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Rain and Energy consumption

Discussion in 'Model S' started by rogbmw, Sep 25, 2014.

  1. rogbmw

    rogbmw Member

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    In our 19 months of ownership, one thing that has come clear is that when the car is driven in rainy conditions, the energy consumption goes up quite a bit. I do not know if it is the added resistance of the rain on the road, the constant use of the windshield wipers, or a combination of both, but I have seen as much as a 30% drop in mileage, driving the same route, in the rain as compared to a non-rainy day. These were on trips of almost 100 miles over the same road.
     
  2. Doug_G

    Doug_G Lead Moderator

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    Windshield wipers are beyond insignificant, as is every power draw in the car except the main drive and HVAC. The main drive uses about 20 kW to keep the car moving at highway speeds. Your wipers use a matter of watts (no kilo!).

    It's the resistance on the road. You'll find that slightly wet pavement does little, but constantly driving through puddles (e.g. due to driving rain) really hits you on range.
     
  3. brianman

    brianman Burrito Founder

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    There was a thread with some math on this topic a bit back.
     
  4. Plug Me In

    Plug Me In Member

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    At least you don't get the cold rain combo in Florida! I theorize that the battery also needs to be warmed a little more actively in the rain.
     
  5. invisik

    invisik Member

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    Would Opticoat or other exterior product reduce rain resistance because the exterior of the car would be more slippery?

    -m
     
  6. pgiralt

    pgiralt Active Member

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    Doubt it would help much, if at all. I think most of the resistance is the additional rolling resistance on the tires, not the rain hitting the car (although I'm sure that has some impact as well).
     
  7. Genebe

    Genebe Member

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    The same thing happens to every ICE car. I've noticed it for years on all the cars that I've had (since I became conscious about fuel economy).
     
  8. wycolo

    wycolo Active Member

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    A 30% hit seems excessive. Are your tires >45 pounds? Headwinds might be hard to observe during heavy rain. Do you steer away from puddles and watertroughs-made-by-semis?
    --
     
  9. ckessel

    ckessel Active Member

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    Yep, rain is brutal. It doesn't get terribly cold in Oregon for much of our rainy season, not cold enough to kick in the "warming the battery" bit. I average ~330 and that jumps to 430 in the winter. A big chunk of that jump is added rain resistance.

    It's just as bad on a gas car, but folks don't typically track gas mileage changes.
     
  10. scaesare

    scaesare Active Member

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    While I can see the effects of driving through HEAVY rain, I've noticed it to be much more dependent on the temps associated with the weather during moderate rains.

    Light-medium summer rains don't have much significant impact. When the temps drop below 60, then I get a jump up in energy usage. When it drops even further below that (below 50) I get an even larger increase....

    This morning it was 59 for most of my drive in to work under light rainy conditions... my energy usage was ~10% higher than "normal".
     
  11. AudubonB

    AudubonB Mild-mannered Moderator Lord Vetinari*

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    30% may be at the top end of the change-range, but it's not, in my experience, excessive. I consistently drive multi-thousand mile trips, which give me the ability to factor out load, tire conditions and other variables, and year-in, year-out, the three most significant factors in fuel economy variation are road surface, rain and wind. The latter is a roulette wheel, however, in that if it's on your back it gives you a fine boost. I just can't figure out why it never seems to be other than coming right at me!
     
  12. rcc

    rcc Model S 85KW, VIN #2236

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    Besides higher rolling resistance due to rain, I think a major issue is having to use power to heat the battery. That'll happen at cooler temps. Also, at highway speeds, the air moving under the pack may be cooling it even more and that has to be counteracted by electric heat.
     
  13. Brass Guy

    Brass Guy Member

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    Speculation: as wind resistance is the primary energy use factor at highway speed in fair weather, it is more likely that the rain in the air has more effect than rolling resistance - at highway speeds. Excluding puddles, of course. (Most highways have very good drainage.) If high density due to cold is a factor, air doesn't get much more dense than when it is raining.
     
  14. AudubonB

    AudubonB Mild-mannered Moderator Lord Vetinari*

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    I wasn't clear in my first entry: my experience is from driving ICE vehicles, which means that the results are independent of any of the effects on battery heating/cooling, etc., that may or may not come into play with an EV.
     
  15. jerry33

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

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    ^^^ This. A small amount of water on the road really increases rolling resistance because the tires have to push the water away. At a water depth of 1 mm, each tire pushes 1900 litres (just over 500 gallons) per km.
     
  16. ChadS

    ChadS Petroleum is for sissies

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    #16 ChadS, Sep 25, 2014
    Last edited: Sep 25, 2014
    If you don't believe standing water is a significant force, try this: after a heavy rain, when it is no longer raining but there is still water on the road (it is more than just damp) , get the car up to speed, and then put it in neutral and note how fast you slow down. Try it again later on a dry road. The difference is dramatic.
     
  17. David99

    David99 Active Member

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    Battery heating isn't a factor unless its very cold. Rain is causing higher energy consumption because it is a significant of mass (compared to air) that hits the car. While each drop seems like to have an insignificant amount of weight, it is literally thousands that hit the car constantly. The water on the road also causes an increase of the rolling resistance. How much depends on the amount of water/rain.
     
  18. Xenoilphobe

    Xenoilphobe Active Member

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    Also realize that the cabin humidity rises and the AC kicks on to lower humidity in the cabin and prevent fogging on the windows.
     
  19. sns

    sns Member

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    I have definitely noticed mileage dropping by about 20% in my MB in long drives in rain vs dry...I was guessing it was the rolling resistance.
     
  20. wycolo

    wycolo Active Member

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    I realize that I slow down in the rain automatically whereas most traffic seems not to. Right there is a good bit of that 30%.
    --
     

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