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Posted mileage wildly inaccurate

Discussion in 'Model 3: Battery & Charging' started by clicketyclack, Oct 30, 2019.

  1. clicketyclack

    clicketyclack Member

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    Perhaps this is normal, but I am not sure. I have about 20K miles on the car as of today and charging to 80% gives me 234 miles. Now as I understand the car is rated for about 241 watts/mile. If I am driving rather efficiently I can get about 220 watts/mile however if drive 10 miles at this rate the display saying how many miles I have driven will decrease by more than 10. Since I am driving under 241 watts/mile shouldn't it decrease by less than 10 or am I missing something?
     
  2. Uncle Paul

    Uncle Paul Well-Known Member

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  3. jjrandorin

    jjrandorin Moderator, Model 3, Tesla Energy Forums

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    To spare you the list of links I have been posting in these threads recently, I will suggest that if you actually want to read about the topic of "rated range", if you just search this forum for "rated range" I guarentee you will find at least 15 threads just for the month of october with lots of responses on the topic. I can say that definitively as I have done exactly that, then posted the resultant links into new threads on the topic.

    To not appear like I am being contrarian, I am not going to do it this time, but encourage you to search if you actually want to read about the topic and not just post about it.
     
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  4. ewoodrick

    ewoodrick Well-Known Member

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    There is startup usage. Measure your usage in the middle of a trip, at least 15 minutes after starting. Even longer if cold
     
  5. mreynolds767

    mreynolds767 Member

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    Are you confusing range with wh/mile?
    Not sure, reads a little confusing to me.

    Anyway, I am not sure your 241 wh/mile is what the EPA uses for the model car you have,
    The LR AWD I have read range rating is based on 230 wh/mile.
    If you have an RWD version than it is more efficient so more like 220 or even 210 wh/mile.
     
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  6. clicketyclack

    clicketyclack Member

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    Why even spend the time responding?
     
  7. AlanSubie4Life

    AlanSubie4Life Efficiency Obsessed Member

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    #7 AlanSubie4Life, Nov 1, 2019
    Last edited: Nov 1, 2019
    Lots of threads on this, but to achieve one-for-one mile rolloff with AWD you need to do about 230Wh/mi, and for the LR RWD you need to do about 223Wh/mi.

    The rated driving consumption is not 241Wh/mi.

    Again, all the details are described elsewhere. I’ll see if I can post a link here.

    List of rated lines and constants

    Note that the discharge constants can vary by about 2% depending on conditions, but it is a pretty small variation relative to using the wrong value (like 241Wh/mi) for that constant for your calculations.

    Your stated example is too short to see the alignment (though it is within one mile of correct). If you drive a bit further and track the numbers you’ll see everything checks out pretty much exactly.
     
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  8. clicketyclack

    clicketyclack Member

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    awesome thank you!
     
  9. AlanSubie4Life

    AlanSubie4Life Efficiency Obsessed Member

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    No problem. Again, there is some small variation on that discharge constant - for my AWD P, I normally see around 230Wh/rmi, but I've seen a range of 227Wh/rmi to 233Wh/rmi (only quoting values from longer trips so it was likely not rounding error).

    I suspect that in the case of a battery that is in the process of losing full charge capacity, you may see values for the discharge constant on the lower end of the range - the BMS discovers that there is less energy than it expected in a rated mile, and on the next charge it makes some adjustments to what it says "full" is in rated miles (and if the adjustments are correct, on the next discharge you might see something closer to the middle). I have no hard proof of this though - really requires a very careful tracking of discharge constants over time and the relationship to what your car says a full charge is, and I only occasionally have opportunity to track such things.

    Other sources of variation are the rate of discharge - higher discharge rates do seem to be mildly correlated to lower values of the discharge constant, presumably due to untracked internal battery losses - but the dependence is pretty small in my experience (I would imagine that such internal losses could be reasonably accurately modeled by Tesla (it's pretty easy to determine what the internal resistance of the battery is with a dV/dI measurement), so there's really no reason why they shouldn't show up in the trip meter, to be honest...but what method they actually use, and whether they decide to include these losses in the trip meter reading, I don't know).

    Regardless of the above, it's really splitting hairs - we're talking about at most in typical situations about +/-1.5% variation in the value of the discharge constant - so for your tracking purposes, you'll find that once you select the correct value of the constant to use, it will explain your observations quite well.
     
  10. vickh

    vickh Active Member

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    I've noticed the range drop on my SR toward the bottom end of the range too.I'll usually wait to charge at home till 8pm and get the scary battery damage email at 17miles remaining. Does that really hurt the battery if I don't get down to the single digits range?
     
  11. ran349

    ran349 Member

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    My guess is that the trip meter is pretty accurate; assuming, of course, that the trip taken is pretty much continuous so that all the energy usage is captured. The slight difference from what the BMS is reporting is probably just the BMS making small corrections for its uncertainty level.
    And although I think in most cases the difference between the trip meter and BMS is a small negative number, I have seen some evidence that it can also be a small positive number. And it makes sense that the BMS should adjust or correct itself in whatever way was necessary to keep its remaining capacity estimate accurate.
     
  12. AlanSubie4Life

    AlanSubie4Life Efficiency Obsessed Member

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    That's what I've been thinking recently, as I alluded to above. I've been able to get some data from a user with CAN bus access recently for the AWD.

    That data strongly suggests to me that the BMS kWh value is firmly fixed to a 234Wh/rmi value (plus the buffer) for the RWD. So for example a battery which shows 205rmi at 66% would be: 234Wh/rmi*205rmi + 3.4kWh = 51.4kWh (remainingkWh) according to the BMS (the size of the buffer does vary from car to car so it won't always be 3.4kWh). In the datapoints I've seen so far it seems pretty deterministic no matter what shows up on the trip meter. (For example the trip meter might show you driving 200miles at 230Wh/mi (46kWh, and showing as using 200 rated miles), but that would show up as a change in the BMS kWh of 46.8kWh.)

    And I have NEVER seen a trip meter consumption constant of 234Wh/rmi - I believe it could happen if I drove really slowly - maybe. But typically I see right around 230Wh/rmi. There might always be some sort of uncounted loss (this uncounted loss seems more common in the AWD than the RWD variants so I wonder if it is something about the front motor energy counting).

    Recently, I got closer to 228Wh/rmi - and it was followed on the next charge by a change in my projected full charge energy from about 304rmi @ 100% to 302rmi @ 100%. So there's something to what you say, perhaps.
     

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