With regenerative braking, what speed does additional KW generated cease?

Discussion in 'Gen 3 Prius Main Forum' started by Johnny Cakes, Jan 24, 2020.

  1. Johnny Cakes

    Johnny Cakes Active Member

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    It seems that you are coasting from 10 MPH you are going to be generating less KW back to the battery then coasting from 40 MPH, just because the rotation of the generator is slower.

    Is there a speed at which you max out the KW being generated back to the battery?
     
  2. jerrymildred

    jerrymildred Senior Member

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    There is a maximum amperage which I've read is 110 amps. Over that and the mechanical brakes take over. I don't have a clue what speed that would require since it's never happened to me.

    Also, if you're going down a long long grade like in the Rockies, the system could stop regenerating to cool off. I've had that happen one time.
     
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  3. Robert Holt

    Robert Holt Senior Member

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    +1 on the regeneration system maxing out and system changing modes. However, in our case we were coming down a very long downhill grade whilst exiting the Blue Ridge mountains in Shenandoah National Park and once the HV traction battery hit the 8-bar (full) level and stayed there for a minute or two, the system seemed to shift to engine compression braking which I had to supplement with intense hydraulic braking. I was not really monitoring battery or inverter temps at the time (concentrating on vehicle control), but I could see the traction battery indicator get to maximum and about a minute later the engine braking began.
     
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  4. jerrymildred

    jerrymildred Senior Member

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    I can sure believe that. (y) I came close to filling up my PiP battery in the Smokies once. And kind of close to filling the Prime in Colorado. Those cars love the mountains. :D
     
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  5. Johnny Cakes

    Johnny Cakes Active Member

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    Thanks -- actually my question is more toward generating electricity for filling the battery than stopping the car.

    I'm assuming that if you start to coast at 40 MPH, you are generating more amps than if you start to coast at 15 MPH. I could be wrong on that, but that's my guess.

    But at some point I would think that additional speed is not going to generate more amps. Meaning that the generator tops out at, let's say 50 MPH, and would put out the same amps coasting at 50 MPH as it would coasting at 60 MPH.

    First, I'm wondering whether that's accurate and, if so, what is the MPH where the generator tops out such that addition MPH isn't going to generate additional amps.
     
  6. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    You can max out regeneration current at a wide variety of speeds, depending on how hard you hit the pedal.
     
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  7. bisco

    bisco cookie crumbler

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    i have read 6-7 mph
     
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  8. fuzzy1

    fuzzy1 Senior Member

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    7 mph is the minimum speed for any regeneration at all. Full regeneration power requires higher speed.
     
  9. Grit

    Grit Senior Member

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    You max out charging at 6-7mph? :eek:
     
  10. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    At higher and higher speeds, the amount of force on the pedal is less to get full amperage. Any more force is friction braking. (Or in rare cases, engine braking) but it never quits doing regenerative braking at some speed.
     
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  11. Grit

    Grit Senior Member

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    Depends on how much brake pedal you apply. If you lightly hold brake pedal at 40 mph vs 10 firm brake right before friction brakes kick in going downhill, you’ll get more regen at 10 mph. I’ve regen 85 mph downhills on the way to and back from Grapevine to from LA which never cutoff, was thinking the amperage charge would be too high but regen never was cutoff at that speed.
     
  12. Johnny Cakes

    Johnny Cakes Active Member

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    I need some educating. What's the brake pedal have to do with it? I thought that re-gen braking was "no pedal" and the braking effect came from the drag of running the generator that is creating the power to the battery. I seem to be missing something.
     
  13. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    In 'gentle' braking, the Prius is always maximizing regen braking. In 'panic' braking it just uses friction brakes.
     
  14. bisco

    bisco cookie crumbler

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    coasting is no feet on the pedals
     
  15. fuzzy1

    fuzzy1 Senior Member

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    The "no-pedal" version is very light, primarily intended to simulate the engine drag of conventional non-hybrids, though is somewhat lighter than them. This improves drivability for people accustomed to conventional cars, and the energy "loss" is recovered and piped into the battery.

