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EPA Model S Charging Efficiency

Discussion in 'Florida' started by GeekLad, May 1, 2015.

  1. GeekLad

    GeekLad Member

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    Using the EPA efficiency stats for the Model S, you can derive the charging efficiency.

    Model S EPA Ratings.png

    If you do the simple math of calculating the Wh/mi by dividing the battery size by the range, you get 288 Wh/mi for the 60 kWh and 321 Wh/mi for the 85 kWh. Those figures make sense, and if you drive relatively conservatively you'll make those numbers in the real world.

    When you look at the kWh/mi figures, on the surface things look a bit wacky. For the 60 kWh they calculate 35 kWh/mi (350 Wh/mi) and for the 85 kWh it's 38 kWh/mi (380 Wh/mi). However, those figures are calculated wall-to-wheel, which accounts for efficiency losses in the A/C to D/C conversion.

    By dividing the calculated the battery capacity Wh/mi by the EPA wall-to-wheel Wh/mi consumption, you can derive the charging efficiency. For the 60 kWh we get an efficiency of 82.4% and for the 85 kWh it is 84.4%. I have my doubts as to whether or not one vehicle is really more efficient at charging than the other, as the charging components of the vehicles aren't any different.

    The difference is likely due to rounding the calculations to the nearest kWh/100 miles instead of going out one decimal place (or calculating in Wh/mi). If the wall-to-wheel Wh/mi of the 60 is more like 345 Wh/mi and that of the 85 is 384 Wh/mi, you'd only have a difference of around 0.1%, and their charging efficiencies would be around 83%. If you want to use this to estimate your wall-to-wheel energy consumption, just take your driving kWh consumption from your trip meter and divide it by 0.83.

    I would imagine the EPA tests were performed using single-charger configurations with the mobile connector. From what I've read, using the HPWC with dual chargers has less overhead and is a bit more efficient. If you have dual charges and a HPWC, you're likely getting slightly better MPGe than what's on the sticker.

    Has anyone out there put a meter on their charger to calculate the real-life efficiency? If so, how does it compare to the 83% EPA efficiency?
     
  2. Larry Chanin

    Larry Chanin Model S Perf Sig 1055

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    Hi Jorge,

    From the Tesla specifications web page:

    Larry

    - - - Updated - - -

    That should say "For the 60 kWh they calculate 35 kWh/100 mi (350 Wh/mi) and for the 85 kWh it's 38 kWh/100 mi (380 Wh/mi)."

    Larry
     
  3. apacheguy

    apacheguy Sig 255, VIN 320

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    Yeah, 83% seems low for charger efficiency. And I don't see why it would vary between the 60 and 85 kWh packs. They use the same charger after all.
     
  4. GeekLad

    GeekLad Member

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    Seems like an odd way to describe efficiency. However, being stated in that manner it would lead one to believe that the average efficiency is lower.

    Oops! Indeed it should.

    It does seem a bit low. If it is actually higher than that, then the MPGe figures could be slightly understated.
     
  5. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    I think that the EPA number is all in, including the Vampire drain. The Vampire, with typical settings, drinks almost 1 kWh per day which really hurts overall wall to wheel efficiency.
     
  6. GeekLad

    GeekLad Member

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    Vampire drain wouldn't push up the kWh/100 mi very much though to make much of a difference in the kWh/100 mi measurement. If they factored in a full day's worth of vampire drain, 1 kWh/208 mi would come to 0.48 kWh/100 mi on a 60 kWh pack, and 1 kWh/265 mi would only be 0.37 kWh/100 mi on an 85 kWh pack. Even if it was factored into their calculations, it would be a rounding error to the nearest kWh/100 mi.
     
  7. Mookuh

    Mookuh Member

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    Doing this, you assume that the Model S uses its entire battery capacity - while I don't have a source to quote, it's unlikely they would do that. All manufacturers build in a cushion of sorts to increase battery longevity, usually both at the high end and the low end (though I believe you can range-charge the Model S to get to a true SoC of 100%). The 85 kWh figure is probably not equal to the actual *useable* amount of energy storage the battery provides. Though I might be wrong, as I said, I have no source, but it would be tricky for Tesla to meet their battery lifetime goals if they allowed the battery to go all the way from 100% to 0%.
     

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