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Actual Battery Capacity and a related puzzler (with answer guess)

Discussion in 'Model S: Battery & Charging' started by wndrful, Oct 17, 2014.

  1. wndrful

    wndrful Member

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    I am curious to know the actual battery capacity. I searched through the forums and I have seen the breakdown analysis with 7104 and 5040 cells, 3100 mAh per cell etc etc. with 85KWh being 86.6KWh.
    However it seemed more of continuing discussion and didn't see a firm answer. If I have missed it then I would appreciate if someone points me to the thread.

    Okay. Now lets talk about how I came about asking this question. The puzzler part:

    I have MS60, 6months old, 11.5K miles. It now charges to 222-223 (100% charge; ideal miles; this is after doing some balancing effort of depleting and range charging). Mileage loss guess: 6-10 miles so 2%-3% of my battery capacity.

    However after the carcharges to 100%, I do the following math from Energy display:
    average Wh/mile * (average projected range) to calculate remaining capacity. And I get 59.8KWh.

    Now the puzzling part: If the battery capacity is 60KWh then I have lost only 0.2KWh which is only 0.3% which sounds too less. (Too good to be true for 11K miles.) Also my loss of range tells me I should be losing say 2%. So why the discrepancy?

    Answer guess: This all adds up if the actualy capacity is more than 60KWh. Say, 61KWh. In that case, there is a capacity loss of 61-59.8 = 1.2 KWh (~2%)

    Do you guys see any holes in this argument? And add pointers to actual battery capacity? Thanks in advance.
     
  2. Bighorn

    Bighorn Member

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    The 85kWh battery only allows use of ~74kWh of energy, so I'm not sure how to square that with your number that is so close to 60 of 60.
     
  3. tom66

    tom66 Member

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    I think it's been determined that you can get 80kWh of energy out of the 85kWh pack if you drive until the contactors in the pack open (this requires a flat bed afterwards.)
     
  4. Saghost

    Saghost Active Member

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    The range you mentioned is ideal miles. You're using your average consumption from when you're driving and multiplying by a projected range to get the battery capacity. Which of the three ranges the car displays are you using?

    (If you're using the ideal miles you mentioned further up the post, that's your problem. The only one that's related to your average consumption in any way is the estimated range - the ideal and rated are both based on test results instead, and are always at the same fixed Wh/mile number.)
    Walter
     
  5. Electric700

    Electric700 Member

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    It would be nice if, in an emergency, you could just stop the car, push or tow it to a place where you can charge, then automatically have the battery contactors reconnected by pressing a button either in the frunk or trunk. Tesla could design it so that the contactors would not reinitialize until the car is connected to a valid charge source, even if it's just 120 V. The charge source would energize a small computer and contactor relay after you press the "reset" button, which would then allow the system to reconnect the contactors and start charging the battery.
     
  6. tom66

    tom66 Member

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    This is how it's currently implemented. If you charge the 12V, you can then begin charging the HV pack from the mains. It takes about an hour to pre-condition the battery when a supercharger is used before actual supercharging can happen. Source:
    http://www.teslamotorsclub.com/showthread.php/28050-Ran-out-of-juice-12V-and-main-pack-drained-need-info/page3?p=592607&viewfull=1#post592607
     
  7. Electric700

    Electric700 Member

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    Ah, that's great. Thanks for the link.
     
  8. wndrful

    wndrful Member

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    Okay, here is picture of Energy plot just after it finished charging to 90%. This is before I start driving. (car is in range mode with AC turned off.)
    It shows that the battery capacity = 0.268 * 199 = 53.33 KWh
    So I have lost 0.67KWh in 90% charge. Converting that to full charge will be 59.26 KWh (53.33/0.9). So I have lost 0.74KWh or 1.24% of my battery capacity.
    IMG_9154.JPG
    However, I think I must have lost (guessing here) 7-8 miles of ideal range which boils to 3-4%.
    What I am trying to figure out the discrepancy between the two numbers. As said earlier, my guess is this because the actual battery capcity might be 1-2% higher than 60KWh.

    Note: I do see similar numbers when I do full charge (100%) as well.
     
  9. Bighorn

    Bighorn Member

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    This isn't the way to solve your question. The three different projected ranges use different Wh/m constants. Did you try the calculation for 5 and 30 miles and get similar results. I'd be using rated miles and the Since Last Charge energy figures to do any sort of realistic analysis.
     
