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Just how much buffer is there? (resolve this > 10 kWh discrepancy)

Discussion in 'Model S: Battery & Charging' started by PoweredByRain, Aug 11, 2014.

  1. PoweredByRain

    PoweredByRain Member

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    #1 PoweredByRain, Aug 11, 2014
    Last edited: Aug 11, 2014
    2014-06-23-05-31-42-2H6A1830.jpg

    I charged to full before leaving. The image shows 406.7 km traveled, 71.8 kWh consumed (177 Wh/km). Extrapolating to 85 kWh that should be 481 km total (74 km remaining). Yet the car says only 15 km remaining.

    (Conversion for USians: 252.7 miles, 71.8 kWh, 284 Wh/mi, should theoretically allow for 299 miles and thus 46 miles remaining, yet the car says 9 miles remaining).

    So, how much is there in reserve? The above photo suggests at least 10 kWh!

    Can someone explain? Is there really 10 kWh of inaccessible reserve? (This is the lowest I've ever drained the battery, and I am not one to experiment on my own, expensive, generally pampered battery.)

    I have a second example below which shows roughly the same thing: the car seems to only want to let me consume 74-75 kWh.
     

    Attached Files:

  2. Matias

    Matias Active Member

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    You can continue driving some kilometres after range displays zero. So it is not all inaccessible.
     
  3. brianman

    brianman Burrito Founder

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    Unfortunately, I can say with high confidence that my vehicle definitely does not have 10 kWh below the 0 rated mark -- at least not that can be used by an owner for propulsion.
     
  4. yobigd20

    yobigd20 Well-Known Member

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    nobody knows for sure but its probably around 7-8kW

    - - - Updated - - -

    there have been several instances of owners car shutting down right at 0. so 0 means 0. don't expect to get any miles past 0. - Caution about Vegas to LA trip!!
     
  5. dennis

    dennis P85D

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    Someone (can't remember who) created this chart which answers your question. It seemed very useful so I saved it. :smile:

    85kwhbatterycapacity_zps4b705da8.jpg
     
  6. PoweredByRain

    PoweredByRain Member

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    That's the sort of experience I was hoping to learn from. :) I was assuming zero meant zero, but I had also mentally done calculations assuming 85 kWh was available. That was naive, obviously, but I didn't expect that over 10% of the "battery capacity" was not actually available.
     
  7. yobigd20

    yobigd20 Well-Known Member

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    NOTE: this is UNOFFICIAL and UNCONFIRMED, & also doesnt explain the instance of what happened in the 'zero means zero' thread
     
  8. PoweredByRain

    PoweredByRain Member

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    Awesome. Thank you for posting that.

    I have no problem with having "only" 76 kWh available for driving, but you have to admit that it's likely to be a surprise to many people that so much is needed for protecting the battery. In particular, I cringe when I hear Tesla showroom people saying "the range" of the Model S 85 kWh is "480 km" (300 miles). Uh... in most conditions, no it ain't!

    Thanks, everyone! I can't believe how quickly the responses came in.
     
  9. Zextraterrestrial

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    I know there was some talk/guessing that some of the power used isn't being added to the kWh trip data? maybe active cooling/heating or other power draw like ac, radio, computer and lights?
    this would make a bit of a difference in the 2-3 hour full range trip #'s
    I have seen ~76kw used from my car, 100%- 0 rated
    driving 242 mi @ 303 Whr/mi then I put 74kWh back in to 90%SOC
    this was early firmware (4.4)
     
  10. PoweredByRain

    PoweredByRain Member

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    But surely the power measurement is done on the current coming out of the battery, not on each one of these separate loads?

    I've never seen a major discrepancy between reported energy usage that would suggest that, for example, heating isn't included.
     
  11. Matias

    Matias Active Member

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    On that thread OP was able to drive at least 4 miles past 0, so 0 wasn't exatly 0. Though it almost was.
     
  12. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    I have always used the number of 17 rated miles below zero, although I have only tested it to a few miles below zero, and always plan to arrive at zero greater. Before they changed the battery display vs miles left, I extrapolated to no battery from the zero miles point on a graph and it came out -17 rated miles at no battery left (I assume still an anti-brick reserve left).

    I have found in my MS that 290 Wh/rated mile is a good conversion factor. If you use that and the "Zero Mile" protection number from the illustration, then 5.1 kWh/290 Wh/rated mile is 17.5 rated miles. All pretty consistent.

