How many amps drawn when stopped with light foot on brake?

Discussion in 'Gen 2 Prius Technical Discussion' started by theorist, Jul 27, 2006.

  1. theorist

    theorist Member

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    I've noticed that when completely stopped at a red light, the MFD shows current flowing from the battery to the wheels if I don't have a really firm foot on the brake pedal. Can someone with CAN-view tell us how many amps are flowing from the battery or to the motors when stopped and how this depends on the amount of pressure applied to the brake pedal?
     
  2. efusco

    efusco Moderator Emeritus
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    <div class='quotetop'>QUOTE(theorist @ Jul 26 2006, 11:19 PM) [snapback]292992[/snapback]</div>
    My recollection is that it's about one additional amp.
     
  3. ScottY

    ScottY New Member

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    I noticed that if I have everything off, no AC, no headlights, and car stopped, current draw is below 2 Amp. If I don't press on brake hard enough, but not till the point that the car's moving, current draw went up to around 5 Amp.
     
  4. hobbit

    hobbit Senior Member

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    5 amps sounds like a lot. I see almost nothing until I actually
    start *moving*. Yes, there's a little bit of torque applied from
    MG2 as hill-holding and creep force, but at locked-rotor if it was
    any more than an amp or two you would have much more trouble holding
    the car still. The way to think about it is this:
    __ power = torque * rpm
    __ power = volts * amps
    if RPM is zero, then power is zero [other than maybe a teeny bit of
    heat dissipated in the windings], and thus amps must be close to
    zero too. Current starts to rise as you begin moving, and when it
    reaches a certain threshold the ICE starts to make up for it. This
    gives rise to the common but incorrect notion that the "engine starts
    above a certain speed" that is less than 42. With steady foot
    demand, it simply starts when battery current goes above 50A [at
    nominal SOC].
    .
    _H*
     
  5. ScottY

    ScottY New Member

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    5 amps is what CAN-View is showing at the condition mention above. How about other CAN-View users, what you guys get?
     
  6. tomdeimos

    tomdeimos New Member

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    I see only 1 amp on my canview with brake on too lightly. Maybe peaks of 2, as it seemed to jump around a bit.

    This was at a draw of 3 amps when stopped with brake on normally. This varies mostly with ac and fan speed.

    This all might vary with battery charge as in creep mode the battery might try to charge a bit too, but in full stop it should not.
    Grade of the road might affect the current needed to hold the car too.
     
  7. ScottY

    ScottY New Member

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    wow... am I the only one with this high of current draw? The readings are pretty consistent since day one I got CAN-View.
     
  8. tomdeimos

    tomdeimos New Member

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    <div class='quotetop'>QUOTE(ScottY @ Jul 27 2006, 11:39 AM) [snapback]293194[/snapback]</div>
    OK last night I measured this again and I got 5 amps this time.

    This was at night with headlights on and AC working hard.
    Maybe the current varies due to charging the 12 volt battery and givine more power to the AC?

    Mostly I just see the 1 amp change between brake full on and on lightly at a stop.
     
  9. ken1784

    ken1784 SuperMID designer

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    <div class='quotetop'>QUOTE(hobbit @ Jul 28 2006, 12:09 AM) [snapback]293176[/snapback]</div>
    Yes, the power output is zero, but the input is not.

    [email protected]
     
  10. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

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    i only see 1-3 amps with no lights on. stereo running. as i put it in park nearly every time i stop. but on places where there are extreme hills, i use the brake and it seems the harder i push, the higher the current pulses. (in any situation, the readings constantly fluctuates.) i have seen it as high as 5-7 amps, but you will notice that if braking just hard enough to keep from moving, the change is very slight
     
  11. hdrygas

    hdrygas New Member

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    I will check tomorrow but with my foot lightly on the break and stereo on, air on 2-3. I am learning to not press the break hard right now so I will have to check but by memory alone it may be 1-2 higher. I will check tomorrow.
     
  12. hobbit

    hobbit Senior Member

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    You're testing with the *AC* on ?! You realize that's a whole
    'nother kilowatt of draw directly from the HV battery, right?
    There's where your 5 amps are coming from.
    .
    Try it with everything else turned off and letting the creep force
    try to work against the set parking brake so you sit still.
    .
    _H*
     
  13. ScottY

    ScottY New Member

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    ok... i went out and confirm it again. The reading is with everything off, current draw 1.x amps. When I allow the car to creep, it's 1.x amp more. Total of 2-3 amps.
     
  14. GreenJuice

    GreenJuice Active Member

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    I'm resurrecting this 2 year old thread to give some additional readings from my ScanGauge II BTA Xgauge in various situations when stopped, and to ask a question of the many gurus who've previously contributed to this thread!

    My additional observations (A/C, lights, interior fan all off):
    If I hold the car stationary in D with a light foot on the brake so that the yellow arrow from the battery on the MFD is still on, it reads around 2 - 3 amps.
    A firm pressure on the brake, drops its down to around 1.5 amps, as the yellow arrow clears. This reading stays the same regardless of whether I am in D, N or P.
    Lift off the brake in D and the reading rises to 2 -3 amps, as the car creeps forward.
    Lift off the brake in P or N and the reading falls to around 0.9 - 1.1 amp range.
    I guess this last figure represents a background or baseline energy use.
    The application of the brake itself appears to consume energy, putting up the current draw.

    My question:
    Does this imply that there might be something (perhaps minimal) to be gained by selecting P and taking my foot of all pedals, if I think I might get held up at a long set of lights?
     
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  15. ken1784

    ken1784 SuperMID designer

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    I also think it is minimal, 0.5 amps between brake-on and off.

    However, following is a ceremony among Japanese hypermilers going to red lights. :)

    1. shift to N below 7mph, no more regen expected and no electric consumption of simulating creeping at lower speed by light braking
    2. apply parking brake when stopped, then release foot brake to save 0.5 amps
    3. shift to D and release parking brake when to start
    [email protected]
     
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  16. David Beale

    David Beale Senior Member

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    With foot hard on brake you are adding the brake lights to the load and intermittently, the brake booster pump as well as making the braking system "think". As these run off the 12V system you may not see an immediate change, as the charger may take some time to compensate. Note that while the brake lights don't draw very much at all, they will trip the charger, and it's not very efficient, thus you can see the difference.
     
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