    Light to moderate pedal braking applies much stronger regenerative braking. If you use the HSI display with the CHG<-->PWR bar, you will see the bar move sharply to the left when the pedal is applied. (With "no-pedal" coasting, this bar is only barely to the left of the zero position.) Maximum regeneration happens about when that bar pegs to the left-most limit. As you press even harder, for moderate to serious to panic braking, the bar doesn't budge anymore because the car is engaging friction brakes, not more regeneration.

    Due to basic physics (power = force * speed, and regeneration is limited to the battery power limit), as speed increases, it takes less pedal force to peg that indicator bar display, i.e. reach maximum regeneration.

    On the Gen3 Liftback and 'v', battery power (both in and out) is limited to 27kW, though temperatures may force an even lower limit. But full force panic braking, on a fully loaded car at 70 mph, exceeds 500 kW. Since that is far above the regen limit, friction brakes must do nearly all of that hard braking work.

    -------
    Do note that for most driving conditions, the majority of battery charge is siphoned off from MG1 being driven by the ICE. Only a small portion of the battery charge comes from regeneration.
     
    #15 fuzzy1, Jan 24, 2020
    Last edited: Jan 25, 2020
  16. jerrymildred

    jerrymildred Senior Member

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    They are the same. The car uses regeneration to slow down.

    Have you ever watched your HSI? The left end shows a graphic representation of the charging. When you let off the gas and coast, it shows a little regeneration. When you brake it shows more. When you brake a little harder it shows more regen. Regeneration is, by definition, "the drag of running the generator that is creating the power to the battery."

    If you're curious, you can watch this neat demonstration.


    You could also use phone app with an OBDII wireless adapter to see the actual amps generated while you brake.
     
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  17. Mdv55

    Mdv55 Junior Member

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    The amount of regen you get depends on battery SOC as well. Below 60%, you'll get more amperage from regen going to the battery and stronger decel. Above 60% the the car doesn't give you as much current to charge the battery from the same speeds, hence a lower decel rate on pure regen braking.

    Above 60% SOC it seems the max is 50 amps or so. Below that the car will give 75-100 amps to the battery. It might briefly spike higher but I've never noticed that on my Scanguage.
     
  18. Ronald Doles

    Ronald Doles Member

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    I have a ScanGauge and battery current is one of my parameters. The image below is a gauge on the dash of my 2015 Prius V. It is the energy bargraph. Your dash information may look a little different.

    prius energy.JPG

    If you were traveling below 45 mph and took your foot off the accelerator after a few seconds the engine would shut off and you would regenerate at about 13 amps down to about 8 mph. This would present a slight drag to the vehicle slowing it down. On my bargraph, the Charge bars at the bottom would be illuminated.

    If you were traveling below 45 mph and released the accelerator for a few seconds, the engine would shut off. Then if you put just a slight pressure on the accelerator, you would see no regeneration. You would see the bargraph hover at the midpoint near the longer center bar. You could coast the greatest distance doing this.

    If you were traveling at 40 mph and put a constant pressure on the brake pedal, you might get 100 amps of regeneration initially and then that value would drop off as the vehicle speed is reduced. The output from the generator is a function of it's rpm.
     
    #18 Ronald Doles, Jan 25, 2020
    Last edited: Jan 25, 2020
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  19. Grit

    Grit Senior Member

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    Now I have an understanding why some people say after after reading the thread, I am even more confused.
     
  20. Johnny Cakes

    Johnny Cakes Active Member

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    Wow, I was way off -- appreciate the schooling.

    I thought taking your foot off the gas was like attaching a generator, so the faster you were moving, the more KW you'd generate.



    Now understand that that's not it at all -- more KW is generated when braking, which was counter-intuitive to the idea that a generator is being run just by the RPM of the wheels.
     
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