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
     
  9. Bigheadrich

    Bigheadrich Member

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    I turned it off on my Tesla. It slows the car down so much as soon as you lift your foot off of the accelerator pedal. Kind've annoying. Also, I wonder if it's really beneficial if you're slowing the car down tremendously to barely add some charge to the battery. Wouldn't the free drifting down a hill or to a stoplight etc be a better alternative...
     
  10. Hmp10

    Hmp10 Well-Known Member

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    Personally, I find one-pedal driving very appealing, but I can understand someone's feeling otherwise. When I'm in my ICE car now I find all-friction braking to be a bit annoying, and I have to remind myself to keep my foot on the brake pedal at a red light.

    EV's are heavy vehicles for their size due to the battery packs. My Model S weighs almost 5,000 pounds. That's a big load on friction brakes. On a Tesla forum several years ago I read that no Teslas produced since 2012 had had to have any pad or disc replacements up to that point, as the regenerative braking did much of the work.
     
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  11. Aurum

    Aurum Active Member

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    @Hmp10 This is definitely my favorite part about regen braking. I love the fact that the energy can be harnessed while at the same time increasing the longevity of the disc brakes by an exponential amount! Makes it feel like I’m cheating.
     
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  12. EyeOnRivian

    EyeOnRivian Well-Known Member

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    A few months ago in attempt to learn about regen braking I had done some research on the internet. So please excuse any improper use of terminology ... and I don't mind being corrected.

    As @Bigheadrich mentioned he turned off his regen braking or at least the feature that causes the deceleration of the vehicle when lifting your foot off the accelerator pedal. (I've driven an EV only a couple of times and I also found this annoying but I chalked it up to something I would "probably" just get used to over time.) A friend of mine I believe mentioned many years ago he could adjust this "deceleration" affect when backing off the accelerator pedal on his Chevy Volt. I suspect there were various settings, perhaps something like 100%, 75%, 50%, 25% and 0%(off), with regards to regen braking (?) that in turn would affect the accelerator pedal when depressed. Does this sound right or close to it, and if so, do many vehicles with regen braking offer this? Can we expect this adjustment feature on either of the Rivian EVs? This is an option/feature I can see early adopters of EV driving using early on while they get use to this new accelerator pedal behavior.

    Given that, I recall also reading something about shifting (activating?) the regen braking to the actual brake pedal. This would allow the accelerator pedal to behave in a more traditional manner, that is, when depressing it the vehicle would coast. Has anybody heard/read anything about this?
     
  13. Hmp10

    Hmp10 Well-Known Member

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    The features of regenerative braking vary a bit by manufacturer. Tesla has a high and a low setting and also a "creep" function you can engage if you want. The latter causes the car to crawl forward a bit when your foot is off the brake to mimic the action of a torque converter on an ICE vehicle. Unless you're on an upward slope, the Tesla cannot be brought to a full stop using only regenerative braking, though.

    The Chevy Bolt can be brought to a full stop with regenerative braking by pressing a button on the steering wheel when braking.

    I know several people who own Teslas (my brother and myself included), and all got used to the regenerative braking feature quickly and now list it as one of their favorite aspects of EV driving. Going over a series of speed bumps in a parking lot or a gated community only requires gently backing off the accelerator pedal. You don't have to hold the brake pedal down at a red light. You can slow when coming up on a slower car from behind without having to hit the brake pedal. In fact, I now get annoyed in my Honda minivan when I have to keep my foot on the brake pedal at a long stop light, and I find the more frequent need to use the brake pedal just to slow down in heavy traffic aggravating.

    At least with Tesla, the transition from regenerative braking to friction braking is seamless. You can detect no difference at all in the cars deceleration characteristics except that you don't have to hit the brake pedal until later if you want to let regen do the first part of the braking. You'll also greatly reduce the wear on brake pads and discs, which is important in vehicles as heavy as EV's are.

    If I could change anything about the Tesla's regenerative braking, it would be to make it bring the car to a full stop without ever having to use the brake pedal except during emergency braking. One-pedal driving is far easier than getting on and off a brake pedal once you get used to it.
     
  14. ajdelange

    ajdelange Active Member

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    Three phase "motors" (Induction, PM and Switched Reluctance) are all four quadrant machines which means they can operate as motors or generators running in either the forward or reverse directions with the torque (absorbed or generated) being controlled, broadly speaking, by the relative rate at which the stator magnetic field rotates with respect to the rotor and, of course, the strength of the magnetic field. With respect to the OP, a four motor car will not recover twice as much energy by regeneration as a two motor car. The power recovered is the product of the torque absorbed by the generator and its shaft speed. A two motor car rolling down a hill at fixed speed generates (or regenerates I guess) as much power as a 4 motor car of the same weight on the same grade. The difference is that each motor in the two motor car produces twice as much power as is generated per motor in the 4 motor car.

