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Thread: Cold Weather Range

  1. #1
    Roadster 919, S 2006 Doug_G's Avatar
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    Cold Weather Range

    Today was the first time I did a longer road trip at below-freezing temperatures. Some interesting results.



    The trip was from Ottawa to Kingston and back to Ottawa again, a total distance of 296.1 km. Except for the occasional small town, all the driving was between 80 and 90 kph. I avoided the faster highways to preserve range because of the lack of charging facilities at my destination. (I actually did pick up a few hours of 110V charging, which was nice because it padded my range margin.)

    The total trip time was 4:05 hours, and the energy consumed was 49.31 kWh, for an average of 167 Wh/km.

    Since the trip was going to be a good fraction of the car's nominal range, and conditions were not ideal, I took it easy on heating. I used the seat heater throughout because it's power draw is negligible (can't even see it on the power meter). When I needed to clear the windows I used the cabin heater very sparingly and only at its lowest power setting. I'd ballpark average heater usage at 3 Wh/km. (Continuous full power could have consumed 40 Wh/km, but that would have been too much heat anyway!).

    The main nuisance of driving with the cabin heater off is that air still blows in via the vents. I closed the vents and put the air on the windshield to keep it clear, and turned on recirculation to reduce the flow rate (usually I avoid that in the winter but the Roadster didn't get humid - it was still sucking in quite a bit of outside air). With the seat heater set to low my back and seat were a little too warm, and with cool air bouncing off the windshield made my hands (no gloves) and face a little too cool. The net result was strange because I felt slightly too warm and slightly too cool at the same time; but overall it was reasonably comfortable and I could have stayed that way indefinitely without getting chilled.

    Tesla's efficiency spreadsheet predicted 147 kWh at 90 kph. I've previously found that their estimates are quite accurate - in summer driving conditions, that is. Today's results imply that power consumption was about 14% above nominal. Accounting for heating we're probably looking at 11% in extra losses.

    The temperature ranged from 0C to -5C. If we assume that Tesla's data is for 20C (room temperature) then the increased air density would account for most of the difference. The ratio is 1.316/1.204 (see Density - Wikipedia, the free encyclopedia) = 9.3%. Pretty close.

    We've had a warm winter, and much of the precipitation has been rain instead of snow. Predictably enough a lot of roads around here are in pretty rough shape due to frost heaving. The secondary highways I was driving on were in really bad shape. I wouldn't be surprised if those thousands of bumps added a bit of extra power consumption.

    I'm not sure about the rolling resistance of winter tires versus summer tires, but I don't think that was a big factor.

    So the net result is 11% range loss, plus about 3% for heating. In comparison, Nissan Leaf owners say that they see a 1% loss in range for each 1C below 20C, not including any cabin heat. This would have predicted a 22% drop in range, and implies that the Roadster handles the cold quite a bit better.

    One note: my battery pack was warm throughout. I have previously found that if your pack is cold-soaked well below freezing, you could lose up to 10 km extra during the warmup period.

    Have other people seen similar results?
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  2. #2
    Roadster 938 Model S 5957
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    Hey Doug,

    I have seen about 15- 20+ percent more energy used driving in cold conditions compared to warm. Hwy speeds 100/110 with heater on.

    In an extreme case I decided to drive 200 km on clear roads but -20 C weather......what a mistake. To begin with I basically had to have my seat heater and full heat blowing continously - this stopped the uncontrollable shivers - but I was pretty much frozen. -20 C cold air literally blew in from the door sill area on both doors the entire way. I actually stopped and bought a blanket to help finish my trip lol.....

    I love the car but I won't take it at highway speeds again if the weather is much below zero. It just wasn't made for Canadian winter weather IMO.

    Kevin

  3. #3
    Roadster 919, S 2006 Doug_G's Avatar
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    Curious I haven't noticed air blowing in over the sills. The cold air I was getting was coming in through the ventilation system.

    Another curious fact... of late my Standard Mode charges have been coming in at 294 km, versus 300-301 when it was new. But after the Range Mode charge and road trip, it's back up to 302 km! I guess the rumors that Range Mode charges do more pack balancing are true. It will be interesting to see if that lasts.

    Looks like I'm going to be doing another trip next Friday. Hope the weather isn't too cold!

  4. #4
    R1211 & S282 NigelM's Avatar
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    Quote Originally Posted by Doug_G View Post
    We've had a warm winter....
    Yeh, I just drove home from work with the top off, it's 86F (30C) this afternoon. My fans came on when I parked in my garage at home, so even if you lose power in colder weather, I'm still using power in hot weather.

    BTW, I've been doing a lot of city driving recently and find my est range is consistently about 10% higher than my ideal range. Got in the car this morning with a full (standard mode) charge ideal range of 191miles and an estimated range of 211miles.

  5. #5
    Roadster 919, S 2006 Doug_G's Avatar
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    Quote Originally Posted by NigelM View Post
    Yeh, I just drove home from work with the top off, it's 86F (30C) this afternoon. My fans came on when I parked in my garage at home, so even if you lose power in colder weather, I'm still using power in hot weather.
    Show-off!

    Quote Originally Posted by NigelM View Post
    BTW, I've been doing a lot of city driving recently and find my est range is consistently about 10% higher than my ideal range. Got in the car this morning with a full (standard mode) charge ideal range of 191miles and an estimated range of 211miles.
    My foot is too heavy for that. Just ask my rear tires...

  6. #6
    R1211 & S282 NigelM's Avatar
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    This is why we live in Florida......


  7. #7
    Roadster 919, S 2006 Doug_G's Avatar
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    Quote Originally Posted by NigelM View Post
    This is why we live in Florida......
    Yes, it's so much better there.


  8. #8
    R1211 & S282 NigelM's Avatar
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    Quote Originally Posted by Doug_G View Post
    Yes, it's so much better there.
    Umm, yeh cos' you never had storms like that in Ontario.....


  9. #9
    Roadster 919, S 2006 Doug_G's Avatar
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    I've been doing some more investigation into winter driving range. I'm not happy with my earlier back-of-the-envelope estimates; I simply assumed aerodynamic losses were the dominant factor; in fact, at this speed they're not.

    I was seeing around 160 Wh/km at 85 kph, with no cabin heat. Interpolating Tesla's efficiency spreadsheet, at that speed I should nominally be seeing:

    Aero 54.1 Wh/km
    Tires 34.8 Wh/km
    Anciliary 2.1 Wh/km
    Drivetrain 47.9 Wh/km
    Total 138.9 Wh/km

    The point here is that at this speed, aerodynamic drag is NOT dominant. Drivetrain losses are almost the same.

    Let's assume the aero goes up by 9.3% due to the denser air. That's only an extra 5 Wh/km. I'm seeing about 20 Wh/km. Where is the rest going?

    The battery pack started at 13C and rose to 26C over the trip. The PEM started at 8.5C and rose to 26C by the end. The motor spent most of the trip at 40C. They're all in perfectly normal ranges; I don't see how they would affect anything.

    I'm wondering if the loss was due to the rolling resistance of the winter tires. Does anyone have any experience with that?

  10. #10
    Senior Member dhrivnak's Avatar
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    I do think it is more due to running cabin heat that anything else. I go from about 250w/mile to 300w/mile running the heat. So my results are very similart to Doug_G.

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