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Percentage indicator and kWh used

Discussion in 'Model S: Battery & Charging' started by rpavlicek, Jan 27, 2015.

  1. rpavlicek

    rpavlicek ***** Neophyte

    Joined:
    Feb 7, 2013
    Messages:
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    Location:
    Santa Clara, CA
    Since the release of 6.1 I use the show battery percentage option instead of rated miles. I have MS60 (19") I purchased in January 2013 with almost 25k miles.

    On a recent trip to Tahoe, I watched the kWh used and compared that to battery percentage used (this avoids converting to/from rated miles since that is affected by driving conditions and speed).

    My basic observation: The car went from 100% to 50% SoC (while driving) but showed only 27.1kWh used. Using simple math: 27.1kWh / 50% = 54.2kWh max capacity of the battery. This assumes that the percentage indicators and kWh used are both correct, for some form of 'correct'.

    With that in mind, possible conclusions are:
    • My battery lost nearly 10% of its capacity in 2 years (90.3%)
    • The battery needs "rebalancing"
    • Either the percentage or kWh indicators are not truly accurate
      • Does driving up/down hill throw one of these measurements off?
    • I need to go from 100% to 0% to get a real measurement

    I believe rated miles assumes 300Wh/Mi. Before 6.1 I would have to drive 275Wh/Mi to maintain parity between actual miles and rated.(I admit I haven't check this since 6.1, but that is my recollection beforehand). Given that 275/300 = 91%, that is somewhat consistent with my observation above.

    I plan on bringing this up with Tesla for my next service (next month).

    Any thoughts on this? Is this normal degradation for a 60? Or am I assuming too much?
     
  2. rdrcrmatt

    rdrcrmatt Member

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    Location:
    Milwaukee
    It has been posted before that you can't get all 60kwhr out of your battery. While it has that much capacity, some is left in reserve to protect the battery.
     
  3. mibaro2

    mibaro2 Member

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    Georgetown, ON
    I suppose you could charge to 90%, and then switch the battery display back to miles in the control panel, and see how many miles you have at 90%. Switch it back to % and drive until it is at 80%, and switch it back to see how many miles you have left.
    Doing that a few times might give you an indication of how many miles a %.

    Personally, I don't like the battery % and switched it back to show km .
     
  4. David99

    David99 Active Member

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    Seems like a very reasonable number for a 60. On the 85 you are lucky to get 77 kW out of it if you drive very carefully. I did that once when my car was pretty new and driving carefully and an average of 55 I got 75.6 kWh out. The car won't allow the full capacity to be used. When the car shows 0 miles there is some energy left in the battery to protect it from aging and damage.
     
  5. rpavlicek

    rpavlicek ***** Neophyte

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    "Driving carefully" should have little or no effect on the equation between kWh and percentage of battery. Temperature, pressure, SoC, time/use and general battery degradation should...

    I was under the impression that 60kWh meant 60kWh of usable battery and anything extra (reserve) was not advertised (actual might be 63-65?)

    I could be wrong (and probably am :p)

    Does anyone have a definite reference on that detail?
     
  6. kennybobby

    kennybobby Member

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    Heart o' Dixie
    This post has a chart from Panasonic concerning the capacity and energy content of the NNP cells:
    Real Usable Battery Capacity - Page 4

    i think the 60kWh pack is a nominal energy rating based upon the number of cells multiplied by the nominal cell ratings. But those ratings are based upon a full charge voltage of 4.2 and discharge to 2.5V. To increase battery life Tesla has margins at both the top and bottom voltage allowed--the top is believed to be at 4.15V, and i'm not sure i've ever seen a measured number for the bottom. The point being there is less usable energy than the nominal rating.

    Here's another post related to cell count in the 60 to determine the nominal rating:
    60Kwh Battery?
     
  7. RyanT

    RyanT Member

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    Location:
    Portland
    On the Volt it was only using 11kWh out of 17. That's what the battery gauge was based off of too (11kWh).
     

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