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70kwh usable?

Discussion in 'Model S: Battery & Charging' started by J1mbo, Feb 20, 2015.

  1. J1mbo

    J1mbo Member

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    Yesterday, I set off to work with 80% charge. When I got home, I had used 52% of that leaving 28% remaining. (Car is showing % instead of range).

    The total amount used since last charge was just over 36kwh, according to the trip meter.

    This suggests a 100% battery capacity of only 70kwh (usable). Now, I know that we don't get the full 85kwh, due to battery protection, but most estimates put that buffer at much less than 10kwh, so 15kwh seems a bit on the high side.

    The car is in Range Mode, with full power saving (mobile access unticked), and was parked in a warm garage all day. I didn't measure it, but usually the vampire loss over the course of the day is insignificant with these settings.

    Anyone else with an 85 who is using the % meter for range able to confirm whether 100% = 70kwh?
     
  2. tomas

    tomas Traded in 9 rep bars for M3, used to be somebody!

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    % remaining reflects loss while parked. kWh used does not. That affects math. Depending on your settings, could account for another 5 kWh.
     
  3. J1mbo

    J1mbo Member

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    As I mentioned: Power saving is ON with Mobile Access unticked. AFAIK this is the maximum power saving mode.

    - - - Updated - - -

    Ah, just seen the chart in this thread. Seems that 70kwh is pretty good after all. :/
     
  4. RiverBrick

    RiverBrick Active Member

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    At the very least they round down the percentage remaining. Otherwise, I wouldn't have been able to drive three blocks with 0% remaining the other day (without running out of juice, so who knows how far I could have gone). So, 28% left might be 28.9%.

    Then there's the running debate as to whether there's a reserve after 0% or 0 km. Some people think there's as much as 5 kWh usable that's "hidden."
     
  5. David99

    David99 Active Member

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    totalCapa.JPG

    Here is a screen shot from a service screen that only Tesla Service people can see. If 82.2% equals 65.5 kWh, then 100% equals 79.68 kWh. I assume that is the total useable capacity. Note that it is from a very new car. Over time the usable capacity will go down a little.
    The highest I was every able to get out of my battery was 76.5 kWh stopping at 0 miles. If going slower and going all the way until the car shuts down it might be more.
     
  6. scottm

    scottm Active Member

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    It's kind of like worrying what the exact octane is for the gas pumped into an ICE car.

    Was that 87 or 94... yes, in some cars, one will drive you further. ... but is it 8% more distance?

    And is it worth the delta cost for the Premium?

    Want to drive further at no cost? Slow down a bit. You just stretched your battery 10%.
    Same trick works for gassers.
     
  7. Damiano

    Damiano Member

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    In addition to the trip meter not capturing vampire losses and losses from HVAC if you turn car on remotely, there's also a sigfig issue and a calibration issue.

    Significant digits in Calculation:
    The fact that the tank is in whole number % makes a difference. Assuming the % is rounded to nearest whole number (not rounded down like odometer), then the lack of significant digits introduces a +/- 1.1kWh error range into the capacity estimate. On the other hand, if the % capacity is rounded down, then there's a +/1.8kWh error range in the capacity estimate calc.

    Calibration:
    The only real way for the system to know how much true capacity the battery has is to charge to max (and wait for balancing to complete) and then discharge to 0 miles while driving (or as far as it can be discharged). If you run your car in the middle of the battery like a lot of folks do, the calibration gets out of whack. For a long time I was just charging to 70% unless I needed more. My RM decreased as a result, but I ended up getting some odd behavior. On a range charge the car would say 252 RM, but then would remain at 252 for the first 10 miles of driving. Recently I switched back to 90% charging and the behavior went away (259RM on last range charge). I wouldn't be surprised if the range calculation algorithm used in Europe is different either, because Tesla has diff regulatory guidelines there.
     
  8. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    Here is a screen shot from my P85D. A 100% charge is 255 P85D rated miles. This was done on a continuous drive with no stops, so there should be no energy that went to vampire losses.

    255/(255-28)*67.1 = 75.4 kWh max usable

    I actually left with 253 RM, so 241.3/(253-28)*278 Wh/mi = 298 Wh pre Rated Mile for a P85D. If I multiple 298 Wh/mi * 255 RM, then I get a usable capacity of 76 kWh.

    75.4 or 76 kWh are pretty close, and the numbers that I would use for the usable capacity of a new 85 battery. The other 9 kWh are used for not charging to the absolute max on the top and anti-bricking reserve on the bottom.


    Pagosa-Silverthorne.JPG
     
  9. wk057

    wk057 Senior Tinkerer

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    I was able to use 73.1 kWh from 100% charged down to 4 rated miles remaining in my P85. It seems this number doesn't take some usage into account (some 12V or some HVAC perhaps, but could be wrong).

