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Just how lossy is regenerative breaking?

Discussion in 'Gen 3 Prius Technical Discussion' started by BrettS, Feb 24, 2011.

  1. BrettS

    BrettS Active Member

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    Before you get to the building that I work in you need to go through a number of stop signs that are only 500-1000 feet apart. When there's no one around I typically accelerate gently and glide or coast to the next stop sign to maximize fuel economy. However in the mornings and evenings there is more traffic and so I usually have to accelerate and decelerate faster to avoid annoying people too much. I'd certainly feel better about having to do that if I knew I was getting much of my energy back as I stopped at each stop sign, but I imagine that there's a lot of loss.

    Has anyone seen any numbers or run any tests to see just how lossy it is? Say I gently accelerate from 0 to 30 on level ground (in electric mode, just to keep things a bit more simple) which would take a certain amount of power from the battery. Then, as soon as I hit 30 I gently brake until I stop. How much power would go back to the battery? 30% of what I took out? 50%? More?
     
  2. ksstathead

    ksstathead Active Member

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    Below 8 mph, 0% regen.
    Above that, I'm thinking 30% about caps it. No source for that, just recollection.

    Not much you can do unless you have an alternate route that is not much longer.
    Better than no regen at all.
     
  3. twittel

    twittel Senior Member

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    On a different thought...if you're using EV mode to manage to stop signs, I'm thinking you'll spend more in energy to replenish the hybrid battery then if you just keep it in ECON mode. I think you'll actually get better MPGs, too. At least, that's been my experience when I've tried the EV mode.
     
  4. BrettS

    BrettS Active Member

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    Oh... no... I didn't mean to imply that I was going through the stop signs in EV mode, but I wanted to take the ICE out of the picture in my example above to avoid any additional complication. It's easy to compare the amount of power spent to accelerate in EV mode to the amount of power generated by the brakes, but much more difficult to compare the energy expended by the ICE and electric motor in a 'normal' acceleration to the amount of power generated by the brakes.
     
  5. sipnfuel

    sipnfuel New Member

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    Efficiency
    Inverter Efficiency = 88%
    Motor/Generator Efficiency = 88%
    Charging Efficiency = 66% (this figure is debatable)

    Losses

    Energy Dump due to overvoltage/overcurrent (45 mph original speed reference) = 25% (75%)
    Mechanical Braking below 7 MPH (45 mph original speed reference) = 3% (97%)

    If you use a braking force above ~23 mph without engaging physical brakes, there is an energy dump to a bank of resistors. This is because the voltage generated is more than the system can handle. Below 23 mph, a high braking force can still generate more current than the system can handle. Again, this is dumped. This is 100% lost to heating inverter coolant. Curiously, 24 mph is the speed at which EV mode drops out (ever wonder why?).

    My assumptions above show 37% recaptured energy.


    Anectdotally, my real life experience show regen is around this efficiency. I have a hill to descend. The potential energy of that drop should be equivalent to about 6 bars of battery charge. But I usually only get 2-3 bars, even when I brake really carefully so I don't use the physical brakes. This is why I try to have a low SOC when I descend that hill.
     
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  6. qbee42

    qbee42 My other car is a boat

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    Your 37% sounds about right. I usually use a ballpark figure of 40%, just to keep the numbers round.

    Tom
     
  7. SageBrush

    SageBrush Senior Member

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    One major limit is power to the battery, limited to ~ 20 kw or 20,000 joules a second.
    A 1400 kg Prius moving at 30 mph before brakes are applied has mv^2/2 kinetic energy, or about 128,000 joules.

    Answer: Brake slowwwly :)

    In similar situations I get up to speed quickly (for me), then glide, then coast, then brake gently at the end. If cars are stacked up at the stop sign I take my time.
     
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  8. sipnfuel

    sipnfuel New Member

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    I forgot about this charging limit

    Do you know if the discharge limit is the same power?
     
  9. twittel

    twittel Senior Member

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    So the "brake slowwwly" advice would be a good application for shifting in "B"?
     
  10. SageBrush

    SageBrush Senior Member

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    SipNfuel: yep, about the same limits in or out.
    Twittel: My understanding of 'B' is suspect -- I thought it is what is sometimes called engine braking, meaning high pumping losses from a tight air throttle.

    Addendum: *Much* better -- read Hobbit's monograph on 'B' braking
     
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  11. sipnfuel

    sipnfuel New Member

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    I looked at Toyota's page and it says power output is 27 kW (36 hp). Engine is 73 kW (98 hp). Combined power is 100 kW (134 hp). Makes sense.

