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What is the Most Efficient Charging Amperage?

Discussion in 'Model S: Battery & Charging' started by bollar, Dec 7, 2014.

  1. bollar

    bollar Disgruntled Member

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    #1 bollar, Dec 7, 2014
    Last edited: Dec 10, 2014
    A question I have not seen answered is what charging amperage is most efficient for the Model S. I think I have the necessary data and I have begun to tabulate it as I charge each night. Before I get too far along, I thought I'd ask the teeming masses if there's anything else I should try to collect. Here's the current table:
    Code:
    [TABLE="width: 975"][TR][TD="width: 65"]Date[/TD][TD="width: 65"]kWh[/TD][TD="width: 65"]Temp F[/TD][TD="width: 65"]Pctge[/TD][TD="width: 65"]Amps[/TD][TD="class: xl65, width: 65"]Start Time[/TD][TD="class: xl66, width: 65, align: right"] Ideal[/TD][TD="class: xl66, width: 65, align: right"] Rated[/TD][TD="class: xl66, width: 65, align: right"] Estimated[/TD][TD="class: xl65, width: 65"]Stop Time[/TD][TD="class: xl66, width: 65, align: right"] Ideal[/TD][TD="class: xl66, width: 65, align: right"] Rated[/TD][TD="class: xl66, width: 65, align: right"] Estimated[/TD][TD="width: 65"]kWh/Ideal[/TD][TD="width: 65"]Minutes/Ideal[/TD][/TR][TR][TD="class: xl63, align: right"]12/7/14[/TD][TD="class: xl64"]30.998[/TD][TD="class: xl64, align: right"]45.7[/TD][TD="class: xl64, align: right"]90[/TD][TD="align: right"]40[/TD][TD="class: xl65, align: right"]1:00 AM[/TD][TD="class: xl66, align: right"] 155.0[/TD][TD="class: xl66, align: right"] 134.0[/TD][TD="class: xl66, align: right"] 116.5[/TD][TD="class: xl65, align: right"]4:06 AM[/TD][TD="class: xl66, align: right"] 261.6[/TD][TD="class: xl66, align: right"] 226.2[/TD][TD="class: xl66, align: right"] 196.3[/TD][TD="align: right"]0.290787992[/TD][TD="class: xl67, align: right"] 1.74[/TD][/TR][TR][TD="class: xl63, align: right"]12/6/14[/TD][TD="class: xl64"]34.387[/TD][TD="class: xl64, align: right"]54.7[/TD][TD="class: xl64, align: right"]90[/TD][TD="align: right"]40[/TD][TD="class: xl65, align: right"]1:01 AM[/TD][TD="class: xl66, align: right"] 144.2[/TD][TD="class: xl66, align: right"] 124.7[/TD][TD="class: xl66, align: right"] 119.7[/TD][TD="class: xl65, align: right"]4:37 AM[/TD][TD="class: xl66, align: right"] 262.0[/TD][TD="class: xl66, align: right"] 226.5[/TD][TD="class: xl66, align: right"] 221.7[/TD][TD="align: right"]0.291910017[/TD][TD="class: xl67, align: right"] 1.83[/TD][/TR][TR][TD="class: xl63, align: right"]12/5/14[/TD][TD="class: xl64"]18.479[/TD][TD="class: xl64, align: right"]61.5[/TD][TD="class: xl64, align: right"]90[/TD][TD="align: right"]80[/TD][TD="class: xl65, align: right"]1:02 AM[/TD][TD="class: xl66, align: right"] 200.0[/TD][TD="class: xl66, align: right"] 172.9[/TD][TD="class: xl66, align: right"] 172.5[/TD][TD="class: xl65, align: right"]1:57 AM[/TD][TD="class: xl66, align: right"] 262.0[/TD][TD="class: xl66, align: right"] 226.5[/TD][TD="class: xl66, align: right"] 226.2[/TD][TD="align: right"]0.298048387[/TD][TD="class: xl67, align: right"] 0.89[/TD][/TR][TR][TD="class: xl63, align: right"]12/4/14[/TD][TD="class: xl64"]38.272[/TD][TD="class: xl64, align: right"]58.3[/TD][TD="class: xl64, align: right"]90[/TD][TD="align: right"]80[/TD][TD="class: xl65, align: right"]1:02 AM[/TD][TD="class: xl66, align: right"] 116.6[/TD][TD="class: xl66, align: right"] 100.8[/TD][TD="class: xl66, align: right"] 95.0[/TD][TD="class: xl65, align: right"]3:07 AM[/TD][TD="class: xl66, align: right"] 261.6[/TD][TD="class: xl66, align: right"] 226.2[/TD][TD="class: xl66, align: right"] 213.5[/TD][TD="align: right"]0.263944828[/TD][TD="class: xl67, align: right"] 0.86[/TD][/TR][TR][TD="class: xl63, align: right"]12/3/14[/TD][TD="class: xl64"]37.16[/TD][TD="class: xl64, align: right"]48.9[/TD][TD="class: xl64, align: right"]90[/TD][TD="align: right"]80[/TD][TD="class: xl65, align: right"]1:02 AM[/TD][TD="class: xl66, align: right"] 133.1[/TD][TD="class: xl66, align: right"] 115.1[/TD][TD="class: xl66, align: right"] 115.1[/TD][TD="class: xl65, align: right"]2:52 AM[/TD][TD="class: xl66, align: right"] 261.2[/TD][TD="class: xl66, align: right"] 225.8[/TD][TD="class: xl66, align: right"] 226.3[/TD][TD="align: right"]0.29008587[/TD][TD="class: xl67, align: right"] 0.86[/TD][/TR][/TABLE]
    Advice I'd appreciate:

