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Trip meter energy usage incorrect

Discussion in 'Model X' started by MaxK, Aug 5, 2017.

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  1. MaxK

    MaxK Member

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    I recently completed a 2,000 mile trip and logged a lot of data along the way. P90DL Sig X.

    The error I am seeing is repeatable. On a non-stop leg, here is the data:
    Actual miles: 139
    Rated miles consumed: 155.9 (rated range at start of trip minus rated range at end of trip)
    The car is rated at 333 Wh/mi
    Rated miles consumed * Rated Wh/mi = 51.9 kWh
    Avg Energy Consumed (51.9 kWh / 139 actual miles): 374 Wh/mi

    I have attached the trip meter for this leg. It shows:
    Miles driven: 139
    Total Energy: 48.7 kWh
    Avg Energy: 350 Wh/mi

    48.7 kWh consumed to go 155.9 rated miles equates to a car rating of 313 Wh/mi. A car rating of 313 Wh/mi and a range of 250 miles would equal a battery of just 78.3 kWh.

    Thoughts? Is there a flaw in my calculations?
     

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  2. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    1. Rated range is just an estimate based on a calculation of how much energy was used. It's very hard to determine the SOC of the battery when running.
    2. A car rated at 333 Wh/mi does not mean every mile will use 333 Wh/mi or that any particular trip will average 333 Wh/mi. (I think perhaps I'm not understanding what your meaning here is.)
    3. The only real way to determine the actual capacity is to do a 100% charge, run the car till it no longer will drive, and then fill it up again measuring how much energy it took to fill. Of course, doing this is not great for the battery, so the only time you would actually do this is to prove something in court (like the Nissan owners did a few of years ago).
     
  3. MaxK

    MaxK Member

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    Thanks for the reply. I am not sure if you agree or disagree with the conclusion that the energy meter is incorrect. I have Xs so I am familiar with the system and 100% charging etc.

    This problem is repeatable and the energy meter total usage divided by the rated miles consumed equals 313 Wh/mi on all legs of the trip. It is too repeatable to be anything other than intentional. The rated usage is not 313 Wh/mi. If it was, the battery would have started out at 78.3 kWh when new (for a 90 kWh battery). The car is actually rated at 333 Wh/mi (333 * 250 miles range when new = 83.35 kWh battery capacity when new - leaving the buffer of 6+/- kWh).
     
  4. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    I don't believe that I've ever had entirely repeatable sections of a trip. Now my experience is with the S85, not the P90DL, but it now has has 94K miles on it about half of which are long distance trip miles, so I have a lot of experience with it. Different trip legs can be close, but they are never equal except by coincidence. If I understand correctly, you're saying that the Wh/mile in the "since last charge" section is always 313? That seems very weird to me.
     
  5. MaxK

    MaxK Member

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    No not all, what I am saying is that the reported kWh for Trip B (reset each leg) divided by the rated miles consumed equaled 313 on every leg. In the example provided, the total kWh consumed as reported by the Trip B meter was 48.7 kWh. The actual rated miles consumed was 155.9 (starting battery meter minus ending battery meter) and confirmed by Teslafi - note the miles driven only equaled 139. The math is pretty simple from there. The total energy consumed (as reported by Trip B divided by the rated miles consumed equals 313 (this is consistent on every leg of the trip).

    So the math says that the trip meter is saying that 48.7 kWh was used to travel 155.9 rated miles. Dividing the two equates to 313 Wh/mi as the rated usage of the car.
     
  6. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    Are you aware that the energy used when stopped isn't counted? (at least according to some folks).
    But actually, I'm very confused. You travel miles, not rated miles. Rated miles are basically EPA numbers, which are only relevant if you drive like the EPA five cycle test.
     
  7. animorph

    animorph Member

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    I'm not going to worry about the kWh numbers. They are kind of secondary, though maybe someone knows what the primary units (kWh, rated miles, joules) the car uses are. Rated miles are a little more relatable for me. The calculation of energy in the battery, with any units, is just an estimate.

    I would think the trip calculations would be doing the same rated miles calc (start - end) and multiplying by the kWh/rated mile factor to display the energy used in kWh. And I suppose it is possible they screwed up and used the wrong factor when doing trip calculations. Or they do something else like not including climate control consumption, or when the car is not moving. That just makes it a pretty fuzzy number to pin any comparisons on. Plenty of threads have noted discrepancies.

    If it shows the correct rated miles for your car at 100% charge you have the right battery.
     
  8. BrettS

    BrettS Member

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    I think part of this confusion comes from the fact that typically we don’t use range to figure out when we need to refuel a car. In an ICE car you know how much gas you have left by looking at the percentage of gas in your tank. A lot of ICE cars have a miles to empty indicator, but I think everyone who has used one knows that the number can vary a lot based on factors such as speed, wind, and hills. I don’t think anyone with an ICE car would look at an indicator that shows 100 miles to empty and plan to stop for gas in exactly 90 or 95 miles. It’s a nice tool to use to help get an idea of how far you can go, but people know you can’t trust them to be totally accurate.

    The range indicator on electric cars will also vary based on the same factors... the amount of energy a car uses (whether electric or gas) can vary considerably if you’re going uphil at 80mph or downhill at 40mph and the range indicator can only estimate based on the energy usage for the past 20 or 30 miles of travel, but if the driving conditions change significantly then it will be very wrong.

    I think electric car manufacturers have made the range indicators very prominent on electric cars because they want to help lessen range anxiety, but I think it also causes more confusion as people are tracking a number that can be constantly changing. A few weeks after I got my model S I set it to display the battery level in percentage and I think this feels much more natural. Once I made that change I never looked back.
     
  9. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    What I've found is that I hardly ever look at the range numbers. In daily driving there's never an issue because I start with a full daily charge every day. On a trip, I just check the trip graph to see how I'm doing compared to the estimate, and that's really only if there is some weather happening (wind, rain, snow). If I remember, I check when pulling into the next SC to see how I did (I forget to do this more than half of the time). So I don't worry about whether the numbers match anything or not. Most of the time I get better than Ideal miles anyway.
     

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