    Toyota Prius - 2011 Performance & Specifications

    It could be power input to the batteries is still limited to 21 kW. I haven't found anything that says one way or another.



    However, there is a strange discrepancy for the Gen II. Gen II technical literature says the battery power is limited to 21 kW (>28 hp). The Gen II Engine is 57 kW (76 hp). This ought to give a combined power of 78 kW (104.5 hp)

    However, stated combined power for the Gen II is 82 kW (110 hp). Any guesses on where the extra 4 kW (5.5 hp) is coming from?

    http://en.wikipedia.org/wiki/Toyota_Prius_%28XW20%29
     
  12. mikewithaprius

    mikewithaprius New Member

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    Gen II 86 hp...Prius gnomes cycling in the engine?

    Tom - I think by brake slowly he means gently apply the brake so that you provide a constant source of energy to the battery while gradually slowing. If you brake just before the light you only get some of the input apparently, plus potential use of friction brakes if it's quick enough. I do this down the backside of bridges for full green bars by lightly keeping the foot on the brake so that I maintain the same speed essentially, but get more charge than just foot-off-the-gas regen.

    B mode will charge the battery less than regen in drive (Hobbit link is great with details...).
     
  13. pEEf

    pEEf Engineer - EV nut

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    I think the THS is around 85% efficiency total. This includes the MG, the Inverter, and the boost converter. The battery is different, as it's efficiency changes depending on the total current and several other factors, but you could be close on it, at least in some conditions.
    This is incorrect. There are no resistors in the THS anywhere! Regen is capable of delivering a peak of about 120A to the battery, and sustaining around ~20kW continuous. This is a limitation of the boost converter, not the inverter or the MG. (Though the battery would also limit it even with an improved converter.)

    What happens if you request more braking force than about that ~20kW, the friction brakes take up the slack. Also, at 7mph the system transitions to friction.

    The stock EV mode cuts out in 34mph on the Gen II or 24mph on the Gen III. This is a software limit likely done to dissuade you from trying to use EV mode much, as it will reduce fuel economy. THS can safely run pure EV mode up to 52mph, and can run a fuel-cut EV mode at any speed. Some people call this "Warp Stealth" mode.

    Because of the I2R losses in the battery, you generally want to stretch out your regen stops to be as gentle as possible. It's a false assumption that if you keep it under 20kW you are getting the same amount of energy captured.

    Keep in mind, regardless of how much you get from regen, it's energy that would otherwise be wasted, and that's the beauty if it!
     
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  14. twittel

    twittel Senior Member

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    SB, thanks for the easy read reference debunking "B" brake myth. I've learned that B brake is not a bad thing, but you're right, it's use does carry a lot of misconception about regeneration efficiency.
     
  15. sipnfuel

    sipnfuel New Member

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    pEEf,

    Thanks for the clarification. I misread/misunderstood some stuff in the ORNL reports that led me to believe this may be the case. In my work, all the motors we use have have VFDs using dynamic braking with braking resistors . I read the Prius had some resistors, I assumed this was the same thing. It seems the resistors in the Prius only bleed the caps after the Prius is shut down, not enough to do what I described.

    Also the numbers I used for the Inverter & Motor efficiencies was based on a rough integration I did in my mind, it did not turn out very accurate. I looked at the charts again and the combined efficiency should probably be more around the number you gave.

    66% is a generic number for NiMH batteries. I tried to see if anyone has ever calculated the charging efficiency of a Prius NiMH battery, but I didn't find any.
     
  16. SageBrush

    SageBrush Senior Member

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    Sorry guys, I should have clarified earlier --

    Max power in/out of the battery is 21 kw for G2, 26(?) kw for G3 Prius.
     
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  17. pEEf

    pEEf Engineer - EV nut

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    Actually, max (peak) battery power on a Gen II is close to 30kW, but can sustain about 21kW.
     
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  18. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

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    i am shocked that regen returns 30+%? is that right? i assumed it to be much lower than that. even in a zero gravity, frictionless environment, i thought that 100% recapture was not possible or even close to possible

    hmmm...
     
  19. David Beale

    David Beale Senior Member

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    Well, regenerative "breaking" is total loss. Regenerative "braking" is not. ;)
     
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  20. 2009Prius

    2009Prius A Wimpy DIYer

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    I don't know the system well enough to do the math but I thought someone tested his (gen2 ?) Prius and found that the highest energy return happens at some intermediate level of regen current, definitely not with the lowest or the highest regen current. I am not sure how well controled that experiment was though.