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This is why you can't get 'rated range'

Discussion in 'Model S: Battery & Charging' started by David99, Aug 15, 2018.

  1. David99

    David99 Active Member

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    Yes I know this subject has been beaten to death, but I have finally been digging into some data and figured out why you can't get the range the car tells you even if you use exactly what is considered 'rated range' consumption.

    My car is 85 and the energy graph shows very clearly that 300 Wh/mile is the rated range. It literally says 'Rated' on the right side of the line and '300' on the left side. My car will not get it's rated range when I drive exactly at 300 Wh/mile. Jason looked into the software and found that under the hood the car actually used 295 Wh/mile to calculate rated range. I trust him so I will use that number. Now my theory is (and what I found supports it) that to calculate rated range Tesla is using the entire battery capacity, all the way down to true zero. Technically it's not a lie, but it is most likely that the car will shut down before it reaches true zero to protect the battery. But for the EPA numbers they can use the entire capacity as a basis. I did the math and it actually works out that way. When using the entire battery capacity reported by the BMS and dividing it by the 295 Wh/mile rate range energy usage, I get exactly what the car reports to me as rated range. (it's off by 0.1 kWh probably due to rounding errors in all these calculations)

    Here is the BMS reported full pack capacity.
    fullpack.jpg

    My car reports 242 miles of rated range when fully charged. When divided by 295 Wh/m it comes out to 71.4 kWh.

    So yes, Tesla is kind of lying to us about the range as you can't discharge the battery all the way. Here is more evidence. This is a graph from the CAN bus charging the car from 2.5% to 92%. I captured two values. "State of Charge" (what is shown to the user in the car) and "State of Charge Min". I assume this value represents the true full to empty range as a percentage. At the start the battery was at 2.5% (shown on the main screen) while the SoCMin was 10.3%. As the battery fills up the values get closer. I assume at 100% they will match.

    2.5-92Percent.JPG

    So the car tells you you have X amount of range but as you drive, the percentage that is shown to you on the screen counts down at a faster rate than what is used to calculate the range! Technically the range is in the battery but the BMS (= battery management system) will protect the battery from being discharged all the way to zero and shuts down the car before it reaches that. In my car the difference is aprox 18 miles. So if the car shows me 242 miles of range it should rather show 224.

    I hope this helps explain why the car doesn't get the true range even if you drive at the rated consumption. BTW this has nothing to do with degradation. The BMS keeps track of degradation is adjusts all values down. Based on what I found there doesn't seem to be any trick to hide degradation. I would certainly notice as my car has 172k miles and a good amount of degradation already.

    Just for reference, here is Jason's post about the true consumption numbers used by the car
    Calculate usable battery capacity based on rated miles values
     
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  2. David99

    David99 Active Member

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  3. TLej

    TLej Little-Known Member

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    One reason that I display battery % instead of range on my instrument display.

    Recent trip as an example. I drove the furthest I ever have on a single charge - 405.7 km. For this trip, my actual consumption according to the car was 157 Wh/km, which means I used 63.7 kWh - math on the instrument display checks out. I started the trip at 96% and ended at 8%, so I used 88% to travel those 405.7 km. That projects out to something like 460 km of total range if I went from completely charged to completely drained (405.7/88*100). It also implies that my battery capacity should be 72.3 kWh (63.7/88*100).

    The last time I charged my car to 100% it read 437 km of total range - 437 km at 180 Wh/km (rated) should give me 78.7 kWh in the battery. If I had 78.7 kWh available, and I averaged 157 Wh/km, I should be able to go 501 km (78.7/0.157).

    Jason's numbers in the post you referenced above give a value of 81.8 useable kWh in my 90D (I'm pre-refresh, so that number might be off by a bit). 81.8 at the rated 180 Wh/km would yield 454 km; it would give me 521km at my actual useage for that trip of 157 Wh/km. My car's a couple years old so I will have some degradation from that number; from 521 down to 501 is 4%, which seems like reasonable degradation over 2 1/2 years and 64,000 km.

    With all of the various numbers above, who's to say what the "right" value is? On the other hand, when I'm driving and I see that the past 30 (or 50 or 100) km has eaten up x%, I can work some quick math to project out the distance that I should be able to travel - all things being equal (temp, wind speed and direction, etc.). I usually watch to see when I've use 10%, and how far I went on that 10%. If I travelled 30 km on that 10%, then I figure I should be able to go 30 x (whatever % is left in my battery/10) km. It's usually pretty accurate, and then I don't worry about all the other possible calculations one can get into as above. Other factors like speed, how much weight is in the car, flying speed of an unladen sparrow etc. are automatically worked in when using the % method instead of relying on rated range.
     