    Only someone who has never driven an EV would find regen annoying. It is, indeed (see No. 15) one of the best features of an EV. I would strongly advise anyone acquiring an EV with adjustable regen settings to set it for max and learn to use it. Within a very short time period you will find yourself touching the brake pedal seldom. Not only does this minimize wear on the pads and discs but saves energy. Your Wh/mi readings will go down noticeably. At max regen you still have full control over the amount of regen. Back the pedal off a wee bit and you decelerate slowly. Back it off a lot (or take your foot off the pedal) and you decelerate faster. The pedal becomes an accel-decelerator and the job of controlling speed becomes easier as the only time you have to move your foot to the brake pedal is to bring the car to a complete stop when that is necessary. I wish Tesla would introduce firmware to bring the vehicle to a complete stop using the motors.

    In using regen you convert kinetic energy into electrical energy instead of heat (for the most part - conversion is not 100% efficient). The only way to do better is to put the car in neutral at such time as will result in speed of 0 at the place you want to stop. This is, at best, difficult to do and requires that you be on a positive grade.

    It is true that less energy can be recovered by regen at slow speed on 0 grade and that is simply because there is less energy to recover. The kinetic energy of the car is half the mass times the square of the velocity. But potential energy can be recovered at low speed. My Tesla X100 taken to the top of a moderately steep hill (3% ?) rolls down it at about 15 mph generating about 15 kW (IIRC).

    All braking needs can be fulfilled by "regen". The car can be brought to a complete stop by adjusting the torque appropriately as it slows.

    The major issue in regen with EV's is that you have to have somewhere to put the regenerated energy. The amount of energy generated depends on how fast you are slowing down so if you can only absorb a certain amount of energy you can only decelerate so fast. On locomotives they just dump the regen juice into big resistors. In a BEV we don't want to do that - we want to recover that energy and so are limited by how much we can put back into the batteries. This becomes a problem when the batteries are cold as charging them at too fast a rate is detrimental. Until the battery is warmed, regen is limited so that in cold weather for the first 15 minutes to half hour or so of driving when you take your foot off the pedal the car does NOT slow down the way it usually does. This has been cited as a problem with regen. Obviously, drivers should be aware of this (and the power display on Teslas shows this limitation) and be ready to apply the brake. This is about the worst aspect of regen that I can think of. There's lots of interest in super caps which, while they can't hold much energy, can absorb (and release) it fast. Perhaps these could be used to capture the energy from regen at times when the battery can't take it at a very high rate and then transfer it to the battery more slowly.
     
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  15. Bigheadrich

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    @ajdelange ...I have owned a couple EVs and am standing by MY opinion that i can not stand regen and believe its worthless. My Tesla P85D was purchased by me for the performance and technology. Not to try to regenerate 1 extra mile of travel. I think this will be a thing of the past also as the vehicles surpass 400 mile ranges. I never bring it home empty now so what would be the difference if I have an extra 2 miles in the battery.
    On the other hand I purchased a Volt when they first came out to run around in for work. I definitely tried to keep that damn little green ball centered on the dash for efficiency. Whatever engineer invented that is genius as it was a game lol.
     
  16. Hmp10

    Hmp10 Well-Known Member

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    #16 Hmp10, Aug 5, 2019
    Last edited: Aug 5, 2019
    Agree 100% with the first point (with the exception of Bigheadrich, of course).

    Capacitors, which can store electricity far faster than batteries, are ideal for capturing regenerative braking energy. While they are not ideal long-term storage devices, they can be used to augment acceleration with stored braking energy in stop-and-go driving, considerably reducing the drain on the batteries. This is suspected to be one of the reasons that Tesla was interested in acquiring Maxwell and is rumored to be experimenting with combining capacitors with batteries in their next-generation power packs.
     
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  17. Bigheadrich

    Bigheadrich Member

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    Lol of course...as long as they have an adjustment or off switch we will all be happy.
     
  18. cllc

    cllc Member

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    Regen braking is awesome. No doubt about it.
     
  19. skyote

    skyote Well-Known Member

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    In my opinion, the benefit of regen braking is the ability for one pedal driving, along with saving wear & tear on the mechanical brakes. Any added charge is definitely secondary to me, but why waste it?
     
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  20. Lmirafuente

    Lmirafuente Member

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    This was a highly technical discussion and very much appreciated. As long as there are options for the driver on what to use is great.

    I can see a use-case if you are running low on energy and you have regen braking on can help get you get to a charging station, something an ICS vehicle cannot do to add more gas/energy until you hit a gas station--but some can say there are more available gas stations than charging stations...but that is changing everyday--and should be a non issue in the next five to ten years.

    I know this is not a like-for-like comparison but as a golfer going from a two pedal (brake and accelerator) to a single pedal is awesome, I love the self park of the golf cart. I am sure the golf carts are probably not regenerating any power when stopping but I assume it is the same sensation at a smaller scale.

    Hope this helps the non-technical persons on this forum.

    Great discussion!
     
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