Regenerative braking and full battery

Discussion in 'Gen 2 Prius Technical Discussion' started by whodat, Jul 19, 2006.

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  1. whodat

    whodat Member

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    What happens to the charge (coulombs) during braking when the battery is full?

    I drive a hilly route each day (HWY 17 in CA) and going down hill the battery in my 2006 Prius gets charged to the maximum about halfway down from the summit and I'm wondering whether I am still getting regenerative braking with a full battery or whether I am burning up the disc pads instead? If I am still getting benefit from regenerative breaking, where does the charge flow to if the battery is full? During the battery full state, I would presume the charge controller electronics would shunt the current away from the battery. I understand Toyota limits the charge and discharge to 80% and 40% respectively of the battery capacity to improve the battery life for the high voltage battery (220V) - not to be confused with the tiny 12V auxilary battery.

    Does anyone know?

    Thanks
    Dave
     
    #1 whodat, Jul 19, 2006
  2. efusco

    efusco Moderator Emeritus
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    Once the battery starts approaching 70% SOC regeneration falls off dramatically. Above 75% SOC it's gone completely and the ICE will begin to spin solely to drain off excess charge. And yea, you're using friction brakes only to slow/stop at that point.
     
    #2 efusco, Jul 19, 2006
  3. hdrygas

    hdrygas New Member

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    Anticipate on the up hill "burn" up that charge and get to the top with a pink battery if you can, then go for the green!
     
    #3 hdrygas, Jul 19, 2006
  4. whodat

    whodat Member

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    <div class='quotetop'>QUOTE(hdrygas @ Jul 19 2006, 07:28 PM) [snapback]289137[/snapback]</div>
    I've tried that and even then I still max charge the battery. There are about 50 other Prius drivers that drive CA 17 and my guess is they experience the same as well.

    Dave
     
    #4 whodat, Jul 19, 2006
  5. Bill Merchant

    Bill Merchant absit invidia

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    This is a perfect use of the much misunderstood B mode. Shift into B as you crest the hill and you'll save your brake pads by burning kinetic energy spinning the ICE. And it's always a good idea to use as much battery as possible going up the hill, since excess charge coming down is turned into heat.
     
    #5 Bill Merchant, Jul 20, 2006
  6. efusco

    efusco Moderator Emeritus
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    <div class='quotetop'>QUOTE(Bill Merchant @ Jul 20 2006, 12:27 AM) [snapback]289215[/snapback]</div>
    Yea, but I'd rather use my brake pads a little at the bottom, they're probably going to outlast the rest of the car anyway, than use gas/energy from the top of the hill to the bottom.

    But yea, if you're gonna use B-mode this is the situation to do it.
     
    #6 efusco, Jul 20, 2006
  7. whodat

    whodat Member

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    <div class='quotetop'>QUOTE(efusco @ Jul 20 2006, 04:58 AM) [snapback]289276[/snapback]</div>
    Seems like using B mode when the battery is full rather than at the crest of the hill is the best thing to do. This this confirms that I am burning up the disc pads rather than charging the battery when the battery shows full.

    I wonder how much error there is in the 60% capacity algorithm in the battery versus what is shown on the screen in the car with the battery green bars fully charged. My guess is its not perfect.

    Dave
     
    #7 whodat, Jul 20, 2006
  8. Trollbait

    Trollbait It's a D&D thing

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    I'm pretty sure that the regenerative braking still operates in B mode. The braking won't charge the battery as quickly as in D-mode, but that just means your charge won't reach full until after the halfway point of your descent. Depending on the grade, B-mode and regenarative drag alone may be enough to keep you at the proper speed.
     
    #8 Trollbait, Jul 20, 2006
  9. cagemo

    cagemo Member

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    <div class='quotetop'>QUOTE(ShellyT @ Jul 20 2006, 08:21 AM) [snapback]289373[/snapback]</div>
    I'd love to have a definitive answer to this one. I live in Northern CA, and go up and down long hills all the time. I've "shifted" to B mode on steep downgrades and wondered what happens to the regen braking then. Would love to know the "best" braking strategy based on different battery level / grade conditions that minimizes component wear, excess heat gain, etc.

    Hello, TMS? :D
     
    #9 cagemo, Jul 20, 2006
  10. efusco

    efusco Moderator Emeritus
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    <div class='quotetop'>QUOTE(cagemo @ Jul 20 2006, 02:46 PM) [snapback]289529[/snapback]</div>
    This is probably something outside of TMS's expertise.
    What, exactly, do you want a definative answer to?
    Regenerative braking does still occur in B-mode, but it won't be as effective b/c some slowing is being caused by the ICE. Once the battery is full there is no significant regenerative braking and I've actually seen battery drain occuring while my foot was on the brake with a full battery.

    So, I ask again, what, exactly, do you want answered definatively?
     
    #10 efusco, Jul 20, 2006
  11. KMO

    KMO Senior Member

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    B is there to help slow you down and save the mechanical brakes, as well as helping control. Same as switching to low gear downhill on a normal car.

    As soon as you start worrying about "maximum generation", it's you that's making things complicated. The car interface is supposed to be simple for ordinary drivers - D for normal drive, B for extra hybrid system braking downhill.

    If you're going to worry about it, the executive summary is: for maximum regeneration, always use D. Hobbit's article has the full details.

    On a really long downhill run, you're going to fill the battery no matter what you do, no need to worry maximising regeneration, so you may as well use B. On a shorter downhill run, stay in D if you want maximum regeneration. But use B if you're ordered by signs to select a "low gear".

