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Discrepency Between MS and Chargepoing on Energy Used.

Discussion in 'Model S: Battery & Charging' started by democappy, Oct 10, 2016.

  1. democappy

    democappy Member

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    On the trip meter inside my model S it says I have used a total of 444 kWh for the entire time I have owned the car. About 90% of my charging happens at work through a ChargePoint station. ChargePoint says I have charged through them for a total of 500 kWh. How do I reconcile these numbers? When I got the S it was already 90% charged plus I do some charging at home and my current battery is only at 40%. I can't think of any reason the ChargePoint number should be higher.

    The charger is free at work so I am not worried about getting ripped off. I am just curious to understand what I am missing here. Some sort of electricity loss between the charger and my battery?
     
  2. McRat

    McRat Active Member

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    The amount of electricity pumped into your car does not equal the amount of energy stored in the battery.

    There are losses. Now you know what the losses are.
     
  3. democappy

    democappy Member

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    This would probably work out to a little over a 20% loss rate. Is that about what you would expect?
     
  4. McRat

    McRat Active Member

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    500/444 = 13%, about right.
     
  5. democappy

    democappy Member

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    I don't have my exact % of the 444 that came from ChargePoint. I am going to assume ~52kWh came from the battery originally charged at delivery. I have probably gotten ~100 miles from charging at home. I average 316kw/m so I will say another 32kWh from home charging. However, I still had roughly 40% of 60 battery left so I will say 24kWh charged, but not used yet.

    With that I am thinking 500kWh charged for (444-52-32+24) = 384kWh of actually usable kWh. That puts me at about 23% so let's say 20-25% since the math was a little fuzzy and I used some rounding and best guesses. 20-25% seems sort of high, but I don't really have anything to compare against. It has no impact at all to me really, but I thought it very interesting so I just want to make sure I understand.

    One more question along this same path. The charger is a 6.6kWh charger, but I always get ~5.75kWh. In my mind I had always sort of put that as maybe the drop there was the loss rate (it would be ~13%), but I just assumed it was more of limited by the car only wanting to take 30A at a time from it.

    Still learning the basics of charging. I know more about electricity in a couple months of ownership than I ever thought I would.:D
     
  6. McRat

    McRat Active Member

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    The onboard charger has design losses which are turned into heat. The battery itself has internal resistance which is turned into heat.

    Any time you feel heat in electronics, you are converting electricity into heat. ie - If you have 13% losses, when you feed it 8kW it is creating 1,000 watts of heat that must be removed.
     
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  7. swaltner

    swaltner Member

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    I'm not familiar with the ChargePoint gear, but my OpenEVSE system at home also keeps track of a running total of power distributed to my Nissan LEAF. They never match up because the OpenEVSE doesn't have a voltmeter in it, so it simply assumes that the voltage is steady at 240V. It never is, so the kWh calculation is always a little off. You *may* be running into something similar.

    Consumer electronics making poor calculations for kWh consumed/delivered is one reason that some states ban the sale of electricity by entities other than the electric utilities.
     
    • Like x 1
  8. DarkMatter

    DarkMatter Member

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    It's a 30 amp charger, not a 6.6 kW charger. All of the equipment involved on that side is rated in amps. Since it's a commercial site the voltage is 208 instead of 220, making it at best a 6.2 kW charger. If the voltage is a little low you could easily be below 6 once you count the conversion inefficiency and the battery cooling overhead.
     
  9. democappy

    democappy Member

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    Being 30 amp makes a lot of sense. ChargePoint lists the chargers as "Level 2, J1772, 6.6 kW" so that is where I was getting my 6.6kW from. I assume that is a generic label they put on all J1772s. Maybe they think users understand kW better than amps.
     
  10. jdw

    jdw Member

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    #10 jdw, Oct 10, 2016
    Last edited: Oct 10, 2016
    The battery DC current is different than the AC input current, which can introduce some confusion.

    If you're charging at 240 V @ 30 A on the AC side you are inputting about 7.2 kW, but on the DC side, you would see something like 21A at 340V which is happily, about 7.2 kW. Charging at 240v/30A will be around 85%-90% efficient +/- so in reality you will see power losses on the DC side.

