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Charging while preheating cabin w/ HVAC

Discussion in 'Model S: Battery & Charging' started by scaesare, Jan 5, 2014.

  1. scaesare

    scaesare Active Member

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    I am using the excellent "Visible Tesla" app to manage/monitor my car from my desktop, and notice something interesting yesterday:

    It's been cold here recently, so I opted to start warming the cabin via HVAC before a trip. At the same time I decided to try and get the pack up to temp as well (to avoid regen limiting), so I bumped the charge slider up a bit, and started the car charging.

    So my surprise, the Battery Current dropped in to the negative, about -3.6A or so. Normally my battery current is about 21A and my charger power is 9KW (both about correct for my 240V/40A setup). In this case that implies I was using all 9KW of my charger power, as well as an additional 1.4KW of battery current on order to run to just the HVAC & whatever battery heaters that car may have enabled.

    I've seen my car pull about 3-6KW before, but never that much, especially as it wasn't the coldest temps the car has been in (probably high 30's/low 40's in my garage). As a matter of fact, I've heard that 6KW is the max the car will pull when heating (although I'm unclear if that's battery heater alone)

    In any case, it seemed rather high, so I toggled the HVAC off, then back on, and sure enough my battery current jumped back up to about 15-16 amps, with is what I expected.

    Weird car behavior... not sure what happened there.
     
  2. jerry33

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

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    You set the Amps on the charging screen and that's all the amps that the car will pull. It's then divided between charging and preheating. Perfectly normal. It should go back to positive before the charge is completed.
     
  3. Vger

    Vger Active Member

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    I have seen this too. My tentative observation is that the cabin heater goes full blast when pre-conditioning remotely, even if you have the "Eco" mode set for driving. This makes some sense, since Tesla presumes you are in a hurry to get the car ready to go. Once you hit temperature set point, I believe it backs off quickly, and you will get more shore power for charging.
     
  4. jerry33

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

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    I'm not sure about that. I've found that when set to ECO the preheating from the App stops at about 5C lower than when in standard mode.
     
  5. scaesare

    scaesare Active Member

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    #5 scaesare, Jan 5, 2014
    Last edited: Jan 5, 2014
    I understand that.

    My point was that I was surprised that the HVAC + battery heaters (assuming they are on) take 11KW by themselves.... and that toggling the HVAC off then on changed it significantly.

    - - - Updated - - -

    The 6KW I heard for pack heaters must be in addition to whatever the HVAC system can draw.... which appears to be another 5KW or so...
     
  6. cinergi

    cinergi Active Member

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    FWIW, I've seen the car pull 28 amps at 240 just for cabin heating alone (6.7 kW).
     
  7. Doug_G

    Doug_G Lead Moderator

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    I can guarantee that's not the case. On my recent trip to Toronto I had to go out to the car at my charging stopover and switch off "Range Mode" because the heating power was insufficient to warm the cabin. I'd rather have it heated up from AC power, thank you very much!
     
  8. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    Here are some examples that I grabbed with my TED 5000 measuring what the MS was using from my HPWC. The first example shows the power needed when preheating from a cold soak in a 50˚F, 10˚C, garage. Notice the the preheat was still drawing power after several 1/2 cycles. (That power spike at midnight is the vampire taking a drink, and the Voltage drop at 11pm is the start of my TOU cheap rate and the ETS heater starting to charge.)

    After I saw this, I decided that during my range charge for a long trip, that I would start pre-heating as soon as the taper had more than 5kW margin from the 20 kW output of the HPWC and continue until the range charge was complete and I was ready to leave. You can see in the second plot, the preheat was still drawing power after 2.5 hours; I had the internal temp set to 79˚F, but the interior rose to 90˚F. My theory is that the car is still heating the battery and that the waste heat from the battery is heating the cabin; at least the HVAC did not turn on cooling.

    BTW, after this 2.5 hour preheat in a 50˚F garage and outside temps at about 14˚F, -10˚C, my additional Wh/mi had only a minor increase. It seems that you really need a multi-hour preheat, even starting in a 50˚F garage to really mitigate losses to the battery and cabin heaters.

    Preheating.png

    Range Charge Complete w heat.png
     
  9. scaesare

    scaesare Active Member

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    Yeah, it looks like cabin HVAC + pack heaters can consume 12-KWh just themselves... that's a pretty hefty draw...
     
  10. tezco

    tezco Sig P85

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    Wow, that is an amazing amount of power still going to heat the pack after a charging cycle, although I guess the cycle must have been relatively short since I'm assuming it was just a top-off. How much time between the last charge and the top-off? Wonder what this looks like with a cold-soaked battery starting at 0°? Sometime I think I'm using as much electricity preheating the battery and cabin as I am for the actual drive. Sorry Mr Polar Bear....I'm trying to be Green.....really.
     
