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Could the Wall Connector deliver more than the charging limit while preheating?

Discussion in 'Model S: Battery & Charging' started by David29, Dec 12, 2016.

  1. David29

    David29 Member

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    Going into my first winter with home charging and have been thinking a lot about the interplay among charging, preheating, Time of Use Rates, and so on.

    My Wall Connector is set up for 40 amps, which is all my car can use for charging.

    It is my understanding that, when preheating, the car allocates some of the charging capacity to the cabin and/or battery heaters. If the two heaters are both on, it looks as if that could take the full 40 amps (or more if it were available).

    But it occurred to me that the WC can deliver up to 80 amps for cars that can accept it, so the box itself and the cable and connectors can obviously handle that current. And when preheating, the car needs as much as 11 or 12 KW for the battery and cabin heaters, as I understand it. (I have seen conflicting figures in this forum for the KW ratings of the heaters, but they seem to be in that neighborhood.)

    So, practical question: If the WC were on a circuit that can deliver more than 40 amps, say 72 or 80 amps, could the car use that excess capacity for preheating so that the full 40 amps can be used for charging? And if not, wouldn't that be an advantage? I do not see any discussion of this point in the WC installation instructions as a basis for choosing what capacity your circuit should be.

    Probably this has been addressed in past posts but a few minutes did not turn up the answer. Discussion I have seen seems to imply that the car will limit the current to the maximum allowed for charging, without any allowance for the heaters. That seems to be a big disadvantage in cold climates....
     
  2. MP3Mike

    MP3Mike Active Member

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    No, because the heaters are DC, and the AC to DC converter in your car is limited to 40 amps. (There is no where to put the extra AC that the WC could supply.)

    However, you could add the second charger so the car could handle 80 amps, then you could still charge at 40 amps, and have the heaters on at the same time. (At least I think that would work.) I'm not sure if you would have to set the charge rate at 80 amps, and then know that the heaters are drawing down that current so that it doesn't all make it to the battery.
     
  3. David29

    David29 Member

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    As a related comment -- When ordering a new car and deciding upon the car's configuration, buyers in colder climates would do well to think of the higher-capacity charger not only as a means to charge faster (which by itself might not be needed or might not seem to be worth the price in their circumstances), but also as a means to enable full preheating plus a reasonable charging speed in cold weather. Tesla does not mention this as an advantage of the optional larger charger in their configuration page, and I will admit that it did not occur to me when I ordered my car 18 months ago.

    And I suppose that experienced cold weather owners long ago figure out what I have now realized, namely that it is smarter to schedule your charging to complete before you would want to start preheating, to avoid dividing the limited current between the two functions (especially with only the 40-amp on-board charger my car has). I have been trying to charge just before I might need the car, with the idea that charging helps to warm the battery. But now I think that is not the best strategy.
     
  4. ig_epower

    ig_epower Member

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    I have the dual chargers in my car but my home HPWC is only set to 40A. I find that my charging is usually finished by 2 or 3am when I start at about 7pm the night before. When I leave at 7a, I usually preheat the car to melt the snow and warm the cabin at 6:45a. The HPWC is off at the time so no problem in heating and my battery was ready to go long before. It will be different for others of course.
     
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  5. Boatguy

    Boatguy Member

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    First, the HPWC should be set to the size of the circuit to which it is connected which in turn is limited by the size of the wire that was used when it was installed. The car will take what it wants but there is no reason to limit the HPWC to below it's wired circuit. You might have a friend who will use your HPWC someday and would appreciate using all it can provide.

    Second, I seriously doubt the heaters consume 11-12kW. An HPWC set to 40a on a 240v circuit can only put out 9.6kW and that kind of consumption would mean that with the charger going full blast it would still be drawing down the battery to run the heaters, and when driving it would dramatically impact range if sustained for more than a few minutes. The impact on consumption shown by EVTripPlanner when changing the ambient temp from 55F to 20F was only about 9w/mi.