    DC current going into the battery will always be less than the AC current since the battery (DC) voltage will always be higher than the AC input voltage. That DC current will also drop as the state of charge and battery voltage increases, making measuring power more useful than measuring amperage.
     
  11. Cyclone

    Cyclone Active Member

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    It is indeed the discrepancy between ChargePoint trying to be "user-friendly" off a generic-calculated number and the actual number.

    My car is plugged into a ChargePoint 30 amp, 200v outlet right now

    IMG_3572.jpg
    The first image (above) shows my car in the plug and offers 6.6 kW. The second image (below) shows I am actually getting 5.95 kW.
    IMG_3573.jpg
     
  12. AWDtsla

    AWDtsla Active Member

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    Chargepoint should never list their chargers at 6.6kW. They are almost always coming of industrial 3-phase, which makes them ~200V under load, not the ~240 you'd get at home. But hey, at least you found one that wasn't out of service :rolleyes:
     
  13. chipmunk

    chipmunk Member

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    If you join teslafi.com, it will show you statistics on your individual charges that will show the loss. In general, the slower it charges, the more inefficiency there seems to be.

    upload_2016-10-12_9-44-19.png
     
  14. AWDtsla

    AWDtsla Active Member

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    It's computing *something* but I wouldn't use that as an absolute number. Especially because the draw numbers that are reported by the API seem to have large errors. But yes, the slower you go the worse it is, because the fixed draw while charging is so high, there's less power available to go into the battery.
     
  15. chipmunk

    chipmunk Member

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    Right. It's certainly not perfect, but it's the most accurate and easy way I know of to get information for those of us without chargers that report energy usage.
     
  16. bkp_duke

    bkp_duke Member

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    Actually that's not true. There is an efficiency curve, if you charge too fast it is also inefficient, sometimes more so than slower charging.

    Rule of thumb, if you hear the BMS (Battery Management System) spin up the AC fans to keep the pack cool, you are charging outside of optimum efficiency. This is rarely a problem with single charger equipped cars, but I have seen it on dual charger cars frequently. Depending upon the ambient temp, we usually charge at about 65A to keep the fans from coming on (they almost always do with 80A).

    Someone around here actually graphed this out, I'm just having trouble finding it right now.
     
  17. AWDtsla

    AWDtsla Active Member

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    Too many conditions. Current ambient temp/humidity, starting pack temp, starting SoC, total amount SoC added.
     
  18. hacer

    hacer Member

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    #18 hacer, Oct 13, 2016
    Last edited: Oct 13, 2016
    Here https://teslamotorsclub.com/tmc/posts/1639573/ is where I posted a curve on that (note that many data points added since I wrote that text - the graph is a live google graph but the post text is static).

    This was done with a refreshed model S with an on-board 48A charger. The line energy was measured with a power meter, but I compared that to the integration of the API charger voltage times charger current sampled every 3 seconds and they agreed very closely. I don't know how teslafi does the calculation nor how often it queries the car but it is certainly possible to get fairly accurate figures on the energy consumed from the wall from the API. Note that there is some significant scatter - in at least one case with low-current charging (everything is 240VAC) I noticed the cooling system was running because it was quite a hot day and it was hot in my garage. All of this data is from the summer time during mostly mild temperatures, I think there were few instances where the cooling system turned on at the start of the charging. I don't know whether or not the cooling turned on later since I didn't attend the whole charging, just let the computer gather the data and record the meter at the beginning and at the end.

    Note that this is the efficiency of the charger only. There are also losses in the battery itself. Adding 50kWh to the battery does not mean you can draw 50kWh out and end with the original state of charge.
     
  19. hacer

    hacer Member

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    May data comes from a wide variety of conditions. I do think that high ambient or battery temperaures (which I believe rarely were seen in my data set but I never measured them) cause lower efficiency. Even so, there is an obvious trend that shows peak efficiency at 25A (the car current setting which is AC current, not battery current).
     
  20. AWDtsla

    AWDtsla Active Member

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    Not in my car, I tried many charges below max. And I see people think the reported charge current is accurate. It's not. It's like +/- 1A in my car. i.e. at a certain setting the actual draw is -1 from reported, at another setting it's +1!!!! Of course the behavior seems to change over time with software updates as well.
     

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