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  4. supratachophobia

    supratachophobia Active Member

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    The right value isn't some arbitrary number. For all intents and purposes, if you are going to tell the driver that 0% is where they can't drive any farther, than you better make damn sure you aren't blowing smoke at them for how far they can drive . That's called lying.
     
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  5. TLej

    TLej Little-Known Member

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    @supratachophobia I don't disagree that 0 should mean 0, it's just how far you can go before you get to 0 that seems highly variable. I have read your posts on range vanishing suddenly as you get to a low SOC, it was actually in my head as I was approaching 8% and hoping it wouldn't magically become 1% or 0%.
     
  6. David99

    David99 Active Member

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    @TLej Yes using the trip planner in the car we can plan trips really well. It works great and it's a feature no other EV has in that way. I love my Model S and it works amazing even after driving it for more than 275000 km. But the way the calculate range and advertise it is shady. Of course one can always go slower, drive more conservative and actually get the range, but that's not the point. The number they advertise is based on the EPA test cycle so people can compare cars. That's the law in the US. Using an unusable part of the battery capacity when calculating range is cheating the system IMHO.
     
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  7. ran349

    ran349 Member

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    So I have a couple questions about your numbers since I don't have the CAN bus data myself.
    If you charge to 100% you will show 71.5 kWh nominal full capacity, and usable full capacity as 67.5 kWh. So if you then drive continuously until you are down to 0% reported, will your dash odometer show 67.5 kWh used (assuming no vampire loss)?
    And then if you recharge back to 100%, how much does your car(not CAN bus) show for kWh added after charging? I would guess it would report .295 x 242 or 71.4 kWh to match your nominal full reading. But that doesn't make sense, since you didn't use all of the nominal pack capacity, correct?
    It just seems like something isn't adding up right or I am getting the numbers wrong.
     
  8. supratachophobia

    supratachophobia Active Member

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    I also do not wish upon you range suddenly disappearing.
     
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  9. supratachophobia

    supratachophobia Active Member

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    Please see attached photo. On the 90's, at least, the car starts out calculating based on nominal, then over the course of the drive changes the calculation to usable.

    You are correct, if you drive to zero, you would not use that 4kw at the bottom (nominal minus Usable).
     
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  10. chillaban

    chillaban Active Member

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    How does the regulation on range / fuel capacity work for gas cars? Most of my recent ICE cars have strong warnings in the manual saying that sloshing at low fuel capacity could cause serious damage to the car due to the HPFP sucking in air.
     
  11. David99

    David99 Active Member

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    @ran349

    The BMS knows the total capacity of the battery and it knows how much it is charged. When fully charged (100%) 'nominal available' and 'nominal full pack' will show the same number. In my case 71.5 kWh When I drive at rated range consumption the numbers add up. I just did a longer drive. At the end it showed: 198 miles, 58.5 kWh, 295 Wh/mi. 58.5/198*242 = 71.5 which is exactly what the BMS reports as full capacity. That means if I start with a full battery I will not be able to drive 242 miles as the car says. Instead it will show 0% on the battery when there are a little over 4 kWh left. The BMS also shows a value called 'Usable remaining' and it will show 0 when you reach zero % on the car's screen. In other words the car shows you the range of the full capacity when it's full but then it counts down at a faster rate. You won't see any sudden drops. They are hiding it continuously over the entire discharge cycle.
     
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  12. DB 2

    DB 2 Member

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    Has anyone ever seen documentary evidence (photos, screenshots, etc.) of a car shutting down right on cue when the dashboard hits 0 miles and the BMS shows 3.5 or 4 kWh nominal remaining capacity? I haven't. But I've seen plenty of evidence of that not happening. I've posted some of it, and I've got lots more. I simply cannot believe that my car is somehow not representative of the design intentions of Tesla.

    The primary purpose of putting some of the usable capacity below 0 is to prevent or at least minimize events of the car shutting down and stranding people while the dash says 3 miles, or 11 miles remaining, due to the imprecise nature of any EV BMS. If that happened with any frequency, the PR would be terrible. So Tesla has to put some perfectly usable capacity below dashboard zero. Then, if they had a hard and fast rule that the car will shut down when it reaches dashboard zero, even if the BMS knows there is some usable capacity remaining, that would strand people for no good reason. Tesla does what it can to discourage excursions below dashboard zero, but not to the extent of stranding people unless they really have to.

