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How does the car calculate Wh/mi (twice)?

Discussion in 'Model S: Battery & Charging' started by aviators99, Feb 13, 2013.

  1. aviators99

    aviators99 Model S - R140

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    So, I've been seeing some odd inconsistencies on the trip computer, and have been paying extra attention to it.

    In at least one thread it was calculated that the Wh/mi to achieve rated mileage is somewhere around 304. Sometimes that seems to be pretty close, and other times it seems that I need closer to 285 to get rated mileage. This brings me to my first question, which is: What is included in the calculation for the Wh/mi on the trip computer? This is also the same question: What is included in the calculation/measurements for kW/h used in the trip computer? (Since the answer to the Wh/mi is really a calculation of the kW/h divided by miles, and I'm not questioning the calculation of the miles (for now)). For example, perhaps the calculation of kW/h used is only based on drivetrain usage and not accessories? That might explain differences I've seen. Or does it measure directly from the battery? Has anyone left their car unplugged for a while and noted that the kW/h in the trip computer (since plug-out) has gone down? I keep meaning to do that.

    Today I was monitoring the energy meter (on the center display), which was set to 30 miles. I watched my trip computer and waited until it was at exactly 30 miles. There was a 10+ Wh/mi difference between the projected (based on average) on the energy meter and on the trip computer (283 vs 295). Why would these be different? I noted it between 29 and 31 miles and the difference was there the whole time.

    For completeness, I ask the specific question: What is included in the calculation for average Wh/mi on the energy meter?
     
  2. ChadS

    ChadS Petroleum is for sissies

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    I have no answers for you; only more questions.

    When I compared the car's Range chart display on a road trip for wh/mi, and then compared it to actual consumption based on Rated Miles consumed...it just didn't seem to line up. I have no idea why not.
     
  3. ken830

    ken830 Model S (Res#P12,447)

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    I was also thinking about this very thing today. I also have no answers but would like to throw in one more question:

    What about vampire load, pre-heating/cooling, and other usage when the car is "off"? Are those included in the calculation? Meaning: Is there always a spike in Wh/mi when you first get into the car and start driving after having it sit unplugged for a few hours?
     
  4. tezco

    tezco Sig P85

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    I almost always see my remaining miles drop like a rock when I head down to Denver, and often return the same 35 to 40 miles with the remaining range not changing, or even increasing. No significant change in altitude, different driving habits, winds etc. Car usually sits for several hours in Denver, and it is usually colder by the time I head home, so I'm confused as well. (Never had the sleep mode software, so that also doesn't factor in.)
     
  5. kcveins

    kcveins delivery 2/7

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    No answers either, but how on earth are you getting wh/mile in the 200s?? I am lucky to get <380!

    update:
    Oops, I see that you are in sunny FL :)
     
  6. XrstalLens

    XrstalLens Model S P1327 VIN P01867

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    From my observations it definitely includes climate usage and other loads while driving. You can see this in heavy traffic on a cold day with the heater running - the Wh/mi shoots through the roof due to not traveling very far but using all that energy running the heater. I've seen it go up past 900 Wh/mi when I've been creeping along in a terrible highway backup.

    As far as I can tell, it seems to stop tracking data when the car is not in Drive. So energy consumed while parked is not factored in. The Instantaneous estimate is based on current energy usage vs. speed, but the average is done using only the data points it has accumulated over the previous 5/15/30 miles (which does not contain any data points from when the car was not driving).

    This is purely based on my observation of the graph, so I could certainly be incorrect on one or many points. Also, I have not done any serious observation of the trip computers, so I can't provide any insight on how those correlate to the energy graph.
     
  7. Beavis

    Beavis Signature 991

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    I am betting that if you were to ride a bike on the same route, you would figure it out pretty quickly. I've done it and eventhough it seems to be flat, it's not. The wind in the afternoon is usually coming from the South. A slight change in elevation and wind make a major difference on the bike and to the watts/mi. In my case, the drive into Denver is ever so slightly downhill and I get around 310 watts/mi. even in the cold. On the way home, it's much higher and my roundtrip average watts/mi. is about 350.
     
  8. aviators99

    aviators99 Model S - R140

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    This doesn't explain the difference I noted in my OP between the trip computer and the energy meter (when both are at 30 miles). I understand that you may not have been trying to explain it, but it underscores the issue.

    - - - Updated - - -

    I think this is the same question, not more questions. ;-)
     
  9. tezco

    tezco Sig P85

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    By flat, I mean that starting and ending elevation is similar. I'm at 5270 feet at the start, so about even with downtown. Some trips with a west wind, some with a north, and some with a south. Maybe the battery pack stays warm a lot longer than I'm giving it credit for.
     
  10. ModelS1079

    ModelS1079 Member

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    Ahh, dear NY Times...

    I had a beautiful ride today. Route 89 in New Hampshire - smooth, sunny, mountains glistening in the recent blizzard's snow. Allman Brothers on Slacker. And a little John Denver. Embarrassing, I know, but it was just right at the time. 115 miles to Boston, 333 wh/mile average, average speed over 72mph. Started out with 228 miles range, finished with 109 miles range. Another Gorgeous day in a Model S. Gee, I think I'll plug in tonight. Novel idea: NY Times, please take note...
     
  11. aaron.s

    aaron.s Member

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    Jim

    So.... are you going to join us on our recreation of the NY Times trip and debunking venture??? I think I may have Ben in on this one!

    Aaron
     
  12. aviators99

    aviators99 Model S - R140

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    I'm starting to think that perhaps the car is not calculating regen correctly. This is all speculative and gut feeling, but based on the stuff I put in my OP, and watching what happens when I get to the end of my long commute, which includes a lot of regen for a short period of time, the Wh/mi seems to go down way too quickly for the comparatively small number of miles.
     
  13. ModelS1079

    ModelS1079 Member

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    Aaron, just saw this. As you now know, thousands of miles away. Will be heading to DC with kids in April. so psyched you guys pulled it together.

    Avaitors99 I don't have any info on this. Will follow your comments/ responses.
     

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