    I think your strategy is sound since charging does warm the battery, though it may cool back down before you get to the car. You should be able to remotely check the cabin temperature before pre-heating and verify the impact of charging. Pre-heating does not take very long and given the day to day variation in the amount of charging needed combined with the fact that the Tesla only only allows selection of charging start, not finish, means you have to set the start time so that it could complete a full charge by your usual departure (unless you change the start time on a daily basis).

    Your S70D would rarely need more than 60kWh of charging, which is roughly 6hrs at 40a. So if your TOU rate ends at 7am, you set your start to 1am and on most days it's finished by 4-5am which means that your pre-heating at 6:30am (just a guess) is not competing with charging in any meaningful way.

    FWIW, I've noticed that irrespective of the charge start time setting, if my car is plugged in and I sit in the car, without "starting" it, it will start up the HVAC and begins drawing from the HPWC immediately (the charge display shows a 2-3a draw). This is kind of annoying since it's usually outside my TOU rate times, but it does illustrate that the car will take electricity when it wants it for other than charging. You might want to go down and observe the HPWC when preheating and see if it's delivering power.

    The bottom line is that you're probably overthinking this and there is no real problem with your approach. :)
     
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  6. ig_epower

    ig_epower Member

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    Just this weekend by the way, I was preheating during the day before I had to run out to do some errands and I noticed that my 40A HPWC was also charging the battery. That makes sense because I read somewhere that the Tesla's have 2 x 6KW heaters. One for the cabin and one for the battery. They are independently operated by the operating system to get the right temperature for the battery when charging AND to heat the cabin as required. If both heaters are on (12KW), that would indicate that my 9.6KW HPWC is insufficient to run everything . In reality, the battery heater is not on all the time - only to get the battery pack up to a proper temperature to attain optimum charging conditions. The same with the cabin heating requirements - it is only set to the last 15-20 min before I head out. Often times, cabin heating schedules and battery heating requirements are never at the same time.

    I never bought into the ICE pre-heating routine because I did not want to waste gas just to step into a warm car but with the electric solution - I now find I love this feature on a cold Canadian winter! I wish in future iterations of the software I could control the mirror/wiper heaters remotely as well. Perhaps even the seats.
     
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  7. dhrivnak

    dhrivnak Active Member

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    At least in the Roadster the charging does not start until my battery is warm. So I am unable to heat AND charge at the same time.
     
  8. davewill

    davewill Member

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    It doesn't work that way. The charger has no clue you are preheating, it just delivers the current that it is programmed to deliver to the battery. The heater is simultaneously drawing whatever amount it actually needs, so that the net amount of charge going into the battery is reduced. It's entirely possible that if you were plugged into a low enough amperage EVSE (6a 120v perhaps), that you could be losing charge during a pre-heat cycle, even if you're plugged in.
     
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  9. David29

    David29 Member

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    FWIW -- I based my statements of heater sizes (2 @ 6 KW each) on statements in various posts on this forum and on an old video by Bjorn Nyland where he measured the energy consumed by his Model S.

    I wondered if one of the reasons Tesla increased the standard charger capacity from 40 to 48 amps was so that an associated WC could be set to 48 amps and that would be about enough to run the two heaters when needed, without draining the battery. It certainly is an advantage of the larger charger, in any case, even if that was not the intent.

    I still think it would be advantageous if the car could draw more than the charger can use, and direct the excess to the heaters. If the supply circuit and the WC and the cable can handle it, why not?

    But maybe there are good reasons and that's why I am not the designer!
     
  10. MP3Mike

    MP3Mike Active Member

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    It isn't possible since the charger is what converts the AC to DC so that the parts in the car can use it.

    If the heaters used AC then you would have to have another invertor in the car that would create more loss and waste more power.

    But, if you want to do that you have options:
    • If you have a classic Tesla you can add a second charger and be able to supply the car with up to 20kW.
    • If you have a refreshed Tesla you can upgrade to the 72A charger and be able to supply the car with up to 17.2kW.
    So if you really want to you can.
     
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  11. David29

    David29 Member

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    Very good point! I was implicitly assuming that the car could use the extra current for heating directly without taxing the charger. Doh!
    Thanks for gently correcting me on that vital point.
     

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