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Wh/mi increasing with age?

Discussion in 'Model S: Battery & Charging' started by apacheguy, Sep 2, 2014.

  1. apacheguy

    apacheguy Sig 255, VIN 320

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    Does anyone else notice a correlation between age of the battery pack and energy consumption? My MS exhibits a positive correlation - that is, Wh/mi as read on the dash has been steadily increasing. Does that make any sense?

    IIRC, resistive losses within the pack increase as cells degrade. Could this lead to higher energy usage? Obviously it could be any number of things such as tire wear, different driving habits, climate, etc. But for me, the only correlation I seem to observe is with age. Anyone else?
     
  2. tom66

    tom66 Member

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    How many miles...? Cell resistance will increase by about 25-50% over cell age, but we are talking 250,000miles+

    You could just be enjoying the car more, learning what you can get out of it for certain distances.
     
  3. efusco

    efusco Moderator - Model S & X forums

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    I'm approaching 39,000 miles and my Wh/mi continue to improve--most likely due to relaxation of my driving style. In any case, it certainly isn't getting higher. and I'm on an "A" pack.
     
  4. ThosEM

    ThosEM Space Weatherman

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    This raises the interesting question of what to believe among all the quantitative data that the Model S generates for us. My suspicion is that the consumption values are among the most reliable data (tied to actual current and voltage measurements done by the controller), along with distance travelled. Together they imply energy values, which are otherwise difficult to determine, since the energy gauge is given in rated or ideal range distances. The latter are presumably directly proportional to energy content of the battery, though the proportionality constant is somewhat arbitrary and unknown to us.

    This suggests that any given trip can be used to determine the energy storage capacity of the battery as follows:

    Make note of rated range when performing a range charge to full
    Take a trip
    Make note of: distance travelled; average consumption; rated range at start; rated range at end.

    Then battery capacity (above 0 rated range) is:

    distance x consumption x rated range-full / rated range-used

    Example: My battery tops out at 417 km rated range. I recently travelled 402 km at avg. consumption of 175 Wh/km, beginning at 417 km rated range and ending at 24 km rated range; so my battery holds 402 x 175 x 417 / 393= 74.6 kWh when full. This is in line with estimates others have provided of the amount of energy in the battery above the zero rated range state of charge, the remaining ~10kWh being divided between brick protection and some amount of reserves.

    My suggestion is that the above method be used to estimate at any time what the capacity of the batter is currently, and that a trend in that capacity would then be evidence of an actual degradation of the battery that is tied to actual performance on the road.

    On the other hand, if the consumption values do vary with battery condition as suggested by the OP, that would invalidate this procedure, and make it very difficult to monitor the battery condition over the life of the car.

    Thoughts? Does this seem like it's on the right track?
     
  5. tom66

    tom66 Member

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    It's easier than that... we have this BMS screen shot:
    https://plus.google.com/photos/102621178199196500076/albums/5776377343146944705/5776377358860084130?pid=5776377358860084130&oid=102621178199196500076

    Ideal energy remaining = 65.5kWh for a SOC=82.2%

    Quick math shows 100% SOC = 79.7kWh or ~80kWh, in line with estimates of 5kWh of bricking protection which should be sufficient for over 3 months in low power mode. (We know the Roadster BMS eats 2W with no extra pack load, assuming Model S is similar, this gives 2500 hours or about 3 months.)
     
  6. JohnQ

    JohnQ Active Member

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    @ThosEM: Indeed, that's precisely how I'm measuring my battery health. A full range charge, followed by an immediate departure. Drive down close to 0 miles and the energy used is displayed on the screen. On a trip in April I measured my battery as containing 67.7 kWh + 10 miles (approx 3 kWh) of usable energy. My next significant trip will be in 2 weeks and I'll be able to perform the same measurement to see if there is any appreciable change. This all assumes that energy usage is measured accurately on the "trip" screen.

    As for the OP's question, I haven't noticed any correlation but haven't paid close attention..

    - - - Updated - - -

    That assumes the pack can still hold 85kWh worth of energy. We're interested in measuring the actual energy available on our aged packs.
     
  7. ThosEM

    ThosEM Space Weatherman

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    It's not easy if you don't know how to find that screen...?

    Also, this is really just more data presented by the system, and its accuracy or proper interpretation may be uncertain. For example, this method may include a "reserve" amount (below zero rated range), whereas the one I described certainly does not. I'm aware that the existence of a reserve below zero (other than bricking buffer) has been called into question since some recent revisions of the software. I don't see any quick way to verify the existence of a reserve but to drive the car beyond zero range, for which I have not yet found the time.
     
  8. ThosEM

    ThosEM Space Weatherman

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    Good point that the total energy from the trip display is likely just as accurate as the average consumption, so my formula could be simplified:

    capacity (above 0 range) = total energy (trip record) x rated range (full) / rated range (used)

    I'll try tracking this and report back if it changes significantly...
     
  9. Vger

    Vger Active Member

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    Exactly my experience. My lifetime average Wh/mi has dropped from 333 to 326 over the past year. In fact, on my last long trip, I did 305 Wh/mi over about 3,500 mi, despite mostly mountain driving, 80-85 mph stretches, and using the a/c a lot of the time! I have also about 39,000 on an A pack.
     