  11. tezco

    tezco Sig P85

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    I was just thinking, my charging peaks never show the sawtooth patterns that you experienced, so I'm wondering if all this power is just going to cabin preheat? Here's a typical charge from last week with the axes adjusted so that even minor fluctuations of less than 0.1 A are visible. (Amps is the black line.) Battery temp should have started out at 40° F since the car sat in the garage for 16 hrs prior to the charge. If there is any battery preheat in my garage during charging (at 40°), it seems to be continuous during the charge. (I suppose there could be a different setpoint for ideal battery temperature during charging only, vs during charge + preheat.)

    But, I've never seen a charging curve where it appeared that battery charging or cooling started first by itself, followed by a larger peak due to a delay in the start of charging. (Note: My garage thermostat is set at 32°F, so it doesn't get any colder unless I run out of propane.)

    Typical MS Charge.JPG
     
  12. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    I was not sure when I was going to leave that morning, so I charged to 90% or about 225 rated miles the night before; a full range charge for me is 255 miles after 12,000 miles and 16 months. As you can see from the plot it took about a 1/2 hour for the taper to go from 20 kW to 10 kW charging. It really is amazing how much power it takes to warm up a cold soaked battery. I'm not sure getting the battery really warm is worth it unless you are about to start on a range-challenged, winter, hypermile adventure like I did that day. :wink:

    - - - Updated - - -

    The difference is that I turned on the cabin pre-heat with the App after the taper started. The on/off cycling that you see is the car turning off the App external pre-heat request after a 1/2 hour and then me turning it back on to keep the pre-heat going; it takes way more than 1/2 hour to warm the battery!

    Let's do some round number, ball park estimates. I don't intend these to be exact numbers, but hopefully are +/- 50%.

    Assumptions:
    1. Battery thermal mass is equivalent to 360 kg of water. The electrolyte is mostly water.
    2. Preheat power is 4,200 Watts.
    3. There are 4.2 Joules per calorie (1 g water changed 1˚C.) Now you see why I chose 4.2 kW to make the numbers simple.
    From this:
    1. 4200 Watts is 1,000 calorie/sec
    2. It takes 360 seconds or 6 minutes to raise the battery temperature 1˚ C.
    3. 4.2 kW of battery heat will raise the battery temperature 10˚C or 18˚F per hour.
    As you can see, it takes a lot of energy (power over time) to warm up that battery pack.
     
  13. tezco

    tezco Sig P85

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    Do we know what the setpoints are for active battery heating or cooling?
     
  14. mknox

    mknox Well-Known Member

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    I'm currently limited to a 20 amp 240 volt outlet in the garage. In this very cold weather when I pre-heat the car, I have noticed that I also lose a couple of miles of range in doing so because 3.8 kW from the wall (16 amps * 240 volts) just isn't enough, so some comes from the battery too.
     
  15. neroden

    neroden Happy Model S Owner

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    I did some testing in the recent 10 F weather and the car can pull 32 amps at 240 just for cabin heating. I simply could not convince the car to preheat the battery (I don't have the app), so I don't know how much it can pull for that.

    (Actually, that was seriously frustrating. It should be possible to go into the touchscreen and say "Preheat battery starting at this time.")
     
  16. Forty Creek

    Forty Creek Member

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    Or even better (if you have enough juice), have the battery preheated by this time.
     
  17. seanmccutchan

    seanmccutchan Member

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    @Cottonwood

    I get somewhat different estimate for pre-heating the battery (perhaps I'm missing something). The following article gives Specific heat (Cp) for the Panasonic 18650 cell (http://jes.ecsdl.org/content/146/3/947.abstract) as 0.96 J/(g-K) or 960 J/(Kg-K). Since one Joule equal one N-m or Watt-s we have 960 Watt-s/(Kg-K). My estimated 85 kWh battery mass is 344 Kg for a total thermal mass of 344 x 960/3600 = .092 kWh/K. For your example, to warm a battery 10C / hr would require 0.92 kW.

    I believe the heating occurs much more quickly when not in range mode. To warm a battery by from 20F to 50F (16C) over the course of 15 minutes would require 0.092 kWh/K * 16K / 0.25hr = 5.9 KW. This is inline with other comments that battery heating can draw 6 kW.
     
  18. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    Good reference! Thanks.

    I made the (apparently poor) assumption that the batteries had the same specific heat as water. From your reference, the specific heat of the batteries is about 0.23 (0.96/4.2), so the batteries will heat a little more than 4 times faster than my calculation and your calculation is correct.

    Even with the correct numbers, there is heat lost to other components in the battery pack (structure, coolant, etc) and the rest of the car. Many people have been starting at temps well below 20˚F. this winter, and there is the mass of the cabin and its surrounding structure. To prepare for a long hypermile trip, I would still preheat on shore power for a minimum of 30 min in a 50˚F. garage, an hour starting from 20˚F. and two hours from -10˚F. That may be conservative (probably too conservative for daily commutes), but the last thing I want to do is run low on battery on a cold winter drive...use shore power when you can.

    Yes, I agree that the car preheats faster when range mode is turned off, even on shore power, but why is that. Even in range mode, the car knows that it has shore power and does not have to conserve battery power.
     

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