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Does there exist a charging efficiency curve or table?

Discussion in 'Model 3: Battery & Charging' started by jebinc, Sep 12, 2019 at 7:44 AM.

  1. jebinc

    jebinc M3 LR AWD w/FSD and white premium interior

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    I've heard that it takes a lot more energy (power in per percent gained) to go from 90% to 100% than it does, say, 70% to 80%. Does there exist a Power input percent gained) chart or table? I'd like to know how much more energy is consumed to get those last 10%, for example. Ideally, the chart would be on a power input units per percent charged increment.
     
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  2. woodguyatl

    woodguyatl Member

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    That is not true. Charging is slower as you approach 100% charge but not less efficient.

    edit: It is very marginally less efficient as charging slows due to increased % of overhead but it is insignificant.
     
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  3. jebinc

    jebinc M3 LR AWD w/FSD and white premium interior

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    So, I will see the amperage drop from 48amps normally (HPWC) as it approaches 100% charge?
     
  4. AlanSubie4Life

    AlanSubie4Life Active Member

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    Charging efficiency plot

    If the kW charge rate drops below the supply capabilities then the efficiency will be worse than suggested in above plot due to overhead (it will actually follow this plot, but you need to plug in the effective input kW you are getting at any point in time as the x-axis value to look up the y-axis efficiency). But that will only happen above about 95%. So it is pretty irrelevant.
     
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  5. AlanSubie4Life

    AlanSubie4Life Active Member

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    #5 AlanSubie4Life, Sep 12, 2019 at 8:39 AM
    Last edited: Sep 12, 2019 at 8:45 AM
    Yes. The exact point, exact amount of taper, never paid attention...it is above 95% I am pretty sure. Could be 97%...just picking random values.

    Good to know of course if you are charging in preparation for a trip at the last minute. But from an efficiency standpoint it obviously does not practically matter since no one ever charges to that level routinely.

    I do believe it was the source of some of the error in some efficiency comparisons that have been done by a British website between Teslas and other brands though. The methodology for measuring and comparing wall-to-wheels efficiency was thrown off because they charged to 100% which is not a realistic use case and introduces a lot of overhead and penalized higher range vehicles more due to their methodology.
     
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  6. jebinc

    jebinc M3 LR AWD w/FSD and white premium interior

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    Thanks, but I'm really after a chart that shows units of input power per percentage of battery (or miles of range added). For example charging at 48amps, how much power is needed to go from 50% to 51% charge... Same for all percentages, including 99% to 100% as well as 1% to 2%. Make sense?
     
  7. darth_vad3r

    darth_vad3r Well-Known Sith

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    The overhead of the 3 being "on" is ballpark ~250 W +/- if you aren't sitting in the car. More if you are in the car with screen on, more again if fan, AC, or heat is on (in increasing order).

    L2 charging from 95% to 100% can definitely slow down especially from 11 kW levels ... so if it takes 3x as long to get from 95 to 100 as from 90 to 95, then you are 'burning' an extra 250 W for that extra amount of time.

    Say it's 20 minutes at 240V / 48 A to get from 90 to 95%, then 60 minutes to get from 95% to 100%.

    the overhead from 90-95% is < 100 Wh (~83 in this case), while the overhead from 90-95% is ~250 Wh.

    5% of ~75 kWh is 3,750 Wh, so 83 is 2.222% and 250 is 6.666% ... 4.44% worse.

    Is this significant? That's up to you to decide.

    The car is less energy efficient to drive at 100% because you will have absolutely zero regen. It's possible that lack of efficiency in driving might actually be worse than the charging inefficiency.

    Add on top the fact that charging to 100% isn't the greatest for your battery, and I say, why bother going that high ever. Do we really ever need the full 100%?

    Certainly not at the start of a trip where you're hitting a supercharger within a couple hundred miles you don't ... charging the extra 5% overnight before you leave will save you a few seconds (minute or two tops?) at the first supercharger.

    If your destination is like 94-99% away and you want to try and eek it out without stopping ... MAYBE I'd charge to 100%. I'd prefer to just charge to 90 and stop for a couple minutes to add 10% though. That's my thinking anyways.
     
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  8. AlanSubie4Life

    AlanSubie4Life Active Member

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    #8 AlanSubie4Life, Sep 12, 2019 at 8:46 AM
    Last edited: Sep 12, 2019 at 8:54 AM
    Yes. This shows it but you would have to know how much the charge rate actually tapers. And then you can plug in your input power and look up the y-value for instantaneous efficiency. However, once you get above 95%, the amount of input energy per % will be constantly changing all the way up to 100%.

    I would guess you’re at about 92-93% efficiency normally, and to charge from 99 to 100% you are probably about 60% efficient. Input power drops from 11kW to more like 1kW or less (you might see 5A drawn from your wall connector, only 3 of which are effectively going into the battery). Just plug those rough estimates in on the x-axis.

    This is just a guess on my part. Not sure how low the taper actually goes before the car gives up.
     
