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Charge time. Am I missing something?

Discussion in 'Technical' started by Dutchie, Apr 13, 2018.

  1. Dutchie

    Dutchie Active Member

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    There is something I don't understand. Perhaps someone can help me out?

    Today I have used almost 48 kWh. I charge at 6 kW and the car says it needs 10.5 hours to charge to 90%. I always charge to 90% so when I started my day today it was also on 90%.

    If I used 48 kWh and I charge at 6 kW why does it need 10.5 hours and not 48/6 is 8 hours to charge? Are there so many losses during charging or am I missing something?
     
  2. IdaX

    IdaX Member

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    Two things:

    (1) yes, the charger eats up something like 400W I think

    (2) you may have used 48 kWh driving, but you may have lost some energy due to phantom drain -- the losses that come in when the car uses energy to keep the battery pack warm or perform other tasks while parked and not driving. These don't get counted in your odometer I find.
     
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  3. David99

    David99 Active Member

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    You have 20% losses when charging. Both the charger and the battery itself isn't 100% efficient. The battery is 93% efficient. The chargers around 85%. The car itself also needs some power (coolant pump, computer, cooling fans). The charge time estimate is also not 100% accurate. So all things considered, what you see it realistic and correct.
     
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  4. boaterva

    boaterva Supporting Member

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    It’s also not linear. It will slow down and taper charge at a slower rate at the end like supercharging does. At least mine does!
     
  5. David99

    David99 Active Member

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    Up to 90% there is no reduction in charge speed when charging at only 6 kW.
     
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  6. boaterva

    boaterva Supporting Member

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    When I turn mine on at 80% and try to charge it to 90 or higher for trip it definitely doesn’t charge very quickly. May be case dependent.
     
  7. David99

    David99 Active Member

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    On L2 30 Amp, there is no reduction in charge speed up to 95% or so. If you sit in the car and have the AC or heater running, the car will still show 30 Amps but part of the power will be used for the HVAC thus the battery will charge slower. It's a common point of confusion with Tesla owners. You watch the car charge from the app and see an estimate how long it will take. You go up to the car and sit it in and the time goes up all of a sudden. It's because when you are in it the AC or heater turns on taking up some of the energy.
     
  8. boaterva

    boaterva Supporting Member

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    Nope, no one in the car. Nothing running. I know quite well what is using what.
     
  9. mal_tsla

    mal_tsla Member

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    Yes, some energy is lost as heat in both the battery and the charger, just like when charging a phone.

    Unlike the phone, more energy can be lost, if for example the battery arrives hot and needs to be cooled during charging to prevent degradation.
     
  10. aesculus

    aesculus Still Trying to Figure This All Out

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    Does it seem to vary by temperature? ie slows down when cold or very hot?
     
  11. SageBrush

    SageBrush 2020: Drain the Sewer

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    How do you know ?

    If the car shows Amps in during charging, that will be the number to use and not e.g. a rating.
    You can also buy an inexpensive Ammeter (inductance sensor) that is easy to install at the EVSE or the panel.
     
  12. David99

    David99 Active Member

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    If you are charging on a HPWC with the 72 Amp charger in the car, you will see a reduction earlier. But the OP is talking about 6 kW charger. There is definitely no reduction up to 95% because the power is so low anyways.
     
  13. Dutchie

    Dutchie Active Member

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    The app tells me that.
     
  14. SageBrush

    SageBrush 2020: Drain the Sewer

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    #14 SageBrush, Apr 15, 2018
    Last edited: Apr 15, 2018
    The app ... that talks with what ? And reports what ? There is a path from EVSE to on-board-charger to battery. Losses occur along the way.

    As I told you earlier, if you want to have a somewhat accurate accounting you should place an inexpensive ammeter in your panel on one of the 120v lines attached to your EVSE. I say somewhat because you will still be guessing at the voltage but that is likely to be within a few percent of actual.
     
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  15. Uncle Paul

    Uncle Paul Well-Known Member

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    When you first plug in your Tesla it will throw up an estimated charge time to reach your selected fill. It will take into consideration several factors to post this estimate.

    It will see how much current your socket is providing as well as the charge rate you selected. Will look at the temp of the battery and ambient as well as the condition of your battery.

    From time to time it will reevaluate it's estimate and post what if thinks will be the time to charge.

    It would not suprise me that in a few minutes it is calculating closer to what you are expecting.
     
  16. qadaemon

    qadaemon Member

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    The Tesla app, I'd imagine. When the car is set to display battery % instead of miles, mine shows kw while charging instead of mph. It doesn't display any decimals, so it's only +/- 1kw accurate.
     
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