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Best way to brake / decelerate?

Discussion in 'Gen 3 Prius Fuel Economy' started by Dr.Mikki, Oct 30, 2010.

  1. Dr.Mikki

    Dr.Mikki New Member

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    Assuming we have to come to a full stop, what would be the best way to do this in terms of charging the battery? Do we want to
    a) Glide and then perform sudden breaking to a full stop (not exceeding the maximum charge)
    b) Coast with the leg off the gas (engine breaking) and then perform sudden breaking to a full stop (not exceeding the maximum charge)
    c) Perform continuous breaking with constant deceleration

    Or are the above options all equal in terms of charging the battery? I could not find a thread of this topic, but I am sure there should be one.
     
  2. adamace1

    adamace1 Senior Member

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    I think C would answer your question.

    So you just want to try to charge the battery or are you trying to get the best mpg?
     
  3. Dr.Mikki

    Dr.Mikki New Member

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    I am just trying to break and at the same time I of course want to charge the battery as much as I can. It is not always possible to glide until full stop :-(
     
  4. dcscm1

    dcscm1 New Member

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    Good question!

    I normally just coast to bleed off speed, and then do final braking as I approach the final stopping point. It may not be optimal, but it's most convenient with a lot of stop and go traffic.

    To maximize regenerative braking, I think the goal would be to maximize the time where the CHG bar is over half full (but not maxed out). At low charge rates I haven't seen the battery charge increase much at all, presumably because losses in the system for voltage conversion result in little real charge returned to the batteries.

    This, of course, assumes that you KNOW you will need to stop. If you're able to decrease speed until the light turns green or until traffic clears, then so much the better since you can avoid the need to accelerate from a standstill.
     
  5. spiderman

    spiderman wretched

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    Someone else mention to drive like you don't have brakes. Of course traffic these days doesn't always allow for this but it does work good when you can glide.
     
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  6. Much More Better

    Much More Better Active Member

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    I have always wondered at what point on the charge bar or at what speed does the friction brake kick in? This way I can keep it in regen when possible.
     
  7. spiderman

    spiderman wretched

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    The switch over is around 7 mph. Oh course you slam on the brakes and the switch over is instantaneous.
     
  8. darelldd

    darelldd Prius is our Gas Guzzler

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    Adamace1 asks a relevant question that you should hear again:

    Do you want to charge the battery as much as you can, or do you want to maximize fuel savings? They are NOT the same. Recall that hypermilers take the battery out of the equation as much as they possibly can. There are inefficiencies in charging and discharging the battery. The momentum of the car is totally wasted with the friction brakes, of course. But understand that you still loose MOST of that momentum even with regen. You get just a fraction of your energy back out of the battery as motive force. So the bottom line - use your brakes (including regen) like they don't exist. ;)
     
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  9. Dr.Mikki

    Dr.Mikki New Member

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    Darelldd, I am aware that breaking and especially stopping is bad for fuel economy. However, there are situations were you MUST stop or almost stop. So I want to figure out what is the best way to decelerate when the break pedal must be pressed. Dcscm1, already indicated that breaking with "CHG bar is over half full (but not maxed out)", would be most effective.

    Is there any way to get data of this?
     
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  10. Dr.Mikki

    Dr.Mikki New Member

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    I think there actually are quite many cases where you KNOW you must stop or almost stop: Slow traffic light that turns red 50m before you (I have one in my daily commute that takes almost 3 minutes to change), yield signs in poor visibility junction, stop-signs, steep downhill with nasty speed pump at the end, etc
     
  11. Hal W

    Hal W New Member

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    If there is a stop sign and you must stop I use c as you get more regen with that system of braking.I keep the braking as light as possible. H
     
  12. qbee42

    qbee42 My other car is a boat

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    As others have posted, try to coast and avoid the stop. That is your best bet. If you can't do that and must use the brakes to slow down, try to use moderate braking. Heavy braking may exceed the capacity of the regenerative system, forcing friction brakes to come on line. Very light braking, at least with the Gen IIs, is less efficient since there appears to be some fixed overhead in the inverter system. In very light braking a significant portion of the regenerated power is consumed by this fixed overhead. Braking a bit harder puts more into the battery. Of course it's possible that this has changed for the Gen IIIs.

    Tom
     
  13. ken1784

    ken1784 SuperMID designer

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    Assuming we have to come to a full stop...
    d) Perform continuous breaking with constant deceleration until 25 mph, shift to B for max regeneration, then shift to N at 7 mph to eliminate the power consumption of creeping.

    Ken@Japan
     
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  14. bisco

    bisco cookie crumbler

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    charging or mpg's? must be lost in translation.:rolleyes:
     
  15. cproaudio

    cproaudio Speedlock Overrider

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    I always brake using the C method. I never brake with HSI maxed at regen if I can help it. Of course if someone cut in front of me or if light turns yellow, I brake hard if I have to. Using the C method, I constantly have 7 bars on the battery charge.
     
  16. qbee42

    qbee42 My other car is a boat

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    A constant 7 bars is not a good sign. Did this start happening recently?

    Tom
     
  17. cproaudio

    cproaudio Speedlock Overrider

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    It's a great sign. my mileage is higher with 7 bars than with 6 bars. I accelerate into the pwr bar on HSI and use only regen for braking. I avoid friction braking if I can. I can easily go 2 miles on EV and still have 3 to 4 bars. Ok it's not a constant 7 bars but I get 7 bars half of the time in city but never 7 bars at highway speed. I'm only getting 7 bars now that I'm not using AC so yeah it happened recently.
     
  18. kgall

    kgall Active Member

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    For those who say you should not maximize charging when stopping--I think that's not quite so. As I understand it, when you slow, you usually do not burn gas--so the real question is how do you use the energy from the gas you have already burned--and saving as much as you can in the battery is a good idea.
    I suspect that your choice C is the best shot there, as it maximizes regeneration.
    I would also say that dcscm's point that strategizing to keep your minimum speed as high as safely possible is a good one--even though your hypothetical is the case where this is difficult or impossible.
     
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  19. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

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    if you must you must. what others are implying is that if you must stop, that usually means you are driving too fast. there is no reason to drive normally up to a few hundred feet of a light you know will be red.

    that is not a great sign. 7 bars usually means your battery capacity is failing. in city driving for me, i try to stay at 6 + bars, but that is hard to do and drive efficiently. i am frequently in the 4-5 bar range.
     
  20. fuzzy1

    fuzzy1 Senior Member

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    Who would they be?

    I see more of a message to think back to the last fuel burned before needing to stop -- could some of the burn been avoided by some better planning? For typical driving styles, the answer is very frequently 'yes'.