    - How many samples are required at each selected amperage?
    - Should I try to collect other data?
    - What metrics / ratios are most interesting?
    - And other things you think of.

    Sources:

    - kWh: Powerhouse Dynamics SiteSage
    - Charging Times & Mileages: VisibleTesla
    - Charging: Tesla HPWC (80 amp)

    Code:
    [TABLE="width: 410"][TR][TD][/TD][TD]Summary[/TD][TD][/TD][TD][/TD][TD][/TD][TD][/TD][TD][/TD][/TR][TR][TD]Amps[/TD][TD]# Samples[/TD][TD]Min kWh/Ideal[/TD][TD]Avg kWh/Ideal[/TD][TD]Max kWh/Ideal[/TD][TD]StdDev kWh/Ideal[/TD][TD]Avg Minutes/Ideal[/TD][/TR][TR][TD]40[/TD][TD="align: right"]3[/TD][TD="align: right"]0.292[/TD][TD="align: right"]0.295[/TD][TD="align: right"]0.303[/TD][TD="align: right"]0.006[/TD][TD="align: right"]1.8[/TD][/TR][TR][TD]80[/TD][TD="align: right"]3[/TD][TD="align: right"]0.264[/TD][TD="align: right"]0.284[/TD][TD="align: right"]0.298[/TD][TD="align: right"]0.018[/TD][TD="align: right"]0.9[/TD][/TR][TR][TD]60[/TD][TD="align: right"]2[/TD][TD="align: right"]0.282[/TD][TD="align: right"]0.290[/TD][TD="align: right"]0.297[/TD][TD="align: right"]0.010[/TD][TD="align: right"]1.1[/TD][/TR][TR][TD]Grand Total[/TD][TD="align: right"]8[/TD][TD="align: right"]0.264[/TD][TD="align: right"]0.290[/TD][TD="align: right"]0.303[/TD][TD="align: right"]0.012[/TD][TD="align: right"]1.3
    
    [/TD][/TR][/TABLE]
    [TABLE="width: 410"][TR][TD][/TD][TD]Deciles[/TD][TD][/TD][TD][/TD][TD][/TD][TD][/TD][TD][/TD][/TR][TR][TD]Amps[/TD][TD]# Samples[/TD][TD]Min kWh/Ideal[/TD][TD]Avg kWh/Ideal[/TD][TD]Max kWh/Ideal[/TD][TD]StdDev kWh/Ideal[/TD][TD]Avg Minutes/Ideal[/TD][/TR][TR][TD]40[/TD][TD="align: right"]3[/TD][TD="align: right"]0.292[/TD][TD="align: right"]0.295[/TD][TD="align: right"]0.303[/TD][TD="align: right"]0.006[/TD][TD="align: right"]1.8[/TD][/TR][TR][TD="align: right"] 40[/TD][TD="align: right"]2[/TD][TD="align: right"]0.292[/TD][TD="align: right"]0.292[/TD][TD="align: right"]0.292[/TD][TD="align: right"]0.000[/TD][TD="align: right"]1.8[/TD][/TR][TR][TD="align: right"] 10[/TD][TD="align: right"]1[/TD][TD="align: right"]0.303[/TD][TD="align: right"]0.303[/TD][TD="align: right"]0.303[/TD][TD="align: center"]#DIV/0![/TD][TD="align: right"]1.9[/TD][/TR][TR][TD]80[/TD][TD="align: right"]3[/TD][TD="align: right"]0.264[/TD][TD="align: right"]0.284[/TD][TD="align: right"]0.298[/TD][TD="align: right"]0.018[/TD][TD="align: right"]0.9[/TD][/TR][TR][TD="align: right"] 30[/TD][TD="align: right"]1[/TD][TD="align: right"]0.298[/TD][TD="align: right"]0.298[/TD][TD="align: right"]0.298[/TD][TD="align: center"]#DIV/0![/TD][TD="align: right"]0.9[/TD][/TR][TR][TD="align: right"] 50[/TD][TD="align: right"]2[/TD][TD="align: right"]0.264[/TD][TD="align: right"]0.277[/TD][TD="align: right"]0.290[/TD][TD="align: right"]0.018[/TD][TD="align: right"]0.9[/TD][/TR][TR][TD]60[/TD][TD="align: right"]2[/TD][TD="align: right"]0.282[/TD][TD="align: right"]0.290[/TD][TD="align: right"]0.297[/TD][TD="align: right"]0.010[/TD][TD="align: right"]1.1[/TD][/TR][TR][TD="align: right"] 30[/TD][TD="align: right"]1[/TD][TD="align: right"]0.297[/TD][TD="align: right"]0.297[/TD][TD="align: right"]0.297[/TD][TD="align: center"]#DIV/0![/TD][TD="align: right"]1.1[/TD][/TR][TR][TD="align: right"] 40[/TD][TD="align: right"]1[/TD][TD="align: right"]0.282[/TD][TD="align: right"]0.282[/TD][TD="align: right"]0.282[/TD][TD="align: center"]#DIV/0![/TD][TD="align: right"]1.1[/TD][/TR][TR][TD]Grand Total[/TD][TD="align: right"]8[/TD][TD="align: right"]0.264[/TD][TD="align: right"]0.290[/TD][TD="align: right"]0.303[/TD][TD="align: right"]0.012[/TD][TD="align: right"]1.3[/TD][/TR][/TABLE]
    * Deciles represent roughly how much of a charge was completed. 10 < 30 miles, 20 < 60 miles, etc.
     