    2014-06-24 01.59.40.jpg
     
  10. Muzzman1

    Muzzman1 Member

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    When I saw the picture @david99 posted it reminded me that I had taken a similar set of shots about a yr ago. My calculated battery capacity comes out to 74.37, and that was when the car had 1280 miles....so who knows what the actual capacity really is??
    Screen Shot 2015-02-20 at 10.23.07 PM.png
    PS: I cropped the photo a bit too tight....it actually says "ideal energy remaining"
     
  11. wk057

    wk057 Senior Tinkerer

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    Keep in mind that at higher discharge rates there is less energy available due to internal resistance losses in the battery. I'm pretty sure Tesla's BMS is able to pretty accurately calculate this and takes it into account in the Wh/mi and kWh used numbers... not 100% sure though. However, but it doesn't change that less energy is available at higher discharge rates. Basically, getting a higher Wh/mi = less total energy available. The lower the average discharge rate the more energy is available.

    I think this is also why a speed of 65 is used for some of Tesla's estimates because this puts the pack right around a 1/4C average discharge rate if rated miles are achieved at that speed which makes the internal resistance losses very low.
     
  12. kennybobby

    kennybobby Member

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    Why do they list local sunrise/sunset times on the service screen?

    What is SOE/ USOE?
     
  13. jerry33

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

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    WAG is that it helps with the auto headlight algorithm. If it's darker than normal daytime light levels and it's around sunset, time to turn on the headlights. As opposed to, it's daytime and your just passing under a bridge for a couple of seconds.

    SOE = Standard Operating Environment
    USOE might be UnStandard Operating Environment
     
  14. ArtInCT

    ArtInCT Always Learning

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    I forget now who created and posted this graph??
    I saved it and am posting it here... This graph really shows the theoretical usable capacities of the batteries....
    I found it quite useful in understanding the drive battery pack..

    Battery Range Histogram Explained 85KW.jpg
     
  15. scaesare

    scaesare Active Member

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    Intresting... I wonder what SOE/USOE is? Usable State of Energy"

    The 4.7% difference between that and the shown SOC equates to just about 4kWh... which is close to the anti-bricking buffer that's been suggested.

    - - - Updated - - -

    Wind resistance has been cited by Tesla as the single largest factor (discounting elevation change, which you can't control) affecting energy usage as you approach and exceed highway speeds. As wind resistance increases approximately with the square of speed, there's ~33% more wind resistance at 75 than 65, despite being only a 15% difference in speed.

    Certainly resistive losses are the square of current as well, but I've never heard Tesla quote that as a factor... just wind resistance. Of course, minimizing speed provides gains in both those areas...
     
  16. wk057

    wk057 Senior Tinkerer

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    Tesla never seems to mention any of the engineering negatives about EV tech in general, especially when it comes to the batteries. The resistive losses are definitely meaningful above 1/4C draw. Wind resistance is certainly the biggest factor in terms of usage, but increased internal losses due to the added power needs at higher speeds (due to wind resistance, for one) definitely adds to the decreased range at higher speeds. At 1/2C vs 1/4C looking at Panasonic's discharge curves as an approximate reference, it's a loss in capacity of nearly 5%... and approaches 10% at 1C. While discharging at 1C continuously is pretty unlikely (85kW), 1/2C for long stretches isn't uncommon at 70-80 MPH in my experience (~43kW).
     
  17. arg

    arg Member

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    Related to the resistive losses, do we know how regen is accounted for in this metering?

    If the 'kWh used' is net energy in/out of the battery - ie. drawing power increases the number, regen decreases it - then there will be charge/discharge losses to account for.
     
  18. wk057

    wk057 Senior Tinkerer

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    I've had negative Wh/mi and negative kWh used showing on the meter before, so it seems to account for it pretty well.
     
  19. scaesare

    scaesare Active Member

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    Agreed.

    I think I've read that a 15kW draw @ 55MPH is what's expected (and works out about right for 75kWh usable from a ~265 mile range pack).

    At 70 it's more like 20-25kWh in my experience... so the current draw is going to be non-linear, and thus resistive losses correspondingly greater.
     
  20. arg

    arg Member

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    OK, that says the regen is counted in the total, but if it is a net total in/out of the battery, regen will tend to cause the total to be smaller than it otherwise would be due to charging losses.

    Say you draw 100Wh accelerating the car, then (helped by a slope) manage to regen 100Wh back into the battery: the total will now read zero. However, the battery will no longer be fully charged since it takes more than 100Wh of charging to offset 100Wh of discharge. I don't think anybody has managed to measure it accurately, but there have been estimates in the region of 10's of % loss on the round trip.

    Of course, the total amount of regen in a long trip is normally a very small proportion of the energy used - though maybe in city driving it could become significant.
     

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