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What am I missing?

Discussion in 'Model 3' started by gowarriors, Oct 22, 2019.

  1. gowarriors

    gowarriors Member

    Joined:
    Jun 13, 2019
    Messages:
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    Location:
    San Jose
    I have a LR RWD purchased in June 2019. I have 1622 miles on it so far. My efficiency is 257 Wh/mile. I charge my car almost exclusively at work and can see that I have used approx 600 Kwh to date for charging.

    Per my calculation, I should have only used 1622 x .257 = 416 Kwh so far. Where did the other 184 Kwh go? Phantom drain?

    Thank you!
     
  2. HelixSpiral

    HelixSpiral Member

    Joined:
    Oct 19, 2018
    Messages:
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    Location:
    Cincinnati
    Vampire drain and charging loss.

    The trip meter doesn't include energy usage while the car is in park.

    And you always get less energy into the battery than what comes out of the outlet.
     
    • Like x 2
  3. AlanSubie4Life

    AlanSubie4Life Efficiency Obsessed Member

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    #3 AlanSubie4Life, Oct 22, 2019
    Last edited: Oct 22, 2019
    What is your charging setup (how many amps, and at what voltage - probably something like 6kW?)? That will allow us to roughly gauge your charging efficiency and tell you more precisely where the "missing" energy went.

    Let's assume 6kW charging:

    For 6kW charging, you'll get about 93% efficient charging. So you've added 0.93*600kWh = 558kWh to the battery. (Per the charging screen in the car.)

    Of that you've used (on the trip meter!) 416kWh, so you're "missing" 142kWh.

    However, the trip meter reads ~4.7% low. That means you have actually used (referenced to the 93% charging efficiency number above, which is based on the charging screen information): 436.5kWh.

    Calculation: 416kWh (trip) * 234Wh (charging) / 223Wh (trip) = 436.5kWh (charging).

    So you're actually missing: 558kWh - 436.5kWh = 121.5kWh (charging kWh)

    You've had the car for about 120 days, so that is 121.5kWh/120 days = 1.01kWh per day (charging kWh)

    This is 1.01kWh/234Wh/rmi = 4.3 rated miles / day (using the charging constant of 234Wh/rmi here since the units are charging kWh)

    This is fairly typical. A little high but totally reasonable (well, reasonable is probably the wrong word ;) - I should just stick with "typical") given there may be some sitting around in the car when you're in park, etc.

    EDIT:
    Screwed this up...modified numbers above to account for trip and charging efficiency meter differences. Confusing point, but true...totally verifiable if you care to pay attention to the trip meter and charging screen. This is where the empirical 234Wh/rmi (charging) and 223Wh/rmi (trip meter) constants come from - these are valid only for the LR RWD.

    It is valid to only use the 223Wh/rmi constant and say the charging efficiency is only 88.6% (0.93*223/234), as a starting point, if you prefer. And assume the trip meter is actually reading accurately (not 4.7% low). But the final answer will be the same; it makes no difference.
     
    • Informative x 1
  4. gowarriors

    gowarriors Member

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    Location:
    San Jose
    Charging setup at work is 6kw (30A). Chargepoint station
     
    • Informative x 1
  5. AlanSubie4Life

    AlanSubie4Life Efficiency Obsessed Member

    Joined:
    Oct 22, 2018
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    8,089
    Location:
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    Ok. See above. Your results are pretty typical as far as I can tell.
     

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