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Estimating battery capacity based on Supercharging readout

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
9,644
11,554
San Diego
Thanks for the info, though it sounds like the info is inconclusive so far...
Perhaps at some point you’ll visit a pay-per-kWh charger.

I’m not surprised to hear there is a discrepancy between miles added displayed and miles actually added, transiently, but I wonder for a longer charge (so more significant digits to work with) or with accessory use (say heat) on whether the difference becomes larger. Something to play around with.

In the meantime, would still be interesting to know what your trip meter Wh/rmi works out to be.

As far as I'm concerned we're always talking about the number available to us as drivers from "100%" displayed and charge stops, down to either "0%" or "car shuts off below 0%
Agreed. As you said, it is usable capacity that matters (some of which may exist below 0 rated miles, so it is not expected that a 100% to 0% discharge would equal the EPA discharge amount).
 

darth_vad3r

Well-Known Sith
May 6, 2019
1,574
1,123
Canada
In the meantime, would still be interesting to know what your trip meter Wh/rmi works out to be.

Yes, me too, unfortunately I only have good data on short trips.

For any long trips the data I have includes a bunch of time in Park with or without AC on for several stops of varying length (including stops where vampire drain or Sentry Mode may have come into play) because I didn't diligently record data points on those longer trips on any individual legs.

I initially only started recording data to track what % was mapping to what km or mi displayed since I noticed it mapped exactly to rated for the first few weeks or first 1000mi or so, and then it dropped off (I expect this is pack/BMS calibration and not actual degradation so quickly, but who knows. I know the numbers aren't to be trusted anyways really, but right now they are usually showing about 2% lower than 'rated').

Anyways, I started recording more datapoints because hey why not, and I've now got more stuff to "play" with :) I've been tracking the last x, y, z km average efficiency + range estimate numbers too. Not sure yet what I'll do with those, but just getting an estimate for kWh remaining and dividing by the displayed % seems to give me ~51.5 kWh of estimated "100%" display. Unfortunately 3 inputs (Wh/km, km, %) leads to a wide margin of error.

I have a lot of commute trips though ... I plan to analyze that at least and try to come up with a range on the Wh/km (Wh/mi).

We are talking about (trip km) * (trip Wh/km) = (trip Wh), then "Wh/rkm" (rkm being 'rated km' the number we think is 'rated' anyways that's shown on the display gauge), right? (or equiv in miles)

For this data to be accurate I'd want to have snapshotted the start/end rated/displayed km/mi/% immediately before/after the 'trip' info is recorded and not include any intervening time spent in Park. Unfortunately, even some of my commutes have that due to stops at a drive-thru and pause to eat, LOL :)
 

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
9,644
11,554
San Diego
We are talking about trip km * trip Wh/km = trip Wh, then Wh/rkm (rkm being 'rated km' the number we think is 'rated' anyways that's shown on the display gauge), right?

For this data to be accurate I'd want to have snapshotted the start/end rated/displayed km/mi/% immediately before/after the 'trip' info is recorded and not include any intervening time spent in Park. Unfortunately, even some of my commutes have that due to stops at a drive-thru and pause to eat, LOL


Yes that is the formula. The opportunity to be able to measure it accurately for myself is pretty rare. You clearly understand the various pitfalls (like time spent in park). One other thing is to do a pre-drive before doing the measurement - because a cold battery could throw off the delta in rkm. But that is not too hard - you can actually reset the trip meter right as a rated km “rolls over”, after a few miles of warmup (maybe quite a few if it is chilly), and if you keep an eye on the rated km as you are nearly finished with the drive, you can actually get rid of some of the rounding errors on the rated km display and get part of an additional digit of precision on that part. (Because you’ll know approximately how close you are to rolling over to the next rated km.). Another final caveat is to put the car in park to finalize the measurement results in the trip meter (it does not continuously update but putting it in park forces the final numbers to be displayed).

Don’t crash into anything though with all this distraction - not worth it!
 
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AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
9,644
11,554
San Diego
Unfortunately, even some of my commutes have that due to stops at a drive-thru and pause to eat, LOL :)

If you can get away without putting the car in park, safely, you can gather this data on your commutes, even with the drive-thru and eating (assuming you eat in the car of course). Good efficiency is not required for this test. I guess it's possible that some accessory energy is not counted (there is the issue of why the trip meter seems to be a few % off - one is the reserve but that is probably not the only reason), which would introduce some error (and greater error if the dataset is from a drive where a lot of time is spent stationary (but not in park)), if that is actually happening.
 

Zoomit

Active Member
Sep 1, 2015
2,196
4,114
SoCal
This post by @wk057 is worth reviewing for this testing: Tesla's 85 kWh rating needs an asterisk (up to 81 kWh, with up to ~77 kWh usable)

"Next, any power consumed while the high voltage system is engaged is accounted for. This is only not true for around firmware v6.x and below, which would have no bearing on the Model 3. This data isn't accounted for properly in the energy GRAPH still, in all versions on all vehicles, but it is properly accounted for in the trip meters.

(Detailed technical explanation of the above: The BMS keeps a total lifetime counter of kWh charged and discharged. Since v7.0, the trip meters just snapshot the odometer and these two values when reset. To get the trip data, it just does the math and displays the difference. Since the battery updates the kWh values for both counters any time the HV system is engaged, the trip meters will always have updated information, in motion or not. This is why the power usage in the trip meters will still be updated even if you reboot the system while driving.)

HOWEVER... the accuracy of this is greatly exaggerated at lower power usage. The measurements Tesla makes of charge/discharge is very accurate above 10A or so. Below about 10A (which is 3-4kW) the accuracy can vary greatly, as poorly as +/- 100%, and worse when near zero. So basic vampire drain while sitting, or mild cabin conditioning while idle/off, can be read as way less or way more usage than it really is (S, X, and 3). This gets even more exaggerated when you have slight inclines/declines on a trip where the power consumption/regen ends up near zero often. The sum of these errors can come out to huge variances in the trip meters over a long period."
In summary, avoid low charge/discharge conditions, such as being stopped in Drive, slow speeds, traffic that causes frequent light accel/decel or hilly terrain.
 
Last edited:

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
9,644
11,554
San Diego
Next, any power consumed while the high voltage system is engaged is accounted for.

Definitely don't want to wade into that thread...but by the above I assume he meant as long as the car is in drive.

Point taken that the accuracy is better the higher the load though. Makes sense in any case to get a higher signal (a higher consumption type of drive) when trying to measure the Wh/rmi (or Wh/rkm) constant. So probably best not to gather the data while sitting in the drive-thru in drive, after all...
 

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