Range calculation & mileage mis-calculation

[rick423]

I learned that my model 3 LR has a 75KWh battery.
Recently I checked the mileage calculation and found a big mismatch.
Example Discharge form 81% to 40% (41% consumption):
- The Odometer-Display says "26 KWh since last charge, 157km, 168W/km".
- My calculation, confirmed by the real amount charged, says consumption of 41% is equivalent to 30.75 KWh.
For 157 km this is equivalent to 196W/km. This is a substantial difference of about 15%
Do I have a misunderstanding, or how can I "trust" the odometer data?

 

[darth_vad3r]

I learned that my model 3 LR has a 75KWh battery.
Recently I checked the mileage calculation and found a big mismatch.
Example Discharge form 81% to 40% (41% consumption):
- The Odometer-Display says "26 KWh since last charge, 157km, 168W/km".
- My calculation, confirmed by the real amount charged, says consumption of 41% is equivalent to 30.75 KWh.
For 157 km this is equivalent to 196W/km. This is a substantial difference of about 15%
Do I have a misunderstanding, or how can I "trust" the odometer data?

What precisely do you mean by “confirmed by the real amount charged”?

a) 81% and 40% are estimates
b) even if they were accurate, there’s +/- 0.5% inaccuracy due to rounding
e.g. 81.499% (aka “81%”) down to 39.5% (40%) = 42% consumption
80.5% (81%) down to 40.499% (40%) = 40% consumption

Also, it’s Wh/km, not W/km... So 157km * 168Wh/km = 26,376 Wh = 26.376 kWh which matches your display of “26 kWh”.

For more fun and confusion over the estimates, check your km range remaining when you are at “81%” and “40%” and see if they both give you the same total capacity for “100%”, they probably won’t.

pps. I trust the odometer data 100x more than I trust the guess-o-meter’s guess at % remaining

 

[edigest]

"... confirmed by the real amount charged ..." may be a source of confusion. On an AC charge, not all of the electrons are going into the battery. The charger uses some of the energy, on the order of 15% to 20% if you're charging with 120V.

 

[rick423]

Thanks guys for your comments! To make it clear: The odometer data is sound in itself, like darth_vad3r says "So 157km * 168Wh/km = 26,376 Wh"). I would love to believe this data. What I say is, the Wh/km is not correct.
What I demonstrate in my example is: The percentage my car consumed is 81%-40%=41%. This is equivalent to about 31kWh (not 26kWh).
Even if I consider +-0.5 deviation (40-42% consumption), that is equivalent to 30.. 31,5 kWh.
When I charged the battery, it took about 36kWh to come back to 81%. This is pretty much the amount I calculated above, considering the extra for losses that edigest mentioned.
By the way, 15-20% charging losses would be REALLY bad efficiency, unless one uses air condition and/or other energy consuming devices while charging

 

[rick423]

Addendum: Of course that all is if the battery 100% is 75kWh, as bought. If 100% means 65kWh then the odometer is true but the charging loss is tremendous!!

 

[darth_vad3r]

Does your example interval of 81 -> 40% consist of any periods of time in park while on (in the car fiddling with stuff), or even some vampire drain overnight, or is this one single trip? (My reading suggests the trip meters only calculate while driving)

If it’s a single trip, it seems quite off, I would agree. If it’s two data points 3+ days apart you could have easily lost the difference in 2% per day of vampire drain ... or just a few hours of Sentry mode

 

[rick423]

Good idea, yes that might add to the problem. As far as I remember it was two days with about 1% drain each, no sentry no fiddling.
Coming days I will have the chance to drive a longer distance per day and re-charge from day to day.
I will check consumption again and report.
Thanks again!