B gear versus CC

Discussion in 'Gen 3 Prius Technical Discussion' started by vday, Aug 22, 2010.

  1. vday

    vday Member

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    :rolleyes:In my daily commute home I drive about 5.5-6 miles continuously downhill. To hold the car back I sometimes use the B gear and sometimes use CC. In both case I start at about 45-50 MPH and use the brake when I hit 65 MPH which is at the bottom of the steep decline about 2/3 of the total decline. Neither method holds the car back but both help. In both cases the battery easily completely fills up (8 bars) before finishing the steep decline.

    What is better for the car?

    Thanks for your advise;)
     
  2. Colonel Ronson

    Colonel Ronson New Member

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    B gear prevents regenerative braking. it uses engine braking. so basically you are spending energy to slow down.

    as with cruise control, i think it will utilize regenerative braking to the point as if you were simply lifting your foot off the pedal. i would recommend light braking instead to slow down versus the "B" since that is wasting energy.

    65 mph doesn't seem that fast though...
     
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  3. Airbalancer

    Airbalancer Active Member

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    65 mph doesn't seem that fast though

    I GUESS it would depand on what the limit is and if a cop is at the bottom of the hill:mod:
     
  4. qbee42

    qbee42 My other car is a boat

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    This is incorrect, or at least confused.

    B gear allows full regeneration, but in addition it *immediately* invokes engine braking. That is the fundamental difference. Without B gear, you press the brake pedal and regenerative braking slows your Prius and charges the HV battery. When the HV battery gets to the high limit, engine braking is invoked, since the HV battery now has little room to absorb charge.

    With B mode you start engine braking immediately. You press on the brake pedal and you get engine braking *and* regeneration. Since the engine braking is dissipating some of the energy, it takes longer for the HV battery to reach the high limit. This is the main reason to use B mode.

    In either B or D mode, if regenerative and engine braking are insufficient, friction braking is automatically added to the mix.

    In the OP's case B mode is the correct gear. This is because the OP states that either way the HV battery reaches full bars. Starting in B mode reduces the chance that friction braking will be necessary.

    Tom
     
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  5. teeasal

    teeasal New Member

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    I suggest you try to use EV to drain the batt as much as possible before the down slope. Yes using B is good in this case.

    You may want to let the car go a few mph over 65 at the bottom of the slope. That's another way of storing energy in way of kinetic. You'll glide much longer after the slope, flat or even up hill. I don't think the cops will nab you for 68-69 or even 70 for a short distance cause you're slowing down gliding right afterwards?

    What is the normal speed of other cars? If most of them are over 65 at the bottom of the slope, I think you'll be ok too if you're just following them.
     
  6. David Beale

    David Beale Senior Member

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    This is also partially incorrect. He asked "what is better for the car?" Using "EV" mode to drain down the battery and then recharge it is NOT better for the car. The more and harder you use the battery, the shorter its' life.

    I agree with Tom. "B" mode is the better way to go. It reduces the friction braking you would have to use, lengthening the life of the brake pads.
     
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  7. qbee42

    qbee42 My other car is a boat

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    The EV mode suggestion above would maximize the recovered energy, which is good for mileage. As pointed out above, it works the battery a bit harder.

    Tom
     
  8. vday

    vday Member

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    Speed limit is 56 MPH (90KPH) and there is a fairly sharp curve at the bottom of the big slope.
    thanks
     
  9. vday

    vday Member

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    On the subject of EV the car seems to refuse to go into EV of late - could it be because the engine is too hot?
    It has been very hot (even triple digits) of late
     
  10. qbee42

    qbee42 My other car is a boat

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    It's not the engine, but the battery. High battery temperatures prevent
    EV. The control system works very hard to protect the HV battery.

    The HV battery is cooled by cabin air. You can always crank up your AC and see if that helps.

    Tom
     
  11. teeasal

    teeasal New Member

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    I think fully charging the battery too often is NOT better either. By "draining" it before hand, he'll have more chances of the battery not over charged at the bottom of the slope. It allows the car to store more energy using the whole safe range of battery operation.

    Yes it does work the battery harder but I don't think that would shorten its life too much. Toyota won't give us such a long warranty period for the battery without having absolute confidence about the way the Prius' computers control the use of the battery to protect and prolong its life.

    If the battery craps out earlier before the warranty expires because of this normal usage, the better because he'd get a brand new battery replacement from Toyota and it would last another full battery life (although a bit shortened but 2 shortened battery life is still longer than 1 normal battery life.) I still don't think that'll happen though.
     
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  12. tumbleweed

    tumbleweed Senior Member

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    As pointed out it still regenerates in B mode, but you can get a little more regeneration by just using the brake pedal in D if you watch the HSI display closely. It will also use engine braking when you are braking in D after the battery is fully charged.

    In the circumstances described I don't think it makes much difference which you use. I would just pick which ever way I liked the best.
     
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  13. qbee42

    qbee42 My other car is a boat

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    The battery management software will never let you overcharge the HV battery, so that is not an issue. Likewise the effects of running it down and charging back up are very minimal. While it is technically correct that cycling the battery reduces its life, the battery management system keeps the HV battery in a fairly narrow range. The practical implications are negligible.

    Tom
     
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