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  9. woodguyatl

    woodguyatl Member

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    Yes.
     
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  10. darth_vad3r

    darth_vad3r Well-Known Sith

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    Find your charge power in that chart of mine just quoted, imagine a straight line from 0% all the way to 96%-ish and that's your chart.
    Then move down the line to the left a bit on power as it tapers following essentially the same taper as the Supercharger profiles (refer to @Zoomit 's charts for this). My chart stopped at ~7 kW... if you are going up to 11 kW you need to project a bit higher into the low 90's.

    It's going to look like this roughly for 48 A 240 V:
    Code:
    ____________________         94%
                         \       92%    Efficiency
                           \     90%
                             \   88%
    0   25   50   75   95 97 100%
               % SoC
    
    ASCII charts! :D
     
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  11. woodguyatl

    woodguyatl Member

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    Keep in mind that your % efficiency will be low as you approach 100% but that is because the overhead remains the same but the input power drops precipitously. So while the % looks terrible, the actual power lost is minimal.
     
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  12. jebinc

    jebinc M3 LR AWD w/FSD and white premium interior

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    Right, that's why I asked for the chart I did, with the X-Axis = Power in ("P") and the Y-Axis being battery percent (Charge %) or range gained ("C"). This way, all is normalized. Time ("T") become the variable. (e.g., it will take longer to do 99% to 100% than 50% to 51%). I know this andI don't care about T, just P and C.
     
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  13. AlanSubie4Life

    AlanSubie4Life Active Member

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    As a percentage of overall energy added, yes. On a marginal basis, it is pretty significant. More than 30% of the energy you use is wasted. That is the beauty of efficiency.

    From a practical standpoint, it does not matter at all.

    Each 1% is 750Wh. So to go from 99-100% you would waste perhaps 300Wh if the taper really does get to 1kW. Vs. the normal 60Wh of waste for a 1% increase.

    Just rough numbers.
     
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  14. AlanSubie4Life

    AlanSubie4Life Active Member

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    Can generate a plot if you know the actual taper, from the info provided. The problem is we don’t know the exact actual taper. It can also vary from car to car at very high SoC and may depend on the last time it was fully charged.

    I guess I can do a 100% charge on Chargepoint and grab their data at some point. Otherwise you have to be super patient or have meter logging at home.
     
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  15. darth_vad3r

    darth_vad3r Well-Known Sith

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    If you want a chart for power in "P" (kWh?) on x-axis and y-axis being battery percent gained, you actually want 100 different charts for each SoC level. Or one same chart repeated for 0 to 96-ish, and then 4-8 charts for 96.5, 97, 97.5, 98, 98.5 ....

    Based on what you were asking earlier anyways, it sounds like you wanted something relative to SoC, no?

    You were asking about 90-95 vs 95-100% or something like that (EDIT: it was 90-100 vs 70-80). How does a chart you described above help with that?

    Or do you want ONE chart for your power charging rate, and the x-axis is the SoC?
    But someone else would want it for their power charging rate.

    But the charge rate tapers... so you'd want that reflected too.

    I don't think your requirements above are defined well enough to result in something you will be pleased with. I'm sure there are a few dozen Dilbert comics I could quote or xkcd that would be relevant here ;)
     
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  16. darth_vad3r

    darth_vad3r Well-Known Sith

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    OK, so you want a 3D chart with axes for T, P, and C? How does time factor into your first chart description of P vs C?

    BTW, P vs C is the same chart I made that @AlanSubie4Life quoted above which is P vs E(fficiency)

    You just need to multiple E by the power to get energy added. So basically it's the same shaped curve but steeper... or convert the y-axis to log scale.
     
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  17. jebinc

    jebinc M3 LR AWD w/FSD and white premium interior

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    I know... so no chart exists already in the format I was looking for? I do know how to do math, as am an engineer by education. Was looking for a simple chart, no more. Thanks.
     
  18. darth_vad3r

    darth_vad3r Well-Known Sith

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    Please describe the simple chart. What is the x-axis? What is the y-axis?
     
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  19. MaryAnning3

    MaryAnning3 Supporting Member

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    I would be interested to see that too. I think the reason it is difficult to find is that it is a very small effect. You can measure it yourself with a Watt-hour meter (using 110 V charging, for example). You can compare the Whrs used to get from 55 to 55% to what you use to get from 93% to 98%. You will find I am pretty sure a very high efficiency in terms of the ratio. With your 48A Wall mounted it might be even a tiny bit higher efficiency. I don't think the efficiency depends much on SoC, but i would be curious to see the small variation too.

    PS. I understand that what you really care about is the performance of your wall-mounted, but it is hard to put a meter on that in an isolated way and an experiment with 110V (or 220V) with a watt meter will suffice.
     
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  20. MaryAnning3

    MaryAnning3 Supporting Member

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    I think it is power added to battery divided by power coming from wall vs SoC (binned in 1% intervals)
     
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