  10. HankLloydRight

    HankLloydRight Fluxing

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    Just my opinion here, but I think you're trying to hard. I don't think the car provides you with enough raw and accurate data to calculate actual battery capacity in the way you're trying to do it. Especially using the energy graph and figures -- that's more for casual energy monitoring than any real accurate analysis. All those numbers change based on average/instant settings and the mileage setting,and they're all just averages based on fixed Wh/m. Since we don't know exactly what goes into calculating those numbers, I don't think you can use any of them in your calculations. Same goes for the trip/energy odometers. They're more accurate in terms of Wh used and average Wh/m, but I'm sure they're also not totally raw numbers either.

    Here's how I would calculate my total battery capacity (for an 85kWh battery): Take my 100% range charge number (259) times 308 wh/m which is the constant what Tesla uses for "Rated Range", equals 79.7 kWh total battery capacity. My car has 18k miles on it and was built in July '13, so it appears that I have a 6% loss in total range, assuming of course that the battery actually started at exactly 85 kWh.

    I think your 222-223 figure is "ideal range" which I believe Tesla uses a 270 Wh/m constant, so that would mean for you a 59.9 kWh battery capacity. What is your 100% charge in "rated range" miles?

    Also, having the car in "Range Mode" but also having the A/C doesn't make any difference. Range mode only limits the A/C and heating when it's on. Also, I've never noticed the projected range changing on the graph just by toggling "range mode" on or off.
     
  11. Bighorn

    Bighorn Member

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    I recognize that the figures may vary between models, but my P85+ uses a constant of 280Wh/rated mile when counting down the miles and I've been told it uses 300Wh/rated mile when charging up. It's thought that that 20Wh/m delta is related to mileage that is being put in a safety buffer for after zero rated miles is reached. There is definitely a 3+ mile increase in rated range for me when I toggle between range mode off and on. Projected range is so labile, I've not noticed an impact there. I agree that it might not be a real gain if one isn't utilizing any climate control, though that's not entirely certain as it could impact other background systems such as battery conditioning.
     
  12. HankLloydRight

    HankLloydRight Fluxing

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    That sounds really strange to me. Maybe that's what you observe, but I can't imagine Tesla using two different figures to calculate remaining range in the dashboard. That would lead to really inconsistent readings given the same SOC%. And if that were the case, then the battery would be accumulating +20Wh/mile every time you recharged. Or it would have to "clear the buffer" and reset the remaining range every time you recharged. Just mathematically, it always has to use the same Wh/mile constant when calculating remaining range based on the current kWh SOC.

    According to several recent threads, there is no longer any reserve below zero, and "zero means zero".
     
  13. TEG

    TEG TMC Moderator

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    With the old Roadster, the published pack size numbers changed at time. Sometimes you saw 53kWh, sometimes 56kWh. I gather there are different ways to state pack capacity based on the context of how it was being measured.

    I know from talking to LEAF engineers, they talked of some "spooky" properties of Li-ion capacity where attempts to measure remaining capacity would suddenly get unexpected results. I don't know if that applies to Tesla packs, but I gather there are a lot of variables related to things like temperature, and current draw that make it difficult to get extremely accurate, reliable results in knowing how much the pack is storing. From what I can see, Tesla is probably best in the industry with doing this, but my point is that the numbers on the energy charts may still be approximate so trying to extrapolate with expectation of perfect accuracy may be slightly misleading.
     
  14. Bighorn

    Bighorn Member

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    Like I said, the 300Wh/RM figure is hearsay based on a reliable source, though. I can absolutely attest to the 280 Wh/RM figure based on extensive personal travel and obsessive analysis. The 308 figure is no longer relevant. With the current figures, only 74kWh is represented in the rated range. There is an apparent reserve still, though I recognize that's been debated as firmware has been modified--part of it is accessible and the rest is not, to prevent bricking. As one who has driven "past zero", I can attest to that.

    I will be using several superchargers today, so I will try to assess the validity of the 300Wh/m replenishment figure.
     
  15. HankLloydRight

    HankLloydRight Fluxing

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    #15 HankLloydRight, Oct 25, 2014
    Last edited: Oct 25, 2014
    I'm not debating the 300 wh/m figure, in fact, I'm agreeing with it. What I'm saying that it is the Wh/m constant used for all remaining rated range calculations, and it doesn't vary depending if you're "charging" or "driving". It really can't. Take this example:

    - Start with exactly 70 kWh in the battery, so at 300 Wh/mile, that would be 233 miles of rated range.
    - Drive exactly 10 kWh of energy. Using your "driving" example at 280 Wh/mile, that would get you 36 rated miles off the range meter
    - In your scenario, the range would now be 197 rated miles (=223-36).
    - You plug into a SC and recharge exactly 10kWh. Using your "charging" rate of 300 wh/m, that puts back in only 33 rated miles.
    - So the rated range would now be 230 rated miles (197+33) for the exact same 70kWh SOC you started with.

