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Correlating Wh/Miles with rated and actual range

Discussion in 'Model S: Battery & Charging' started by metafor, Dec 13, 2013.

  1. metafor

    metafor Member

    Joined:
    Aug 13, 2012
    Messages:
    169
    Location:
    Portland, OR
    So while driving, I tend to stare at my Wh/Mile rating and wonder about how the numbers correlate. I've noticed that to even get close to the rated miles, I have to hover around (a bit below actually) 300Wh/mile.

    But a back-of-the-envelope calculation says that to get my rated 260 miles at full charge (264 often, but it drops to 260 or so after I unplug), you'd need:

    85000 Wh / 260 miles = 327 Wh/mile.

    Anyone who's ever driven at 75mph and getting ~330 Wh/mile knows they're not getting the rated range.

    So out of curiousity, I decided to watch my battery indicator (the big one on the charging screen) change. The 10% marks are somewhat clear to look at. I started from 90% in the morning and watched how long it takes to get to the 80% mark. I then look at my "energy used" indicator.

    At 7.4 kWh used, it hit the 80% mark. Now granted this doesn't mean it's actually at 80% since it the battery indicator moves in fairly course intervals. So I waited until it gets to one tick below 80%. I was at 7.8 kWh used.

    Extrapolating, this would mean that to get from 90% to 80%, I used roughly 7.6 kWh.

    I'm going to see how much it takes to get from 80% to 70%. Perhaps the battery indicator isn't linear. But extrapolating based on this, it means that the battery capacity is 76 kWh, roughly 10% less than the advertised. Of course, I fully understand that 10% is within reason for any spec given. Between reserved charge (for when the battery is 0 and you still need to move), manufacturing variations and battery capacity decay (I've had it my car for 6 months, with roughly 12.5k miles), having 90% of the specified capacity is perfectly understandable.

    That being said, what's everyone else seeing with their daily commute? Is the battery indicator accurate? Has anyone run it down to 0 from 100% charge and looked at the final kWh used?
     
  2. liuping

    liuping Active Member

    Joined:
    Jul 23, 2013
    Messages:
    1,858
    Location:
    San Diego
    You don't really get access to all 85kWH of battery.

    If you should get 265 miles when using 300wh/mile, you get 300wh/mile * 265mile = 79500wh. That leaves about a 6.5% safety reserve to increase battery life and prevent bricking.
     
  3. Cottonwood

    Cottonwood Roadster#433, Model S#S37

    Joined:
    Feb 27, 2009
    Messages:
    5,062
    Location:
    Colorado
    There is the anti-brick reserve and the few (10-17 rated) miles below zero. In an 85, if you want the traveled miles to equal the number of miles reduced in the display, you need to average 290 W-Hr/mi and not do any pre-heating, feed-the-vampire, etc, while off of shore power, between the end of charge and your driving.
     
  4. lloyds

    lloyds Member

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    Bay Area
    That's what I've noticed as well. With the cold front of late the low 300s not possible. At 70mph avg, I'm hovering around 350
     
  5. balefire

    balefire Member

    Joined:
    Sep 27, 2013
    Messages:
    98
    Location:
    Chicago, IL
    how do you know there is a reserve of 10-17 rated miles below zero?
    I've come to the single digits a few times on my long Chicago to Indy trips and it would be nice to know if I have a few more miles to play with...
    Thanks!
     
  6. Cottonwood

    Cottonwood Roadster#433, Model S#S37

    Joined:
    Feb 27, 2009
    Messages:
    5,062
    Location:
    Colorado
    Sorry, I can't find the nice graph showing how the battery capacity is doled out, but the best reference is to see that the battery still has about 10% of red capacity left when the message says "charge now." 10% of 250 miles is 25. Leave a little for anti-brick and you are left with the 10-27 rated miles left.

    Battery reserve and real world range - Page 3
     

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