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    Oldwolf Prius Enthusiast

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    I’ve searched for an answer to my question but have not found it yet.

    When the HV battery becomes fully charged, which might happen when driving down long mountain grades (that can continue for many miles), what happens? Does the computer just cut off regeneration so you “feel” a loss in regenerative braking?
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    mikewithaprius New Member

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    You should switch into B mode (on the stick with D, R, N) for engine braking. This will help prevent overheating/overcharging of the battery pack (EDIT: last bit is wrong, see post #6) when it reaches the computer-imposed 80% state of charge limit. It will still feel like regen braking, but instead of taking that energy and putting into the battery, the engine throws it off as heat.

    You can still drive normally in B mode, too, accelerate, etc., just the braking is different.
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    2k1Toaster HID Guru

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    When the battery is fully charged (80%), it then cuts to friction brakes. The transition is pretty seamless. When in B mode, it spins the engine as an air pump just to waste energy it captures from regen. That way it doesn't go into the battery.

    The car is extremely careful to protect itself. You cannot kill it. If the terminator was in a Cars movie, it would be a Prius. (With a slight personality change from "death to all" to "look at the fuzzy wabbit").

    I live on a mountain and every morning I start with low SOC and end up fully charged by the time I get to the town's elevation. I don't really notice the switch from regen to friction only when it fully fills up.
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    firepa63 Former Prius Owner

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    The ICE will also spin up to draw off energy when in D. You don't have to be in B, but it helps when going down a long hill.
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    cwerdna Senior Member

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    Agreed w/firepa63. I frequently hit soc above 75% (as indicated by ScanGauge, looks like all bars filled on the MFD) while going downhill to get home and the ICE automatically turns on while in D, even at speeds <41 mph. Sometimes, the revving gets pretty high too, say over 3000 rpm.
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    richard schumacher shortbus driver

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    The reason to shift to "B" on a long downgrade is to save the friction brakes, not the battery. (The battery takes care of itself perfectly under all driving conditions.) And that's not just a matter of conserving the brake pads: it is possible to boil brake fluid, in which case you will have no brakes.

    Don't use "B" when not on a long downgrade because it will only reduce fuel economy.
    1 people like this.
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    billnchristy Active Member

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    We drive up and over Blood Mountain here in GA a few times a year and the first time you use B it can kind of freak you out when you are slowing down but the engine starts revving higher and higher.

    It works like a charm though, I love it.
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    BAllanJ Active Member

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    Is there any other way to use this energy productively rather than let the energy get thrown away spinning the engine?

    I was thinking that maybe if it was hot and you were using the ac minimally to improve mileage and you saw a full battery before a long downhill you could go to max ac and then you could reduce the ac later. Not sure if the ac uses enough to have an impact, but I'm just throwing this out there... wonder if there are any other possibilities?
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    fuzzy1 Senior Member

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    Yes on the AC, because at this point, it is free. Just like going into 'B' mode, I switch it on before the battery fills.
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    cwerdna Senior Member

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    Good idea... I think. Problem is, it's so cold in the Seattle area that AC is of no benefit other than to dehumidify.
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    fuzzy1 Senior Member

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    Fortunately most of my long mountain descent experience has been in warmer seasons outside of Seattle.
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    vincent1449p Active Member

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    Even in cold climate, the HV Battery gets heated up with frequent charging and discharging. So there is still benefit to use A/C for the longevity of battery life.
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    cwerdna Senior Member

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    True... I do have HV battery temp XGauges set in my Scangauge so I'll check them more closely but err.. it's pretty cold for the driver already. I'm using the heater!

    Here's is our forecast:
    KING5.com | Seattle Weather Forecast Maps

    50 F = 10 C and 39 F = 3.9 C
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    Oldwolf Prius Enthusiast

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    To summarize:

    When the HV battery is full or nearly full and the car is descending a grade-
    1) The battery on the MFD turns a Salmon color.
    2) The computer starts to use the engine for a higher percentage of braking.
    3) When the engine cannot provide enough braking (RPM limited perhaps?) the friction brakes are utilized to some degree.

    Do I have the sequence correct? The automatic use of the friction brakes would requires some sort of temperature measurement to avoid overheating the rotors/drums. What am I missing?
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    richard schumacher shortbus driver

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    The car never automatically applies the friction brakes, that's up to the driver. The car will use more engine braking sooner when in "B" than in "D". Shifting is also the driver's responsibility, the car will not shift itself.
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    uart Senior Member

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    Salmon Color? Mine just goes to full green. (8 bars.)

    What's with the Salmon thing, has anyone else seen that?
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    Oldwolf Prius Enthusiast

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    uart Senior Member

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