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range and battery capacity, doing the math

Discussion in 'Model S: Battery & Charging' started by David99, Jun 11, 2014.

  1. David99

    David99 Active Member

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    My 85 shows 270 miles when fully charged. That is based on rated range. According to the energy graph, 300 Wh/mile is the rated usage of the car. 270 * 0.3 = 81 kWh. That would leave about 4 kWh for 'brick protection'. I drove the car down to zero (actually 2-3 miles beyond) and both times it showed that I used 76.5 kWh. That would suggest that I still had aprox 4.5 kWh in the battery before it would shut down. Driving at exactly rated usage (300 wh/mile) you only get 255 miles. That would mean, when Tesla did their EPA cycle test, they ran down the battery to the point where the battery shut down. IOW, they drove it beyond the point when it shows 0 miles. Assuming there are 4.5 kWh left between showing zero and when the car shuts down means you have 4.5/0.3= 15 miles beyond zero. That matches with what people have reported.

    I have no desire to test this out, though :)
     
  2. Mayhemm

    Mayhemm Model S P85+ "Lola"

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    This image breaks it all down. You actually have less battery available than you might think.

    HlX3wIuhGbYQ8lZr3FoI-ZZSa7ZaPFCzqu0ZGTFjxJgIdJyQuoQ_72toEjlB7riEpSDr-NOAx5977epUASH0h2Mv92U1KVqw.png

    If this image is outdated or something, please feel free to correct me.
     
  3. jerry33

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

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    That image is someone's guess on version 5.8 (maybe even earlier than 5.8) based on a few observations. It's not official. It's been speculated that the zero mile protection isn't there anymore (or at least has been greatly reduced in 5.9). It's not a bad image, but I wouldn't rely on it in a critical situation.
     
  4. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    Tesla keeps many sets of books with different conversion factors concerning power, energy, and rated miles. Here are some of the numbers that I have found correlate well in my Sig 85:

    1. 290 Wh/mi - DC out of the battery. If I do a single trip right after charging (no vampire or HVAC loss), then this number makes actual miles driven equal to rated miles used.
    2. 300 Wh/mi - DC into the battery. There appears to be about a 3.3% charge/discharge cycle loss in the battery. If you look at Supercharger charge rates compared to DC kW into the car, then 300 Wh/mi is the conversion; 90 kW => 300 mph charge rate, 120 kW => 400 mph charge rate etc. Just to create some confusion, Tesla decided that kW are displayed as an instantaneous value, while charging mph is displayed as the average for the session.
    3. 333 Wh/mi - AC into the car. The AC chargers are about 90% efficient. 300 Wh/mi / 90% = 333 Wh/mi. To create a little more energy accounting confusion, if you display charging stats in kW while AC charging, the rate (kW) displayed is AC power into the car, while energy accumulated into the battery (kWh) is the DC energy put into the battery.

    The difference between AC into the car and DC out of the battery plus vampire loss create most of the difference when people compare the energy use in the car with energy use measured at an AC Power Meter.

    During the charge rate taper, while Supercharging, the average mph charge rated displayed is always higher than the actual instantaneous mph charge rate. This confuses many MS owners, and at the end of a near 100% Supercharger session, they think that they are charging at a much faster rate than the true reality. I think that this has a lot to do with why some people stay at a Supercharger so long; they think that they are getting miles at a much faster rate than is really true. BTW, the true, instantaneous mph charge rate can be calculated as (Power in kW)/(0.3kW/mi).
     
  5. David99

    David99 Active Member

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    While this image looks nice, we had several posts here that show that it's not correct.

    Getting the accurate number is difficult. Battery capacity is not an absolute number but it changes with the discharge rate. A lower discharge rate gives you a higher available capacity. The voltage is also higher at lower discharge rates resulting in a higher energy output. We don't know at what discharge rate the 85 kWh are rated at. We also don't know how where Tesla has set the upper or lower limits. There is definitely a lower limit that prevents the battery to be discharged to 0% as it would damage it. Also, I read that the upper limit is set to 95% state of charge (meaning when the car shows 100% it really is 95% of the actually battery level), again for better battery life. But exactly where these limitations are is just a guess.

    Another thing that doesn't add up is the difference between the 85 and 60 kWh version.
    265 miles on the 85 would be the equivalent of 187 miles for 60 kWh. (265/85*60=187). The S60 has 203 rated miles, though. Doing the math based on the S60 would come out to 287 miles for the S85 (203/60*85=287). In other words the S60 gets more out of it's battery than the 85 which makes no sense. The numbers don't add up.
     
  6. shelbri

    shelbri Member

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    If the upper limit is set to 95% SOC, then why not charge to 100% all the time since the upper limit is prevents full charge anyway?
     
  7. Benjamin Brooks

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    This is due to the different (higher) wh/m for the heavier 85kWh battery cars.
     
  8. djp

    djp Roadster 2.0 VIN939

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    The lower the SOC the better for battery life. 75% is better than 85%, and 85% is better than 95%.

    Why do Li-ion Batteries die? And how to improve the situation?
     
  9. Matias

    Matias Active Member

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    #9 Matias, Jun 17, 2014
    Last edited: Jun 17, 2014
    attachment.php?attachmentid=23059&stc=1&d=1370224174.png

    This is NCA chemistry, I guess Tesla uses LCO-chemistry, but maybe the relation between SOC and degradation is the same.
     
  10. djp

    djp Roadster 2.0 VIN939

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  11. Matias

    Matias Active Member

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    #11 Matias, Jun 17, 2014
    Last edited: Jun 17, 2014
    Very interesting question is whether Tesla´s 100% charge really is 100% charge or something less. There seems to be a big difference between 100% and 80% charge concerning degradation (of course short time 100% charge is not bad).
     
  12. djp

    djp Roadster 2.0 VIN939

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    On the Roadster a Range charge is 95% and Standard charge is 85%. The Model S is probably similar, with the actual SOC being 5% lower than the slider (at least above 50%).
     
  13. Brass Guy

    Brass Guy Member

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    A battery is not like a gas tank. There is no exact 100% full charge. You charge lithium batteries to a certain voltage, depending on how you want to treat it.
    You can charge the lithium batteries to higher than 4.2V, but that would severely impact the capacity pretty quickly, if not destroy it.
    I'm sure Tesla has chosen a balance of maximum SOC while maintaining a certain battery longevity. I wouldn't be surprised if they could reprogram the firmware to allow 300 rated miles on the 85kWh, but they'd have to remove the battery warranty because owners would be down to 200 miles in short order; then bricked shortly after from needing more range charges.

    My point is 100% charge is whatever Tesla decides it is, and I sincerely doubt it is anywhere near 4.2V per cell with an 8 year warranty.
    Just like 0% is not empty, because as we all know that is a lithium brick. 0% is also what Tesla decides.
    I completely trust Tesla's battery implementation, it's the heart of the company.

    Disclosure: thoughts based on what I've read since I placed my order almost a year ago, but I'm fairly confident. "They can't put it on the internet if it isn't true."
     

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