  2. Thread Summary

    Thread Summary Member Generated Post Wiki

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    #2 Thread Summary, Dec 7, 2014
    Last edited by a moderator: Dec 20, 2014
    This wiki is a summary of my Model S Charging findings first covered in this thread: What is the Most Efficient Charging Amperage?

    In short, I am measuring the kWh required by my car to charge to 90% at various amperages, temperatures and battery discharge states and I'm looking to discover if there is an amperage that uses fewer kWh per Ideal Mile charged.

    My findings are that there is no statistically significant difference in the parameters tested so far (but the experiment continues).

    Data Sources:
    - kWh: Powerhouse Dynamics SiteSage
    - Charging Times & Mileages: VisibleTesla
    - Charging: Tesla HPWC (80 amp)

    Observations:

    - We're currently at 17 samples -- still a small number, but find that the average kWh per Ideal Mile is starting to converge around .287 for all tested charging rates. The differences are statistically insignificant on their own, and are probably within the measurement variance of my equipment.

    Summary Table by Amperage

    Tesla_Amps.png


    Summary by Ambient Outdoor Temperature at Charge Start (Rounded Down to Nearest 10 degrees F)

    Tesla_Temp.png

    Summary by
    Ambient Outdoor Temperature at Charge Start (Rounded Down to Nearest 10 degrees F) and Charging Amperage
    Sorry, I need to make this chart more clear
    Tesla_TempAmps.png

    Summary by Percent of Battery Charged (Rounded Down to Nearest 10 Percent)

    Tesla_Charge.png

    Summary by Day of Week

    Tesla_DoW.png


    Summary by Normalized Time (Time to Gain 1 Ideal Mile Normalized to 80 Amp Charge)


    Norm Minutes




    Amps# SamplesMin of Norm Min/IdealAverage of Norm Min/IdealMax of Norm Min/IdealStdDev of Norm Min/IdealAvg Minutes/Ideal
    2030.830.840.850.0113.4
    4070.850.880.940.0351.8
    6040.840.850.860.0101.1
    8030.860.870.890.0160.9
    Grand Total170.830.860.940.0281.7
     
  3. ibcs

    ibcs Member

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    Great idea - Can't wait to see your results.
     
  4. LMB

    LMB Member

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    (LMB spouse)

    Are you measuring kwh where you connect to the power company or using the car's readout? The reason I ask is that if you are using the car's information, you may also have to consider I-squared-R losses in the line to your garage/charging spot.

    For example, we have a 125-foot run of #6 from the house to the garage. At 40 amps, we see about 400 watts of line loss but at 24 amps only about 150 watts. Since wasted power is proportional to the square of the current but charging time is inversely proportional to the current, wasted line power increases linearly with current. If you have very short lines between the power company and your charging location, this is probably not an issue.
     
  5. jerry33

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

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    Ambient temperature should be included too because the pack may have to heat or cool while charging.
     
  6. bollar

    bollar Disgruntled Member

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    I'm measuring at the breaker box and it's a ~30 foot run between the breaker box and the HPWC.
     
  7. bollar

    bollar Disgruntled Member

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    #7 bollar, Dec 7, 2014
    Last edited: Dec 7, 2014
    Okay, I also have that available to me.

