Regenerative Braking Capacity

Discussion in 'Tech: Batteries, Charging, Alternative Energy' started by Aslan, Mar 8, 2019.

  1. Aslan

    Aslan New Member

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    I was just learning about regen braking and how it works in electric cars. Seems like the fact that the rivian vehicles have four separate electric motors will greatly increase the potential energy recovered by regen braking as well as the braking power that would be supplied by this system, which could reduce even further the wear and tear on the disc brakes that are present.

    Does anyone have experience or expertise in this area who can share more? I wonder if they will take advantage of the quad motors for this or simply have 2 regen units at the transfer cases.
     
  2. CappyJax

    CappyJax Well-Known Member

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    There is no transfer case. Each of the four motors drives one wheel directly. No gears whatsoever. Regenerative braking would come from all four wheels.
     
  3. Hmp10

    Hmp10 Well-Known Member

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    #3 Hmp10, Mar 8, 2019
    Last edited: Mar 8, 2019
    Actually, the Rivians do have single-gear transmissions on each motor. The motors do not drive the wheels directly but through a half shaft coming off a centrally-mounted transmission. The output shaft of each motor faces into a transmission case at the center of the vehicle, not into the wheel. (My Tesla -- and all other EV's of which I know -- use some type of transmission gearing. If I recall, the new Porsche Taycan is going to have a two-speed transmission.)

    Here is a passage from a Motor Trend article on the R1T:

    "Each [motor] drives one wheel through a centrally mounted single-speed transmission. To save weight, each pair of transmissions front and rear shares the same case but no moving parts. Putting the transmission outputs in the center of the vehicle allows for long halfshafts that Rivian says give the R1T greater wheel articulation."

    Different types of electric motors have different abilities to operate as generators. The Rivian is using permanent magnet motors, which are the least complicated to get to behave as generators. In many EV's, the ability of the motors to put energy back into the batteries during regenerative braking is limited not by the motor, but by the rate at which the battery can accept a charge. That is why Tesla is experimenting with adding capacitors to its energy pack, as capacitors can be charged almost instantaneously and would be ideal to store regenerative braking energy.
     
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  4. CappyJax

    CappyJax Well-Known Member

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    In Aspen, I was told they were direct drive. No transmission at all.
     
  5. Hmp10

    Hmp10 Well-Known Member

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    I hear you. You wouldn't believe some of the misinformation Tesla sales people have given me.

    For a brief explanation of why electric cars have transmissions -- but usually just one gear -- check this out:

    https://www.roadandtrack.com/new-ca...-electric-cars-have-multi-gear-transmissions/

    Here's a photo of the Rivian skateboard. It shows inboard motors rather than motors mounted at the wheels, and you can see the half shafts.


    RivianSkateboard.png
     
  6. CappyJax

    CappyJax Well-Known Member

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    I understand the principle of torque multiplication. I am just going by what I was told.
     
  7. Hmp10

    Hmp10 Well-Known Member

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    I don't doubt that at all. But, honestly, I don't know why car manufacturers don't do more to train the people they send out to promote their cars. I once had a Tesla salesperson insist to me that the front trunk in the Tesla Model S was the same size whether the car had single or dual motors. I actually had to haul him out to their parking lot and open two cars to show him the very obvious difference.
     
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  8. Hmp10

    Hmp10 Well-Known Member

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    Regenerative braking in cars is a very complicated topic. Efficiencies (percent of recaptured energy) vary from 16% to 70%, based on an array of factors:

    - different types of motors have different regenerative characteristics (fortunately, the two types of motors most EV's use are pretty good at working as generators)

    - batteries are limited in the rate at which they can absorb and store regenerative energy

    - ultra capacitors, which haven't yet appeared in most EV's (although Teslas is experimenting with their use), are the best current technology for capturing regenerative energy

    - a lot depends on driving style; less energy can be recaptured when braking from low speeds

    - regenerative braking cannot meet all the stopping needs of a car, so friction braking still plays a role.

    Here is a good article on the subject:

    https://www.sciencedirect.com/topics/engineering/regenerative-braking
     

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