    As others have said, always be careful and always plan to arrive at 0 or greater miles. If you do push it, remember that rated miles are not actual miles. For example, because of elevation, even in the best of weather, and the most careful hypermiling, the last 9 actual miles from town (and a 70-Amp J-Plug) to my house in Pagosa requires 15 rated miles from the battery.

    I am very aware of all of this when I decide whether or not to drive past the J-Plug and continue on to home. Yesterday, I did a 255 rated mile charge in Silverthorne and made it to home with 23 rated miles left after driving 239 miles. I had 38 rated miles in the battery when I passed that last J-Plug, but having done the drive home so many times, I was confident in not grabbing any more miles. If I had been at 15 or below in the battery, I probably would have stopped in for a few Joules at the J. It is only because I know this route so well and know that there are several reserve miles that I am willing cut it this close. I have never had to flat bed my S...and I plan to never be in that situation.

    Silverthorne to Pagosa-2.JPG

    Yesterday was an ideal hypermile day, moderate temps and cross or tail winds most of the time. That 318 miles is from Boulder to Pagosa and 295 Wh/mi is as good as I have ever done for this trip. The 273 Wh/mi is pretty amazing from Silverthorne to Pagosa even with 1,000 feet of net elevation loss, crossing three passes, Fremont, Poncha, and Wolf Creek. I even set the cruise control at 73 mph in the San Luis Valley once I knew that I had Pagosa made, and hit significantly higher speeds for several passing events on the two-lane highways with short passing windows. The first half of the trip, until I knew things were going so well, I limited the max speed to 60 mph.
     
  13. arg

    arg Member

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    Some of this discussion assumes that the car can accurately measure the remaining capacity. It can't. There's no really precise way of measuring state of charge for most kinds of cell, specially since we are talking here about displaying it to a precision of better than 0.5%. The displayed remaining range is an estimate, based on battery voltage now, battery voltage when last charged, amount of charge taken out and/or other factors that Tesla hasn't revealed.

    So, of the reserves talked about, the bricking reserve is a genuine reserve in that the software retains a portion of the capacity and doesn't let you use it. Most likely it's a voltage range rather than a particular number of kWh, but we don't know for sure.

    The 'below zero' range seems to be more a case that Tesla have contrived to hide the inaccuracy in the range calculation/display in such a way that you (almost) never get less range than you would expect from the figure displayed(*), and you usually get more. This is a change from the Roadster behaviour which (as I understand it) doesn't make an attempt to display the estimated range as you get close to zero. The Model S behaviour also seems to have changed subtly between software releases, presumably attempting to get closer to 'true' results, and possibly reducing the size of the typical distance you can drive past zero.
     
  14. Zextraterrestrial

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    That is what I'd imagine but I remember some discussion about possibilities. If it is really cold out - when you are at home and charge you car, unplug then preheat your car before driving is your trip Whr/mi the same as if you just drove without preheating while not moving? (or preheating while plugged in ) I'm not sure?
     
  15. FredTMC

    FredTMC Model S VIN #4925

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    Also keep in mind that your battery degrades the fastest when it's new. You probably have a couple kWh less capacity now compared to when the car was brand new.

    A guy at TMC connect showed that his car has 10 kWh less capacity at 75k miles vs brand new.

    image.jpg
     
  16. Zextraterrestrial

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    #16 Zextraterrestrial, Aug 11, 2014
    Last edited: Aug 11, 2014
    I disagree w/ him a bit, early firmware did change this a bit. he has a Sig car. no way it is @ 10kWhr less capacity, different firmware from new.
     
  17. yobigd20

    yobigd20 Well-Known Member

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    And yet, there's another guy that just hit 99k something today and his degradation is only 1 or 2%. This guy will be the first to cross 100k and it will happen this week. Must be because the guy your talking about has a sig with all the prototype parts lol

    Edit: ok my bad it's 5% but still...
    e4erepyj.jpg
     
  18. FredTMC

    FredTMC Model S VIN #4925

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    Two different measurement methods...
     
  19. trigga71

    trigga71 Member

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    As the firmware updates roll out I would hope a more accurate reading with a small buffer of 5-10 miles. One would think to keep a buffer in case you forget to check or as humans do take a chance on only needing a few more miles to the next charger. But this also depends on terrain, speed,weather and batter health. Any chart for the 60kHw battery?
     
  20. bluetinc

    bluetinc Member

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    Degradation from use of the Batteries does happen and we should all expect it to. Tesla does a great job, but it's does not have a Battery that degrades that little. I was on the cross country trip with him when he was near 90k and his 100% charges were at about 245 rated miles, which is just about inline with the slide shown.

    Peter

     

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