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I think my car is lying to me...

Discussion in 'Model S: Battery & Charging' started by supratachophobia, Jun 1, 2018.

  1. supratachophobia

    supratachophobia Active Member

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    So try to follow along with this napkin math. I'm going to define some well known facts to start:

    A. 90D pre-refresh nominal capacity 85.8kwh
    B. 90D pre-refresh usable capacity 81.8kwh
    C. 4kwh brick protection, cannot use (A minus B)
    D. Watts per mile used to calculate EPA rated range for pre-refresh 90D, 290 watts per mile.

    So I've been bringing up with support this issue of disappearing range at the low end of long trips. It was suggested that this was just the BMS recalculating based mainly on pack temperatures lowering, impedance rising, and general tomfoolery of mysterious algorithms. However, I am seeing, both while driving and after short stops of 10-20min, the battery level drop by 4-6% in the blink of an eye. I only notice it when the battery gets down to the teens. It had proven disastrous on two long trips so far. 4-6% literally could make or break getting to the next supercharger since 4-6% is about 13-16 miles of range.

    Now here is the data from the BMS at 62k miles:
    A. Nominal pack capacity 79.1kwh*
    B. Usable pack capacity 75.1kwh*
    C. Full rated range 272-273 miles

    *These numbers drop understandably so by .8kwh when the pack is in the single digits.

    To get 272 miles, you need to multiply 290 watts per mile EPA by the usable pack capacity, right? Wrong. The only way to get to that number is using nominal pack size, of which there are 4kwh you could never actually tap into. So while the car says I can do 273 miles on a charge IF I consumer 290 watts per mile (which I can do on lots of long trips I take), my actual ability to travel is much less. How much less? Well, exactly that missing 4-6% the car says I have then suddenly says I don't.

    So to sum up, the car says I can go 273 miles. But if you do the math, I can only actually go 258 miles (at 290 watts per mile), and eventually the car realizes this, at the end, when I need it....

    Fun fact, if the 75.1 number is to be believed, and it should be because it's from the BMS, then that's 10% battery degredation in 2yrs on a 90kwh pack.
     
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  2. ran349

    ran349 Member

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    What were the BMS reported numbers when the car was new? 258 miles at 290 Wh/mi sounds reasonable, but you should never lose 4-6 % in the blink of an eye. Has that been the case from day 1 with your car?
     
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  3. ewoodrick

    ewoodrick Active Member

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    Battery remaining capacity is generally measured by battery voltage. And the difference between charged and empty is relatively small, so having great accuracy isn't easy. Battery voltage is not linear and therefore conversions between voltage and capacity are often off by a decent percent. Watch your cellphone, you will see that its accuracy is often pathetic.
    The car should be calculating remaining charge based on the battery voltage and the current average consumption. An average is not instantaneous and is taken over a period of time. I'm not sure what the time is in the Tesla, but it may be over 50 miles. On a 3 hour trip, the temperature is going to change, your average speed often changes, and the air density will also change.

    Bottom line? Only off by 4-6%? That sounds like they've done an awesomely great job showing you the numbers.

    Your job, don't take the battery to 0% and since you expect that the number may be off, just factor that in.

    Heck, the speedometer on my last car was off by a little less than 10% and that was within legal tolerance.
     
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  4. davewill

    davewill Member

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    The EPA number Wh/m may have been calculated from the range and nominal pack capacity not vice versa. The total range number is probably what was the actual result of testing.
     
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  5. supratachophobia

    supratachophobia Active Member

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    I'm not sure when it started. But I am going to start graphing it to see exactly where the calculation goes screwy. And BMS brand brand new was ~82kwh usable.
     
  6. supratachophobia

    supratachophobia Active Member

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    With the BMS logging I can see the temperatures. There just isn't enough of a change in pack temp/module temp to justify being so off in the calculation. As for the speed, I can pretty much nail 290 watts per mile for a couple hours. You got me on the air density.

    And no, they've done a terrible job showing me the numbers. The first number it gives you is one thing, then the car pulls the rug out from under you at some random point.

    The goal isn't to take the battery to 0%. The goal is to get to the next supercharger. And if the car says you have 11% remaining when you get there, and then all of the sudden you don't, that's bad. Especially in cold weather. I'm just saying that that 5% is for the unknown you may encounter on the way or at your destination. And if you don't have it, it can be nerve racking.