    I prefer to use B quite a lot on the very few hills I encounter, simply because I feel more confident that the car isn't going to run away so much if I slip off the brake pedal. Control is more of an issue to me.
     
    #11 KMO, Jul 20, 2006
  12. cagemo

    cagemo Member

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    <div class='quotetop'>QUOTE(efusco @ Jul 20 2006, 02:03 PM) [snapback]289586[/snapback]</div>
    Heard ya the first time :rolleyes: ShellyT said "I'm pretty sure regen braking still happens in B mode." So my question was simply whether regen braking was on or not when in B mode. In reading the thread, I couldn't tell if anyone knew definitively.

    Anyway, I got the answer. What B does, driving on hills, etc I know now. Thx Hobbit.
     
    #12 cagemo, Jul 20, 2006
  13. tonyl

    tonyl Junior Member

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    <div class='quotetop'>QUOTE(cagemo @ Jul 20 2006, 04:19 PM) [snapback]289671[/snapback]</div>
    The B mode "engine braking" will bring the MDF to show full charge for me, I have used it several times
    now comeing down canyon's hear in SE. AZ. on my birding outings on steep dirt roads. On my slow climbs up I try to get the battery used as much as possable, comeing down for 8+ miles on regen/B and e-mode
    makes me feel good anyway hoping I have used less gas over all. Avg. 52 mpg in the dirt/county roads.

    Off tread:
    And yes for others the dash jumps all over the place (hope it dose not fail) and I don't need an FJ to go on dirt trails if the car is built well.

    tony l
    wa6qmq
     
    #13 tonyl, Jul 20, 2006
  14. hyo silver

    hyo silver Awaaaaay

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    There's another trick you can use when you've got more kinetic energy than the batteries know what to do with: crank up the air conditioning. My daughter suggests cranking the stereo, too, but Mom's not so keen on that idea. :)
     
    #14 hyo silver, Jul 20, 2006
  15. whodat

    whodat Member

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    Has anyone with the Can-View checked whether the SOC (state of charge) and the fully green battery display have good correlation? Stated differently, as soon as the battery reaches the fully green state with all the bars fully green does charging of the battery stop to zero?

    I'm quoting the following Can-View web site:

    http://www.hybridinterfaces.ca/disppars.html

    "Unlike batteries in power tools which expire rapidly, the Prius battery lasts for years because it is very carefully controlled. You can expect to see a minimum of ~40% if you have been running the A/C while stopped for some time, before the engine starts to recharge, or at the end of an EV run. You can also expect a maximum of ~80% after long downhill braking. Normal operation is within just a few % somewhere in the middle. After you reach ~80% on a long down-hill stretch, there is nowhere to store the excess energy (in the current size of Prius battery) so the engine switches to high rpm compression braking and discards the energy as radiator heat. A bigger battery could store that wasted energy....."

    I would expect that during a long down-hill stretch (like CA HWY 17) after the battery shows full green bars the engine should switch to high rpm and I should either feel or hear this, but I don't. So either I don't feel this in the car and its happening, or I am still getting some regenerative braking benefit while the battery shows fully green bars.

    Maybe I need to buy on the CAN-View devices to experiment with.

    Dave
     
    #15 whodat, Jul 20, 2006
  16. efusco

    efusco Moderator Emeritus
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    I've never seen over about 73% on my CAN-View and was getting negligible regeneration at that point. But, in short, yes, the correlation seems to be quite good.
     
    #16 efusco, Jul 20, 2006
  17. whodat

    whodat Member

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    <div class='quotetop'>QUOTE(efusco @ Jul 20 2006, 08:55 PM) [snapback]289795[/snapback]</div>
    Thanks for the post. I will have to wait until CAN-View is available for 2006 Priuses.

    With CAN-View, I believe I could be able to safely charge the high voltage battery by monitoring the SOC and being careful not to exceed some upper limit.

    You are lucky to be able to play with CAN-view and see data about the battery and engine.

    Dave
     
    #17 whodat, Jul 21, 2006
  18. Bill Merchant

    Bill Merchant absit invidia

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    <div class='quotetop'>QUOTE(whodat @ Jul 20 2006, 09:05 PM) [snapback]289800[/snapback]</div>
    You can safely charge the high voltage battery without a CAN-view: just drive the car.

    Toyota engineers did a lot of programming so you don't have to worry about safely charging the high-voltage battery. They also made sure you can't over-charge the battery. Too much charge, like a completely green battery at the bottom of your steep hill, and the ICE will cycle on and off to burn excess charge to get the HV battery in the optimal range.

    The Prius protects its HV battery well.
     
    #18 Bill Merchant, Jul 21, 2006
  19. Sufferin' Prius Envy

    Sufferin' Prius Envy Platinum Member

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    <div class='quotetop'>QUOTE(hyo silver @ Jul 20 2006, 08:16 PM) [snapback]289778[/snapback]</div>
    Proof the air conditioner trick works:
    http://priuschat.com/index.php?s=&showtopi...ndpost&p=276244
     
    #19 Sufferin' Prius Envy, Jul 21, 2006
  20. whodat

    whodat Member

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    <div class='quotetop'>QUOTE(Bill Merchant @ Jul 20 2006, 10:42 PM) [snapback]289829[/snapback]</div>
    Bill,
    Sorry I was not clear. I am trying to charge the high voltage battery with an external charger, not the ICE. I'm trying to make a plug-in for short trips where I don't want the ICE to even turn on.
     
    #20 whodat, Jul 22, 2006
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