250 and 260Wh/rmi (which would still be helpful to dial in on the actual pack capacity). )
However, I could be wrong, and this value is a) completely dependent on the Monroney sticker values being correct and predicting about 103kWh of capacity, and b) it assumes a 3% buffer, and c) it assumes 1% heat losses.
So, if anyone has a chance with a Plaid to do this procedure, it would help shed light on this. It's super easy to do if you happen to be driving around. Just have to be sure to carefully log the starting and ending rated miles and you can't do any stops (since you lose energy while parked). This is essentially equivalent to driving a really long way and comparing % use to kWh use on the trip meter as people often do - but using miles (or preferably km) rather than % is more accurate, and this allows a shorter trip.
Your data so far has shown that the degradation threshold as currently set is 96kWh. But the pack capacity is likely higher than that, and to dial in on that value without SMT, the only option is to do this metering test. In the end this metering will only give us the energy available above 0%, and for now we'll have to make educated guesses about the buffer size (3% seems reasonable).
That will be nice to see. Should provide some confirmatory data.
Summary:
I realize some of the above is confusing and counter-intuitive, and it requires familiarity with how the Tesla energy displays operate to follow along, probably. Tesla's energy display stuff is a little weird, but so far it is very consistent! It's just my current understanding, though, so I'm open to corrections or thoughts.
1) Evidence points toward unlocked nominal pack capacity right now of ~103kWh on Plaid. However that is based ENTIRELY on the posted Monroney, which is not final. Everything I am assuming hinges on this. Really the only firm data we have and it is preliminary.
2) Degradation threshold is (currently) around 96kWh. This is the nominal full pack capacity where range loss will start to show (a long way off).
3) Even the 21" vehicles seem to be displaying 390 miles of range (contradicts Monroney)
4) For a new vehicle, I expect each displayed rated mile to contain 256Wh, which will show as 253Wh on the trip meter. That would imply 99.8kWh available above 0%. (And I'm assuming a 3% buffer which would take things to 103kWh total.)
5) Overall confidence level here on all claims is quite low.
6) I expect updates from Tesla in future to adjust ranges (constants) of vehicles appropriately, and they may be accompanied by changes to range from 348/390 miles, and also (small) changes in the unlocked available capacity. I also expect a change (increase) to the degradation threshold (this does not adjust the buffer size or the actual pack capacity, but it's the only way for Tesla to make the rated miles AND the constant simultaneously be more representative of EPA results). In the end I expect the threshold to be just a couple kWh below the "full pack when new" value. Might be months though before this happens.
I’m not looking at these numbers for the charge rate - just the constant. But there is really no guarantee the constant is correct right now for this display calculation (it eventually will be). To me it looks like it is adjusted for a 420-mile car (103.3kWh/246Wh/rmi = 420 miles), but I could be wrong.
I kind of think this 246Wh/mi and 103kWh capacity is kind of an Easter egg to work out to 420 miles, but the display not actually showing 420 miles at 100% (because the degradation threshold is set to 96kWh rather than 103.3kWh as would be required) makes it kind of a lackluster Easter egg.
I guess we'll see.
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