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Latest thinking on buffer after "Charge Now" screen appears

Discussion in 'Model S: Battery & Charging' started by MrPinrel, Dec 16, 2014.

  1. MrPinrel

    MrPinrel Member

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    Dec 15, 2012
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    Kansas City
    What is the latest thinking about how far can someone actually drive after the "Charge Now" screen appears (ie reaching 0 miles of rated range)?

    I miscalculated a trip the other day, had to baby the battery capacity on the way back home, and the screen showed up half a mile from my house. I was able to get home without issues but was curious about whether there actually is a "reserve" range built in.

    I have read some of the discussions about 17 miles additional capacity but couldn't remember if that was actually confirmed or whether maybe it was an old feature and had been changed with firmware 6.0

    thanks
     
  2. jerry33

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

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    Don't count on anything past zero.
     
  3. arg

    arg Member

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    My understanding is that there has never been a specific reserve below zero, rather:

    1. The algorithm aims to guarantee that you always have at least the range shown (as an amount of energy, not a specific number of actual miles). In particular, it guarantees that it will never suddenly jump from a positive number down to zero.
    2. The measurement of capacity is inexact, especially in the middle of the range - '100% full' and 'empty' are hard limits, but anything in between is only an estimate, and having estimated the percentage it also needs to estimate what the battery capacity is (ie. it can easily know that the battery is 100% full, but does that mean it contains 85kWh, or does it contain a bit less due to degradation). The system uses historical measurements to guide its estimates - and particularly with cars that are kept around mid-charge (driven short distances, never range charged) it can become increasingly inaccurate.
    3. In order to make the range decrease in a linear and predictable fashion, the estimate needs to be made primarily at the end of the last charging session (possibly it gets adjusted marginally on the way down, we don't know for sure).
    4. In order to make the guarantee in 1), the estimates need to be somewhat conservative. This means that typically when it gets down to zero there is some more left, but only the margin that was left to account for inaccuracies, plus or minus the amount of inaccuracy that was actually present in the estimate. So in the worst case, it could always have stopped dead at zero, but typically didn't.
    5. Over successive software versions, the estimation has improved. The combination of better estimation and less need to be conservative in that estimate means that the typical 'reserve' is probably smaller than it was, but it could always have been zero and is never predictable.
     
  4. MrPinrel

    MrPinrel Member

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    Dec 15, 2012
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    Location:
    Kansas City
    thanks. in my case it went from 2 to 0 in less than a mile, then from 0 to "charge now" in like 200 feet. I was only going about 25 but the temperature was about 32F.


    It was a bit scary there for ten seconds when I thought maybe it was going to stop within sight of our house!
     

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