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Graph of Consumption vs. Speed ?

Discussion in 'Model 3: Battery & Charging' started by neutronjeff, Jan 1, 2020.

  1. neutronjeff

    neutronjeff Member

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    After 3 mo with my Mod 3 P (stealth), I took my first long trip this week. First supercharging, first time sweating reaching a destination. One on leg, I missed the turnpike exit for the supercharger I planned to use because I confused it with the rest stop of the same name a mile away. I looked up the next one and found I could just get there if I slowed down [4%] at destination since traffic there is typically moving 75+. This experience gave me more of the significance of speed on the consumption.
    I was wondering if Tesla ever published or someone has a simple table or graph of the expected Wh/mi vs. speed for the different models under controlled conditions to give a basic guideline.
    Thanks,
     
  2. ewoodrick

    ewoodrick Well-Known Member

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    There is a table somewhere. But it's like 5% for each 5 mph over 60.
    You also learned that it is often easy to skip a charger.
    Remember that the emptier you are, the faster you charge.
    Also, you may not have realized how conservative the car routes
    Congrats on the trip
     
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  3. cwerdna

    cwerdna Active Member

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  4. eevee-fan

    eevee-fan Active Member

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    If I remember right, EV's is generally around 25 mph. Gas cars around 45 mph... higher speed because of the higher cost every second the car engine is running.
     
  5. cwerdna

    cwerdna Active Member

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    ^^^
    The optimum speed for range on EVs will depend on the vehicle. For instance, Tony Williams asserts the optimum for a Leaf is 12 mph.

    BTW, for those who want to look at the Roadster blog entry w/o busted images that the above Tesla blog entry points to, see Blog | www.teslamotors.com.

    Will write more later.
     
  6. ZOMGVTEK

    ZOMGVTEK Member

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    The optimal speed for EV’s is really slow, depending on the car and the loads at the time. Somewhere in the 10-20mph range for most cars without any additional load. With significant heat use, this speed increases quite a bit as low speeds mean longer travel times and higher relative consumption for the heat. Same with A/C, but to a much smaller extent. I’d say optimal winter speed when it’s below freezing is likely around 30ish to maybe 40. 25ish with A/C, probably.

    This is a bit complex though, since temperature has a big impact. Not only outdoor air temp, but the battery temp. The colder it is, the higher the intrinsic consumption, and higher the losses in the battery. Driving hard with a cold battery absolutely murders range until the battery warms up.
     
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  7. AlanSubie4Life

    AlanSubie4Life Efficiency Obsessed Member

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    #7 AlanSubie4Life, Jan 1, 2020
    Last edited: Jan 1, 2020
    There’s no way to do a general model because it is dependent on so many factors. A headwind is REALLY bad because it increases aero drag a lot. For example, a headwind of 10mph is worse than traveling 10mph faster without a headwind because you don’t get to the destination faster.

    Here’s a recent couple attempts at a model which I constructed. It’s important to realize that the optimal speed depends on the load of your accessories (in particular HVAC), which can be enormous. It is definitely possible to get 9kW if draw if you have the heat on with the AC. With all that on, the optimal speed can be as high as 50mph if it is extremely cold. This is for a Performance Model 3 with PS4S.

    Wh/mi vs Speed Models

    There are three primary components in these models:

    1) Fixed watts -> impact inversely proportional to speed. Basically this is from accessory use, which is about 300-350 watts at idle assuming no HVAC.

    2) Watts that scare with speed (rolling resistance, etc.). Establish a flat floor in Wh/mi. For vehicles with MXM4s this floor will be lower by about 20-30Wh/mi.

    3) Drag due to aero - the power goes with the cube of velocity. So Wh/mi goes up with the square of velocity.

    I think the people who hit 600 miles in a Model 3 were going something like 19mph (I don’t remember though - so gotta check that). That’s a good datapoint check to see the accuracy of this model.

    I would not put too much stock in the exact accuracy of this model. It is fit to three data points I took on a fairly warm day in San Diego. So in cold weather the drag component will be higher potentially. Drag coefficient increases linearly with air density.

    You can play with the equations on Wolfram Alpha once you understand what all the coefficients represent.
     
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  8. neutronjeff

    neutronjeff Member

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  9. AlanSubie4Life

    AlanSubie4Life Efficiency Obsessed Member

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    It’s a really tough problem to fit to because the data is so noisy (due to differing conditions and HVAC use). In the end a physics-based model is probably best. But it requires knowledge to use such a model safely. It would be good if they were able to model accessory use (HVAC, seat heaters, fan speed, AC, etc) based on settings and include all that. You can give them access to live data when you are driving which I think helps with their estimates. On the browser in the car it is kind of clunky though. I stopped using it for that reason. Just not convenient and it always has to reload and is a bit buggy.

    In the end their curves look pretty similar to my model though. They did not talk about how they generated this model in the above link as far as I can tell. But a fit is obviously not the best, given the source data! (I don’t think they just did a best fit - I bet they used a physical model.)
     
