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Thread: EV range paradox

  1. #1
    Flashaholic* Juggernaut's Avatar
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    Exclamation EV range paradox

    Ok Iíd like to think I have a great understanding of energy consumption. This question may be better suited on an electric car forum, but honestly Iíve never read anything on one that makes me think they know more about batteries than we do, heck we REALLY push our batteries, and have a far better understanding of what is available.

    Anyways here is my paradox I just canít figure out.
    The set up:

    I am working on converting a small tractor (900 pounds) to electric.
    It was powered by an 8 HP Briggs & Stratton
    I am repowering it with a 10 HP brushless motor. 10 HP limited by Speed controller.
    Thatís MAXIMUM 7457 watts.

    It will be powered by a dissembled Nissan Leaf battery pack . It will use exactly ľ of the packs cells (6 cells vs 24) It will have a potential watt hour rating of 2,880 The full leaf pack has a rating of 11.5k/watts

    SO 7,457 maximum watt hours from a 2,880 watt hour pack should mean I will only get 0.386 hours of run time. The tractorís top speed is 4.1 mph matched to the original gas powered top speed via gearing assuming maximum load which is possible.
    My range at maximum load SHOULD be 1.58 miles correct?

    Ok Here is the paradox. The leaf uses 12,069 watts to maintain 40 mph on level flat ground, and attains a little over 100 miles of range. How can this be? That is more watt hours than itís battery, it should only have a maximum range of 38 or so miles. It uses 16762 watts to maintain 65 mph. We can assume the slower it goes the less air resistance and the better the range, essentially down to a crawl.

    If you reduced the battery capacity down to ľ as I will did for the tractor it would attain at least a 25 mile range (ignoring weight reduction)

    The leaf can drive 25 miles using 12,069 watts to maintain 40mph on a 2880 watt hour pack, yet my tractor will only travel 1.58 miles using 7457 watts on the same pack? How can this be, this just seems straight up paradoxical to me.
    ďWhat do you mean LEDs are more efficient then Incans?Ē, thatís just what they want you to think! Itís a Conspiracy, Man!

  2. #2
    *Flashaholic* StarHalo's Avatar
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    Default Re: EV range paradox

    I was told there wasn't going to be any math
    Last edited by StarHalo; 11-07-2017 at 11:41 AM.

  3. #3
    Flashaholic* Juggernaut's Avatar
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    Default Re: EV range paradox

    Well feel free to try solving this with Art if it makes you more comfortable
    ďWhat do you mean LEDs are more efficient then Incans?Ē, thatís just what they want you to think! Itís a Conspiracy, Man!

  4. #4
    *Flashaholic* StarHalo's Avatar
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    Default Re: EV range paradox

    Your watts@speed numbers are a bit high; assuming ICE + all-season radials' ~20 hp @ 60 mph, that's ~14,000 watts, but super low drag EVs and their low rolling resistance tires can in some driving situations get that down to ~7 hp/ ~5,200 watts, bearing in mind the number fluctuates wildly with a mild breeze/fractional grade/asphalt type/etc.

    Visualise that with some colorful flourishes and happy little clouds, and I think I'll have it..

  5. #5
    Flashaholic* turbodog's Avatar
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    Default Re: EV range paradox

    Quote Originally Posted by Juggernaut View Post
    Ok Iíd like to think I have a great understanding of energy consumption. This question may be better suited on an electric car forum, but honestly Iíve never read anything on one that makes me think they know more about batteries than we do, heck we REALLY push our batteries, and have a far better understanding of what is available.

    Anyways here is my paradox I just canít figure out.
    The set up:

    I am working on converting a small tractor (900 pounds) to electric.
    It was powered by an 8 HP Briggs & Stratton
    I am repowering it with a 10 HP brushless motor. 10 HP limited by Speed controller.
    Thatís MAXIMUM 7457 watts.

    It will be powered by a dissembled Nissan Leaf battery pack . It will use exactly ľ of the packs cells (6 cells vs 24) It will have a potential watt hour rating of 2,880 The full leaf pack has a rating of 11.5k/watts

    SO 7,457 maximum watt hours from a 2,880 watt hour pack should mean I will only get 0.386 hours of run time. The tractorís top speed is 4.1 mph matched to the original gas powered top speed via gearing assuming maximum load which is possible.
    My range at maximum load SHOULD be 1.58 miles correct?

