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A real-world comparison: dry pavement versus heavy rain

Discussion in 'Model S: Battery & Charging' started by Rockster, Oct 13, 2014.

  1. Rockster

    Rockster Active Member

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    On the way from Houston to Dallas yesterday, we encountered a serious downpour that appeared (as Texas rains do) quite suddenly. It was the kind of rain that caused us to slow from 70-75 mph to 50-55 mph and turn our hazard lights on so that we weren't hit from behind by someone not so prudent as to slow down.

    The water was deep enough that we could feel a drag from the water on the road and the wipers were on their highest setting, barely keeping the windshield acceptably clear.

    Prior to the rain, we were averaging 300 to 325 Wh/mi on the flat, dry highway going about 70 mph.

    During the torrential rain, we were averaging about 480 Wh/mi, even after having slowed to 50-55 mph.

    Data point: we currently have factory 19" tires on the car.

    After 20,000+ miles, I'm well aware that heavy rain increases power usage, but this was the first time I quantified it. Fortunately, we were out of the rain in about 20 minutes. If the entire drive from Houston to Dallas had been experiencing rain that heavy, it would have been a more challenging drive home.
     
  2. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    Yes, and wet, sloppy, slushy, winter roads are worse! Winter is on its way...
     
  3. iadbound

    iadbound Member

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    Just curious -- how were you able to make the calculation on Wh/mi while mid-trip? Did you reset the trip computer?
     
  4. Rockster

    Rockster Active Member

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    I was the passenger and I watched the energy display. My wife was holding at rather consistent speed and the rains were constant during that 20 minute stretch, so the display graph was a pretty consistent 480 Wh/mi.
     
  5. iadbound

    iadbound Member

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    Thanks. Did you take a video?:tongue:

    480 Wh/mi does sound like a lot, but I guess the resistance element makes sense. I'll have to remember that for the longer road trips.
     
  6. Rockster

    Rockster Active Member

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    I should have taken a video!
     
  7. tga

    tga Active Member

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    Do you know what the number was after the rain stopped, but before the road dried out? There seemed to be some confusion the last time this came up about how much of the increased energy usage came from falling rain hitting the car vs pushing standing water on the road out of the way.
     
  8. Todd Burch

    Todd Burch Electron Pilot

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    Perhaps it's obvious, but if this were to happen while someone is on a travel leg where their remaining distance is uncomfortably close to range remaining, the best thing to do would be to pull over (preferably off the highway) and wait it out.
     
  9. kennybobby

    kennybobby Member

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    Hey Rockster that is a very interesting observation to quantify the load, thanks for sharing the data.
     
  10. yobigd20

    yobigd20 Well-Known Member

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    No I've hit even higher than that in bad weather when against a huge headwind.
     
  11. rcc

    rcc Model S 85KW, VIN #2236

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    The water splashing up against the battery pack also cools it down a lot more than normal. So in addition to everything else, there's a good chance power was being used to keep the batteries up at operating temperature.
     
  12. jerry33

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

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    Unless it was freezing rain, I'd not think that would be a problem as the pack heating doesn't come into play until the temperatures are low. If anything, that would help on highway speeds. The water depth alone is more than enough to account for the extra power usage.
     
  13. Rockster

    Rockster Active Member

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    It's tough to say because by the time the storm stopped, we were also exiting the area of the thunderstorm, so the "non-raining, yet wet" period was rather brief. (For those of you not familiar with Texas weather, thunderstorms can be so isolated that one-half of a cul-de-sac can be under blue sky and sunshine while the other side is experiencing a downpour.)

    Our best recollection is that after the rain stopped, while the pavement was still wet, we increased speed to about 60-65 and were seeing values close to our dry pavement, 70-75 mph usage: low 300's Wh/mi. The qualifier here is that the wet pavement at this point contained much less water than what was on the pavement during the storm.
     

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