    Regarding your speedo, there is no legal tolerance for under reporting speed. That's why most speedos, like Tesla, register 2mph above what you are actually going.
     
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  7. supratachophobia

    supratachophobia Active Member

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    Well, that's truly disturbing if they are trying to calculate their EPA numbers from nominal pack capacity. That's called lying. :(

    @wk057 has basically confirmed all the BMS pack sizes new, and the watts/mile used to get EPA.
     
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  8. Bet TSLA

    Bet TSLA Active Member

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    Has your firmware version changed over the life of the car? If so, you really can't compare old numbers with new numbers, as there's no way to know if their internal calculations of reality have changed.
     
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  9. mal_tsla

    mal_tsla Member

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    EPA numbers come from test observation (actual driven miles), not math, right?
     
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  10. ran349

    ran349 Member

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    #10 ran349, Jun 3, 2018
    Last edited: Jun 3, 2018
    Are you saying that when you are at 0% SOC(or 0 miles), then the nominal pack capacity would read 79.1 kwh, and the nominal remaining would read 4 kwh, while the usable remaining would read 0 kwh? I'm trying to understand if the 4 kwh buffer actually shows up in the readings when the usable remaining is at 0.
    Another question- It looks like the BMS data (I assume this is from TM-Spy) shows an SOC value and a SOC nominal value. Which of those matches the value that shows up on the battery icon in the car when you are reading in %?
     
  11. davewill

    davewill Member

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    Yes, but.... What would they have observed? They can see how many miles were driven from full charge to empty, and they can measure how much power it takes to charge back up. Unless they add equipment to the car, they can't measure actual Wh used while driving.
     
  12. mal_tsla

    mal_tsla Member

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    Why does it matter, then? The car is sold based on how far it can go, not the Wh/mile.
     
  13. davewill

    davewill Member

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    #13 davewill, Jun 3, 2018
    Last edited: Jun 3, 2018
    Exactly, but the OP is stressing over the Wh/m numbers not being in sync with the EPA range. I think that he can't reconcile them because EPA simply measured the range the old fashioned way... By driving the car until empty.
     
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  14. Brando

    Brando Active Member

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    I ONLY WANT TO KNOW HOW MUCH HE PAID PER MILE EXACTLY !

    I figure mileage cost every time I gas up my Saab. (just not sure why the cost drifts around over time)
    SuperCharger - oh, never mind, I know he paid nothing, duhh.

    </sarcasm alert>
     
  15. David99

    David99 Active Member

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    Not really. The manufacturer can do their own test and they can do it on a simulator. The EPA also has a test lab. It's a real car but it sits on a simulator. Only 15% of cars are actually tested by the EPA, the other 85% is data from the manufacturers.

    The Truth About EPA City / Highway MPG Estimates
     
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  16. supratachophobia

    supratachophobia Active Member

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    At 0%, the nominal remaining capacity is correct at 4kwh and the SOC is 0%. The pack nominal full capacity actually reads 78.3kwh and usable pack reads 74.3kwh. At the start of that trip the latter numbers were 79.1 and 75.1 respectively (not so much worried about that discrepancy since the voltage is going to be all out of wack at such low SOC).
     
  17. supratachophobia

    supratachophobia Active Member

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    Right, but the 290 watts per mile is what the BMS uses to estimate range calculations. That's never really changed since that's what was used in the EPA tests. The dotted line if you will in the Energy screen.
     
  18. supratachophobia

    supratachophobia Active Member

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    That's a good point, but would an EPA test have allowed for that 4kwh reserve to be used? I doubt it, if that wasn't available to production vehicles.

    The stress for me comes from the mysterious disappearance of range while driving. And the expectation based on what the car is saying being changed without warning. I'm not saying that I need to drive the car to 0%, that's foolish. But the unexpected does happen on trips, and I should be able to trust the car to give me the correct info.
     
  19. supratachophobia

    supratachophobia Active Member

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    But they have to derive how far it can go based on watts per mile. It's the equivalent to miles per gallon, and it's the whole reason we have the dotted line on the energy graph.
     
  20. davewill

    davewill Member

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    I don't think so. I think they get the range by running the battery all the way down and measuring miles. The guesstimate on the display doesn't have to coordinate with the EPA stats, only the window sticker and ads.
     

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