  10. mknox

    mknox Well-Known Member

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    I have about 32,000 miles on my car and, if anything, my Wh/mi numbers are improving. I drove home from work yesterday (45 miles with 75% freeway) in the rain with a/c on at 75 MPH and was seeing around 260 Wh/mi when I pulled in the driveway. Conversely, I've noticed that my Rated Range numbers on a full charge have dropped quite significantly. I'm lucky if I see 240 miles. I wonder if there is a correlation between these two observations???
     
  11. apacheguy

    apacheguy Sig 255, VIN 320

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    #11 apacheguy, Sep 3, 2014
    Last edited: Sep 3, 2014
    36 K. I'm not talking 25-50% increases, but rather more like 1-2% over the last year or so. I haven't kept good track of it because at the time I thought nothing of it. But for the sake of discussion, let's say 330 Wh/mi to 336 Wh/mi. Is a 2% increase too large to attribute to degradation?

    At which value does rated = current consumption for you? Mine is 308. I get 250 on a range charge. But it seems that if you were getting lower it would go the other way for you.
     
  12. bluetinc

    bluetinc Member

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    Hi Apacheguy,

    By chance are you still on your original battery pack? I noticed the exact same thing with my energy numbers (A pack, 40k miles) and thought it could be a number of things (as others here have suggested). I also noticed that when I set my climate to AC off, and just vented air from outside, the air sent into the car was quite noticeably warmer than outside air. My battery pack fuse went, and with the new D pack I've noticed an immediate drop in my energy usage back to about where (or lower than) I expected it to be. My drop was in the 4-8% range. Also my incoming air on vent doesn't seem to get warmed up nearly as much. Go figure.

    Peter


     
  13. apacheguy

    apacheguy Sig 255, VIN 320

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    Alright, congratulations on your D pack! Yes, my A is still chugging along. I've never tried what you suggest with the AC. Could it be a battery cooling issue perhaps? Did they have to replace a cooling module at the time of your pack swap?
     
  14. ckessel

    ckessel Active Member

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    I've got 25k on my A pack and the Wh/mile hasn't changed much, though there's no way I'd be able to tell a 1-2% difference. I'll see that much difference just depending on if the average temperature was a couple degrees different.
     
  15. mknox

    mknox Well-Known Member

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    I'm pretty sure I'm somewhere close to 300.
     
  16. efusco

    efusco Moderator - Model S & X forums

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    How many miles on your pack? I suspect pretty low if you saw that big of a swing in your Wh/mi over a year. There are so many variables possible here. Have you had the alignment checked, if so, was it when new tires were put on--new tires may reduce your efficiency as would being out of alignment. Is the weather warmer than last year, has traffic gotten worse, are you driving a new commute? 1-2%, unless an very clear trend and with other variables fairly well controlled seems negligible, particularly given the fact that many of us are improving slightly.
     
  17. apacheguy

    apacheguy Sig 255, VIN 320

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    @efusco - 36 K. Yes, there are a lot of variables which is why I wanted to see if a larger subset of owners experienced the same continual increase. There are mixed reports so I don't see how we can reasonably conclude anything. Although I am curious as to how so many are seeing continual decreases.

    Ah, no. I'm talking lifetime averages here not just instantaneous or 30 mi average. For me, that number doesn't really budge due to day to day or even seasonal variability. Obviously folks in more extreme climates will see variation.
     
  18. ToddRLockwood

    ToddRLockwood Active Member

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    The best way to make a comparison like this would be using cruise control while driving a prescribed route at a specific outside temperature with the HVAC set to the same setting. But even doing this, you might get different results due to changes in firmware.

    But then you might simply be having more fun in your Model S.
     
  19. arg

    arg Member

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    #19 arg, Sep 3, 2014
    Last edited: Sep 3, 2014
    Why do you believe that any of those rated range values are reliable? I am particularly suspicious of rated range close to zero - I suspect that the 'number miles below zero' is one of the most variable elements, on the supposition that the whole reason for having miles below zero is to cover up inaccuracies in the range calculation. And the fully-charged miles is also an estimate based on what the system believes is the battery's health - presumably you are doing your own calculations because you don't trust the rated range (full) to tell you the truth, yet you then factor it into your numbers.

    I agree that the energy figure is probably a direct measurement, but anything involving the range display is only an estimate based on factors we don't know and highly likely to vary between software revisions.

    The extreme version of your test - charge to full, drive past zero until the car stops - is probably a fundamental measurement, since we believe that both the range charge termination and the stop-dead are based on direct voltage thresholds on the battery rather than estimates, though it's a very inconvenient test to make. Unfortunately, I can't think of any shortcut that tells what you want to know - fundamentally, determining the state of charge of a part-charged battery is inexact, which is why we don't trust the numbers the software gives us.

    (edit: re-reading this post, sounds a bit aggressive - please read it in the spirit intended of seeking after the best measurement, not attacking yours as such).
     
  20. ckessel

    ckessel Active Member

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    That's what I meant, lifetime. I really have no way to know if I'm 2% above this summer versus last due to temperatures, where I'm driving, how I'm driving, what tires, etc. Even if I knew I was 2% off, it wouldn't occur to me to think that the battery is the reason.
     

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