  10. AlanSubie4Life

    AlanSubie4Life Efficiency Obsessed Member

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    #10 AlanSubie4Life, Jan 1, 2020
    Last edited: Jan 1, 2020
    For sure the internal resistance is higher but unlike the HVAC use, I would imagine this would improve dramatically over the course of a trip. Are you aware of anyone who has managed to measure this effect? I imagine it would be difficult to isolate given the dense air, flatter tires, and HVAC use that come with winter.

    My general impression is that a 9000W load which does not drop off as the cabin is heated, which is what would happen in absolutely frigid conditions running open loop, is going to dominate everything else (except for plowing snow). EDIT: by everything else, I mean all the other cold weather factors. It would not dominate the aero losses and rolling resistance losses. It’s comparable to those.
     
  11. eevee-fan

    eevee-fan Active Member

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    9KW heater would not be the primary use of energy, though still very significant, if you're driving on the highway... driving at highway speeds without snow on the ground could be a steady 10-20KW, depend on wind speed.
     
  12. AlanSubie4Life

    AlanSubie4Life Efficiency Obsessed Member

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    Sorry...that is not what I meant...of course aero losses dominate everything at speed.

    I meant relative to other cold weather factors.

    Since I get 300Wh/mi or so at 80mph in ideal conditions with no HVAC (3P+), yes, that is 24kW. About 8kW of that is rolling resistance or whatever drivetrain losses proportional to speed there are, and 16kW is aero losses. Roughly, for a simple model.
     
  13. ZOMGVTEK

    ZOMGVTEK Member

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    Yeah, the battery does heat itself relatively rapidly when run hard. I didn’t see anyone talk much about this, and I didn’t see data. I don’t have CAN logging, so I’m just going off the regen dots and extrapolating where they ‘should’ be as the pack discharges to guesstimate pack temps. Most of my drives are short, so the battery is commonly cold. Based on my experience, I see roughly 2-3x the expected percent SOC drop compared to a hot battery in the same conditions. The tricky thing is, it’s not metered consumption, the SOC just disappears. Driving the same route every day in very similar conditions conditions, I see 5-6% SOC consumed in ideal summer weather, 9-10% if it’s quite hot and I have cabin overheat enabled. And in winter I’m seeing 30-35% in bad conditions with ~20F ambient and a cold battery. The car now aggressively preheats the pack when preconditioning, so that helps a whole lot and I’m getting consumption closer to 18-20% in very similar conditions if the battery is warmed up until there’s only a few dots. This is just a commute, not quite track use. But when I leave for lunch, there’s a private road that’s good for warming up the battery. I can use 10-12% SOC going 1.8 miles if the battery is particularly chilly. This isn’t too scientific though since the regen power is so limited with a cold pack, the friction brakes get used.
     
  14. AlanSubie4Life

    AlanSubie4Life Efficiency Obsessed Member

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    :eek: That seems bad. I can’t explain such a thing but of course my cold weather experience is extremely limited.

    I assume for everything you mention here you have the HVAC turned off completely.
     
  15. eevee-fan

    eevee-fan Active Member

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    10-12% SOC is about 7KW!!! If driving at 18 mph, that would take 6 min... inferring a power use rate of 7KW during that period would mean power drain rate of 70KW ?!?
     
  16. AlanSubie4Life

    AlanSubie4Life Efficiency Obsessed Member

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    Yeah, 7kWh+ over 2 miles is kind of a lot. It kind of sounds like a fearsome confused BMS, or a serious imbalance issue. It’s not really physically reasonable to use 70kW for 6 minutes. If the internal resistance of the battery were that high there would not be any voltage left if you pulled any current...

    I have never seen any such thing and in my very limited experience, I only see a modest recovery of miles as the battery warms.
     
  17. eevee-fan

    eevee-fan Active Member

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  18. AlanSubie4Life

    AlanSubie4Life Efficiency Obsessed Member

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    How do you figure?
     
  19. ZOMGVTEK

    ZOMGVTEK Member

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    I suppose I ended up being a bit misleading on that one. The 10-12% is repeatable, but its total consumption including HVAC use when its snowing, cold, and the battery is cold enough there is almost no regen. This means ~10+ minutes preheating on HI, leaving the heat on HI for the 10ish minutes it takes to pick up food, and usually alternating between full throttle and full brake at speeds significantly greater than 18 MPH.

    The interesting bit is that it appears to zap less range now that the car near enough always preheats the battery. It sure looks like i'm seeing a bit less consumption doing the same thing, but with a battery that was warmed from its own energy. I don't think its just the enhanced regen, so it could be the energy inst really fully consumed and the BMS ticks off a bit too much? It's only really super noticeable when the battery is below freezing, right around when the little snowflake shows up.
     
  20. ElectricIAC

    ElectricIAC Devil’s Advocate

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    Something I’ve noticed since we have both a single and dual motor cars is that the consumption goes sky high (370wh/mi+) past 75-80mph on the dual motor cars but that the front motor seems to make a lot more noise as well. Too big of a correlation to be coincidence, I think.
     

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