    Edit: And added to the original table
     
  8. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    The optimum may be somewhere in between 40 and 80 or less than 40 Amps. I suggest collecting data at 20 Amps and 60 Amps as well. If 60 is the best, try 50 and 70 Amps, etc. Depending on measurement error, you should be able to find the optimum current with a near binary search pretty quickly. This will be for your configuration, wiring, etc, but should be a good guideline for others.

    Thanks for collecting the data!
     
  9. qwk

    qwk Model S P2681

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    Tesla had a charging efficiency graph(I'll see if I can find it again), and 80A was most efficient, followed very closely by 40A. Anything less than 30A, and the efficiency took a nose dive.
     
  10. bollar

    bollar Disgruntled Member

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    Good idea. Will at least collect 20 40 60 80 and might as well continue as long as it seems like there's a reason to do so.

    Added current summary to the first message in this thread.
     
  11. kennybobby

    kennybobby Member

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    How are you defining 'efficient'--and for what device are you wanting to calculate the efficiency, the charger or the battery?

    For any device in which efficiency is desired and power is a unit of measure it would seem that you need to measure both the input and output voltage and current independently of the on-board measurement parameters such as turtles or ideal/rated miles, etc. It seems that there are too many unknown variables in the on-board calculatus beyond your control.

    But if by efficient you mean how quickly the pack gets charged, then the pack will be charged up in less time using a higher current.
     
  12. richkae

    richkae VIN587

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    There could be edge effects of using one or both chargers.

    If you have a dual charger car how is the power split between the chargers? If you charge at 40amps, is only one charger used, or is 20 amps consumed by each?
    Is only one charger used up to 40amps, then both above 40amps? Do they split the power evenly after that? What is the effect of that?
    Are the individual chargers most efficient at a certain amperage? Does that make for a more complex curve when combined?
     
  13. AoneOne

    AoneOne Member

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    The 12/4 result is an outlier, taking substantially less energy per mile of charge. From the data, it would seem that this might be lower-charge state of the battery at the start.

    If you have the ability to collect the kWh usage during charging and correlate that to range gained, it might show that efficiency differs with the state of charge. If not, it might be best to group the data by starting charge.

    In either case, I'm really looking forward to further results. Many thanks.
     
  14. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    Good point! From 1 to 40 Amps, the MS uses one charging module only. From 41 Amps and up, the current is split between the dual charging modules.
     
  15. richkae

    richkae VIN587

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    Are you saying that at 41 amps, each charger gets 20.5 amps?
     
  16. tga

    tga Active Member

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    Yes
     
  17. bollar

    bollar Disgruntled Member

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    The question that started me down this path was which amperage selection would result in the fewest kWh consumed at the utility power meter to get to 90% charge. I agree there are lots of types of efficiency and perhaps we'll be able to discover some others as a result of this exercise.

    - - - Updated - - -

    Yes, once we have more data points. we can group the data by starting charge (or probably starting Ideal Miles). The kWh number does represent kWh delivered only during the charging period.
     
  18. liuping

    liuping Active Member

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    They have a calculator here: Tesla Charging | Tesla Motors

    According to it, charging for "300 miles" is 98.7 kWh @ 240/80A, 99kWh @ 240/40A, 100.8kWh @ 240/24A, and 132.6kWh @ 110/12A

    By far the most important thing is to use 240V charging (saves over 30%!).

    The improvement from 240V 40A to 80A is .3kWh, which significantly less than the vampire losses that occur during the charge.
     
  19. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    But I am sure that calculation ignores the wiring losses from the meter to the car. I have a pretty stiff connection with #2 wire to my HPWC; still I see an extra 2-3 Volts drop from 40A to 80A. 3V at 80A is an extra 0.24kW. For a 4 hour charging session, that is about 1.0 kWh of loss in wires beyond the loss at 40A. If the there is a longer run of #3, that loss gets even bigger.
     
  20. ljwobker

    ljwobker Geek.

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    In general you're going to want the highest voltage you can get, at 80A. The wire resistance is a function of the length of the run, the material, and the size/gauge/cross section. The current through the wire does NOT matter in relative terms - it's Ohms per foot based on cross section and material (in reality, always copper these days).

    So your two major efficiency losses are:
    1) the power required to run the coolant/thermal system for the battery. This is at least a few hundred watts based on my data, which sounds about right for the fans and pumps.
    2) the rectification losses when you convert the household AC power into DC power for the battery. With every power system I've worked on, this is *mostly* a straight percentage once you get into a sane power loading.

    So given these two things, you want to amortize the power it takes to run the pumps and fans over as much remaining power (i.e. what's going into the battery) as you can.

    I've got a bunch of data between [email protected] and [email protected] and it all fits this curve. I can't swear on my life that having a second charger wouldn't change it, but theoretically it SHOULDN'T change it - especially if the chargers loadshare (As would make a lot of sense)
     

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