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Calculating and keeping track of my actual number of kWh usable for driving?

Discussion in 'Battery Discussion' started by marcmerlin, Jan 12, 2016.

  1. marcmerlin

    marcmerlin Member

    Joined:
    Jan 12, 2016
    Messages:
    24
    Location:
    Cupertino, CA
    Tesla does not tell you how many kWh your pack has that are usable for driving.
    But it gives you a battery percentage meter and the number of kWh you allegedly used.

    Question #1:
    Is the kWh used value you can display on the center console, the actual amount that came out of the battery, or the amount of power used by the motor (i.e. power out of the battery is higher)?

    I took how many Kwh I used to go uphill to a ski resort taking my battery from 95% to 20%, and expanded that value to 100% (57.4kWh/75% = 76kWh)
    Then I did the same on the way back, 65% to 4%, used 46kWh and expanded the Kwh to 100%, 46/0.61 = 75kWh
    I got 75 and 76kWh respectively, which are pretty close. But it seems like a low number for a brand new 90D. I've even read that 85 owners measured their usable kWh to also be in the 75kWh range, which worries me a bit

    Question #2: are there unofficial numbers for 85 and 90D that I can compare with?

    Question #3: was my measurement method incorrect, maybe because the percent meter is not accurate, or because kWh measures the motor power used and not the power in and out of the battery? Or maybe it doesn't count regen properly? (but that's irrelevant if I got the same error climbing 7000ft up to a ski resort)

    Question #4: in my blog post with longer details, this bit worries me:
    10:55: Left Manteca Supercharger: 0 miles with 95% (trip router claims I'll arrive with 29% battery)
    11:35: Passed by Lockeford Inn: 31 miles with 76% (losing 19% for just 31 miles was both weird and disappointing, by now trip router says I should arrive with 21% battery only)


    It feels fishy that I used 19% battery or 14kWh-ish for just 31 miles of simple freeway/highway driving not going uphill. It should have been around 10%. This leads me to believe that the percent meter may not be reliable?

    Post with more details and data:
    http://marc.merlins.org/perso/cars/post_2016-01-06_Driving-from-the-South-Bay-Area-to-Kirkwood-in-a-Tesla-Model-S-90D.html
     
  2. rabar10

    rabar10 FFE until Model 3

    Joined:
    Dec 3, 2010
    Messages:
    1,333
    Location:
    Indianapolis, IN
    I don't own a Model S but have done similar math with my Ford Focus Electric -- am finding out that usable capacity, especially in winter, does not match 'nameplate capacity' of the pack. Of course winter in the Midwest will be more severe than Cupertino...

    Hopefully others with personal experience from their Tesla vehicles will chime in as well re: data collection methods and results.
     
  3. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

    Joined:
    Jun 21, 2012
    Messages:
    7,019
    For your Q1, the answer is "it depends". My observation is that the kWh used value appears to take into account all power consumption when the car is in a driving gear - including cabin conditioning. However, it does not appear to take into account all power consumption when in park and/or neutral. I've had my car powered up and air conditioning on at a movie while the car was in neutral and parking brake set, yet upon driving away, the Wh/mi didn't reflect the spike of 2 hours sitting with A/C on.

    For your Q2/Q3: There have been people who have tried to get the best number possible -- e.g., unplugging at 100% and immediately driving away. IIRC, that resulted in just over 76 kWh of usage as reported by the meter on an 85 kWh pack. If we go in reverse for an 85 kWh single motor Model S, 265 miles * rated Wh/mi gives you a number somewhere around 75-80 kWh (there are differing thoughts on the rated Wh/mi).

    With that said, others have disputed that, though. Bottom line is that we really don't know.

    As for your driving observation, I've found the trip meter to be relatively accurate in the absence of a mitigating factor - temperature, wind, high speed. The trip planner doesn't seem to take temperature into account, so if you're driving in a cold environment, assume it'll use more than expected.
     
  4. marcmerlin

    marcmerlin Member

    Joined:
    Jan 12, 2016
    Messages:
    24
    Location:
    Cupertino, CA
    Thanks for the replies.
    I indeed didn't consider that my battery pack could give out less than its rated capacity if it's cold. Given that, that's enough to throw the rest of the calculations out.
    Also knowing that it doesn't count battery use when not driving is useful, albeit in my case it wasn't relevant since I was pretty much drivng the whole time. Still, good to know though.
     

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