    Where did the 3 rated miles go? Now repeat this example 2 or 3 times, and each time, the SOC goes down 3 rated miles.

    The only way this works is if the car uses a CONSTANT of 300 Wh/m for its rated range calculations based on the current SOC. My experience is that if I drive a constant 300 Wh/m, my projected and rated ranges are the same.

    And I'm not talking about the reserve held back to prevent bricking of the battery. The 10 to 20 mile reserve below zero that some people have experienced, according to many sources on this forum, is now gone, and that zero really means zero (plus the bricking reserve), and that doesn't mean you have 10 or so miles left past zero. People have posted recently that when they hit zero miles, the car shutdown.
     
  16. David99

    David99 Active Member

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    Are there people who were able to go beyond 0 before and after the update had the car shut down? I only saw one post of someone who went 70 uphill when the battery showed 4 miles left and the car shut down.

    On the topic of capacity, the usable capacity of a battery is unlike a liquid not a simple fixed amount that can easily be measured. It depends somewhat on the way it is taken out of the battery, the environmental conditions and other things. The exact same battery under the same conditions will provide a higher amount of kWh when driven carefully vs hard. This is independent of motor and inverter efficiency. Losses inside the battery vary with charge and discharge rate.
     
  17. wndrful

    wndrful Member

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    I understand that the capacity would differ based on discharge rate, temperature etc etc. However there has to be some reference condition (which could be not real world) such as 55mph, 25 deg C, elevation etc etc which Tesla uses to rate their batteries. The same rating conditions can be used to determine battery capacity loss and % change in "range" defined in reference conditions.

    Note: These reference conditions might not have much connection with my driving habits or might give me incorrect range. That's okay. I am looking for % drop in range (not range) and relate that to battery capacity loss.
     
  18. Bighorn

    Bighorn Member

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    I understand and appreciate the validity of your argument. I can't explain how the computer handles what happens to that 20Wh/RM delta, but I hope I've collected sufficient data on this weekend's trip to convince you of the numbers.

    I started with a range charge of 260 miles which is 5 lower than on recent trips either due to lack of full cycling or possibly temperature. Based on the 280Wh/RM figure, I pre-determine that I will have 72.8kWh of driving range. At 130 rated miles(50%SOC) remaining, I had used 36.2 kWh which would suggest a slightly lower capacity of 72.4 kWh. If 72.4 kWh allows for 260 rated miles, that works out to 278Wh/Rm.

    I only kept track of rated miles consumed between charges that were uninterrupted by any stops, and the total energy used. At superchargers, I recorded the increase in rated miles and the energy delivered.

    For driving, I used 630 rated miles and consumed 174.7 kWh which works out to 277 Wh/Rm.

    For charging, I added 522 rated miles using 155 kWh which works out to 297 Wh/Rm. There is more noise around this figure because supercharger power is recorded in whole kWh, vs tenths of a kWh for consumption.

    Many miles ago, I recognized that trying to intuit any of this from the projected miles graph was futile. Just to convince myself this was still so, I recorded data to support that.

    I charted whenever Projected miles equaled rated miles using the average mode on all 3 distances.
    The 30 mile average equaled the rated miles at 296 Wh/m one time and over a range of 291-295 Wh/m another
    The 5 and 15 mile average both matched rated miles at 292 Wh/m.
    When the 30 mile average rated efficiency was 280 Wh/m, the rated mlles read 164 while the projected miles were 174. I don't know if this suggests that there's a 10 mile buffer below zero or not, driving at that efficiency level.

    I just read of someone driving into a supercharger last month, 3 miles past zero, so there is still somewhat of a buffer, but understandably people aren't going out of their way to find out the exact number.
     
  19. Denarius

    Denarius Active Member

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    I drove about 3.8 miles (mostly uphill) past 0 on 6.0. I continued driving based off the power limiter. Islandbay noted he was limited at 30kW when the car shut down, so I figured I was safe until my limiter dropped further. This was from a full range charge, but the car sat through the day while I worked so it was not driven in a single session.

    image.jpg
     
  20. scaesare

    scaesare Active Member

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    3a5.jpg
     

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