    The size of the buffer is a compromise between having as much range of possible be in the normal (above 0) zone, and having an acceptable (very low) number of shutdowns while the dash shows miles remaining. The below zero zone is perfectly usable, but use at your own risk, because you never know how much you will get to use before the real shutdown condition (whatever that is) occurs. On average it will be around 14-15 rated miles, but with significant variability. It's a crapshoot, and if you do it often enough, eventually you'll lose.

    The EPA range is based on how far the car can go before it can no longer perform the testing cycle. Although the car doesn't have to actually shut down, the performance (especially of a Tesla) has to be seriously degraded. That place will be well below dashboard zero and probably very close to an actual shutdown. Tesla is not cheating because they will allow you to use all of it, they just put some in the "use at your own risk" zone. Tesla calls the measurement units Rated Miles, not EPA Miles, and they certainly never promise that number of 295 Wh consumption miles to be above the dashboard zero mark.
     
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  13. supratachophobia

    supratachophobia Active Member

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    #13 supratachophobia, Aug 16, 2018
    Last edited: Aug 16, 2018
    I have no problem with your defense on Tesla on this one, but they aren't being honest. Knowing that past 0% could cause damage to the battery, I would never willingly take it past 0%. HOWEVER, if zero percent is the point where you *shouldn't* drive anymore, then be honest about it and give the owner the Usable Capacity number at 100%. If the car says I can drive 250 miles at EPA consumption, then you better believe I EXPECT the car to go 250 miles at EPA consumption.

    That being said, I've heard the argument that past 0%, the cell imbalance becomes too hard to overcome and the car could actually lack the power to propel car at normal speeds. This would cause a serious safety issue if you couldn't maintain the speed limit (been there, done that) or you didn't have the acceleration available to merge. Also, because of the imbalance and the variable output of power, you may get 11 miles more, you may get 3 miles more, you may get zero miles more.

    Graph of discharge rate: I think my car is lying to me...
     
  14. supratachophobia

    supratachophobia Active Member

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    Interesting that your analogy is extremely accurate when you compare the brick protection in a battery is there to protect the battery from damage.
     
  15. ran349

    ran349 Member

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    But the question is whether or not that percent nominal (around 5.1%) left after the car reports zero percent includes the anti-brick buffer. If there is still 4 kWh remaining after that 5.1% is gone, then I agree that the 5.1% can be considered usable. But I don't know if that is the actual case.
    Still a mystery to me.
     
  16. ran349

    ran349 Member

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    So in your drive, I assume you used up about 210 rated miles for an actual 198 miles driven?
    Is the usable always exactly 4Kw hrs less than the nominal pack value? It appears that way from the data I've seen.
     
  17. David99

    David99 Active Member

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    I used 198 rated miles, but the car had subtracted more. Driving 198 and using 210 rated miles is when you have a higher than rated energy usage. That's a different thing entirely. I started with 242 miles, drove 198 at the rated consumption but the car did not show 44 miles left, it showed less. The issue is that when you drive at rated range consumption, the car subtracts miles at a faster rate.

    The reason I'm emphasizing 'rated range consumption' so much is because that is the basis all car manufacturers have to follow and advertise. This is about Tesla stating a range that is based on the given consumption determined by the official test cycle. When driving the car in these exact conditions you cannot achieve the rate advertised. Driving at different speeds and having different energy consumption have nothing to do with this subject.

    The BMS reports 4 kWh buffer all the time. I have never seen this number change. I have seen the total capacity drop (due to degradation) over time, though.

    If someone wants to borrow me a power generator that fits in my trunk and charges my Tesla I will gladly do a test to see what the numbers on the BMS do when driving the car all the way down to when it actually shuts down.
     
  18. ran349

    ran349 Member

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    Yes, Tesla is misrepresenting when they show the energy graph achieving rated miles at 295 Wh/mi, when in fact that is not possible.
    But they never actually claim that 295 Wh/mi is rated consumption, they only give an EPA spec for rated miles.
    So the big question to me is whether that 5.1% remaining at reported zero is actually usable( if someone is willing to take that chance), or if it is really mostly the anti-brick buffer, and there is no hidden reserve.
    In any case, the energy graph is incorrect and Tesla should fix that, because it gives drivers a false sense of their range.
    But, when I use the navigation, it does seem to predict the arrival percent remaining fairly accurately, so maybe it is based on something different. I haven't checked it that closely though.
     
  19. supratachophobia

    supratachophobia Active Member

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    But you realize what will happen if they give you the actual miles you can drive? Everyone's range will suddenly be decreased and we'll all cry Mass battery degredation.
     
  20. supratachophobia

    supratachophobia Active Member

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    Preach it.
     

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