    Ok Here is the paradox. The leaf uses 12,069 watts to maintain 40 mph on level flat ground, and attains a little over 100 miles of range. How can this be? That is more watt hours than itís battery, it should only have a maximum range of 38 or so miles. It uses 16762 watts to maintain 65 mph. We can assume the slower it goes the less air resistance and the better the range, essentially down to a crawl.

    If you reduced the battery capacity down to ľ as I will did for the tractor it would attain at least a 25 mile range (ignoring weight reduction)

    The leaf can drive 25 miles using 12,069 watts to maintain 40mph on a 2880 watt hour pack, yet my tractor will only travel 1.58 miles using 7457 watts on the same pack? How can this be, this just seems straight up paradoxical to me.

    You can't convert from gas to electric straight across. There is a conversion between gas, electric, and hydraulic power, with electric coming out on top in efficiency (all else being equal).

    You might also be surprised to find how little horsepower it takes to keep an already-accelerated leaf at 40 mph.

    Your overall logic is in the right direction, but your core assumptions are off.
    This is your life, and it's ending one minute at a time.
    Be prepared for the truth.

  6. #6
    Flashaholic* Juggernaut's Avatar
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    Default Re: EV range paradox

    The watts of motor usage was based off of the average found from a few members on a Nissan Leaf forum conducting tests. They managed 5 miles on 1 Kwatt I believe, with 1.6 for 65mph in the same distance. I no longer have the link, but regardless this was based on their in dash feed back not calculated via aerodynamic drag and ICE power consumption.

    Regardless I think it is safe to assume the Leaf is NOT using less than 10 HP / 7457 watts to maintain 40 mph. Or am I simply underestimating just how little rolling resistance and how good it's drag coefficient is?

    I would think to accomplish 110 miles per charge at 40mph the car would need to drive for at least 2.75 hours, OH malarkey!
    I have found the culprit in my calculations. My apologies guess I just needed to hash it all out to find the far too simple problem in my calculations. The Leaf battery is made of 48 cells! Not 24!
    Thus this isn't a question about the power consumption of the car, that was correct, it was simply being divided up from half the potential energy capacity it stores. As the battery is a 23,000 watt hour deal not 11,500.
    So recapping for my own sake
    23,000 watt hours drives the car for 2.75 hours at 40mph (rounding of course for simplicity sake)
    Means the carís motor is using 8,363.63 watts (thus a tad bit more than 10HP as I assumed)
    With 1/8th (Not 1/4th) the batteries capacity the range should be reduced down to 13.75 miles.
    Thus from a 2,880 watt hour pack producing 11.21 HP it will drive 13.75 miles at 40 mph
    NOW my question still stands or so I say (the paradox) as I always understood it, if we divided the speed we should still get the same range, just at a longer span of time, this is assuming (rounded greatly) at 4mph the Leaf would use 1.12 HP totally believable and would take 3.44 hours to complete the trip of 13.75 miles. So I guess the answer to my question: Is it safe to assume my tractor is really only using 1.12 HP to maintain its top speed of 4 MPH? At which point it too would accomplish a range of 13.75 miles. However when under full load (driving up a muddy hill pulling a log etc) it will still manage the same top speed of 4 MPH (due to gearing) but us nearly 10x more power and thus run out of charge in just 1.58 miles as previously calculated?
    ďWhat do you mean LEDs are more efficient then Incans?Ē, thatís just what they want you to think! Itís a Conspiracy, Man!

  7. #7
    *Flashaholic* StarHalo's Avatar
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    Default Re: EV range paradox

    Check out these guys over on a Honda forum trying to get HP@MPH numbers; pretty amazing how efficient a car as small as a Miata can be..

  8. #8

    Default Re: EV range paradox

    Speaking of EV batteries and efficiency, did you guys watch the Tesla event a couple hours ago? Really impressive numbers!
    Their semi-truck does 0-60 in 5 seconds or 20 seconds with 40k lbs load in the trailer, it has 4 motors and a 500 miles range,
    even more impressive are the new chargers that give it 400 miles of range in 30 minutes!
    Plus the new Roadster, 0-60 in 1.9 seconds and 0-100 in 4.2, quarter mile in 8.9, all with a 200kw battery and 620 miles range

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