Will traction battery charge with downhill cruise control?

Discussion in 'Prime Fuel Economy & EV Range' started by P.Neophyte, Mar 25, 2021.

  1. P.Neophyte

    P.Neophyte Junior Member

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    I live in the Midwest where we don't have long, sweeping arcs of roads out of the mountains so I can't test this hypothesis. Obviously, regenerative braking charges the battery, but if there was a long, moderately steep decline and the Prius Prime had Full-Speed Range Dynamic Radar Cruise Control (DRCC) set to a moderate speed, say 40 mph for a prolonged period of time, would the battery charge from its depleted state? Assume that the grade was sufficiently steep that if put it in free wheeling Neutral, the car would coast in excess of 40 mph.

    P.S. I've never had the occasion to use the "B" setting, which I believe would be useful when I want the engine to brake for me, but not use the regenerative brakes.
     
  2. jerrymildred

    jerrymildred Senior Member

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    I looked you up on the map. Indy suburb. Yeah, that's pretty flat.

    Yes, the Prime will regenerate going down a hill with the cruise control set. (Dynamic or regular, it's all the same in that regard.) However, if you mean will it go from no EV range to some EV range, that takes an actual mountain. There's kind of a dead band in the system such that once the EV range gets used up, it takes lots of juice to get it back, but when it does come back it'll show at least a mile as I recall from my trip to Colorado.

    B mode when you're in EV on the Prime increases regeneration which is a GREAT feature if the slope is the right steepness. Cruise control doesn't work in B mode, though, so you'll need to regulate speed manually. That makes it often easier to just leave it in drive and feather the brake pedal. If you are in HV mode, B just uses engine braking like any other Prius.

    I encourage you to take it on a trip to the mountains. The car absolutely loves mountains.
     
  3. rdgrimes

    rdgrimes Senior Member

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    Went for a 250 mile loop through the mountains here last W/E. It's easy to maintain the EV level of charge but not so easy to increase it. Going down a couple of familiar slopes I was able to gain maybe 20%, where any of the other Toy hybrids would have maxed out their battery and switched to ICE braking. My advice is to start out with maybe 60% EV battery but run in HV mode. Forget EV mode on the road, though EV-Auto does work better.
    If its charging you want, use Charge mode and just let the ICE run. It'll get the job done the quickest.
    BTW, averaged about 80mpg on that trip, with max speed of 75 and average speed around 45.
     
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  4. vvillovv

    vvillovv Senior Member

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    hey neo
    you can get some ideas about the dynamic properties of the car and DRCC by taking the same decent (steepest longest one you've got) at different speeds set and with DRCC off, again, using different hypermiling techniques. Along with all the other tricks mentioned by others here at PC for operating the Prime on the downgrades.
    split screen is also helpful with the miles/Kw pump and dump gauge showing a continually fluctuating blue bar of what the computer data estimates the actual mi/Kw the car is getting in realtime, between O and 20 mi/Kw - much like the Mi/Kwh gauge on the MID that also shows a customize able graph of the data the computers retain for any one trip. Except the graph on the MID show either 1 mile slim bars or 5 miles fat bars. Each bar represents the average mi/KwH for either the 1 mile or the 5 mile previous instead of realtime like on the split screen gauge.


    also Don't let me forget to mention DrPrius if you want to focus in on the difference between regen and discharge when going up or down the same grade.

    More about that split screen gauge and the DrPrius app that are beyond the subject of this thread. Give it a look on the downgrades.
     
    #4 vvillovv, Mar 26, 2021
    Last edited: Mar 26, 2021
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  5. P.Neophyte

    P.Neophyte Junior Member

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    Thank you. I suspect that feathering the brake pedal on a long, steep slope does more for regeneration of charge than B mode, but is there a harm to "riding the brake" other than the obvious of disc pads overheating?
     
  6. P.Neophyte

    P.Neophyte Junior Member

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    Thanks, good advice. How long would you expect that it takes to fully charge the traction battery from normal depletion when you use charge mode?
     
  7. john1701a

    john1701a Prius Guru

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    Here's video of that entire process.

     
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  8. fuzzy1

    fuzzy1 Senior Member

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    From past discussions, I thought that careful brake pedal use and EV-B mode should produce nearly identical regeneration. With a slight edge to EV-B, because it won't be lightly dragging the brake pads the way the pedal does.

    HV-B mode, equivalent to the only B mode in non-plugin Prii, will collect significantly less regeneration because some energy is directed into engine braking.
    If feathering the pedal properly, keeping braking down to the regeneration limits, then the pads should not be overheating as they are taking little of the braking load. Even if done poorly, taking more of the braking load, they should still go much farther than non-regenerative vehicles before overheating.

    Do beware of special cases, such as hot and cold temperatures, when the battery can't accept charge at its full rate. At these times, regenerative braking won't harvest nearly as much energy as you expect, so the rest will be diverted into either engine braking, or friction pad-disc heating.
     
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  9. jerrymildred

    jerrymildred Senior Member

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    In my limited mountain driving experience, I didn't see any difference in regen between B and lightly dragging the brake when in EV. What I did notice was that B mode often made me go slower than I wanted which forced me to hit the throttle a very tiny bit. I found that that took more finesse than just a light tough on the brake pedal in drive.

    If you use Hybrid Assistant like John does in his videos, you can see that the brake indicator is green for regen braking and red for friction braking. I don't know the threshold for when it decides to change colors, but it is still informative.

    Off topic, but this is also telling for those people who think taking two hours to charge at L2 is bad for the battery.
     
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  10. fuzzy1

    fuzzy1 Senior Member

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    If you moved out here, I'm sure you'd quickly get enough mountain experience so that using B-mode and throttle simultaneously becomes second nature, not much different than D-mode plus throttle. Then the only issue becomes remembering to take it out of B-mode when it is no longer useful. And even that is probably less of an issue in EV-B than it is in HV-B and in our non-plug-ins, where it will cost a bit of extra dino-fuel use.

     
  11. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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  12. jerrymildred

    jerrymildred Senior Member

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    I might be living there even now if there were any jobs in the Pac Northwest when I got out of school in British Columbia in 1973. :D What a pretty place!
     
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  13. fuzzy1

    fuzzy1 Senior Member

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  14. SeattleBebop1

    SeattleBebop1 Member

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    I may or may not have cackled manically while going down my first mountain pass in the Prime, shouting at it to "CHARRGGEE!" And the PCT is sick, at least the bits I've been on for other hikes (I'm not a through-hiker).

    We hype up the rain and passive aggressiveness to discourage people from moving here (it's not working, lol!) But yes, it's gorgeous (Seattle since 2010, formerly of Baton Rouge).
     
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  15. jerrymildred

    jerrymildred Senior Member

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    Sounds like me coming down from Grand Mesa last time we were in Colorado. The battery went from 10% to 98%. :p

    Culturally, I like it a lot better here. Scenery-wise, I don't see why anyone lives here. On top of that, we're already getting highs over 90° F. As I recall, Mt. Rainier is a great weather forecasting tool. If you can't see it, it's raining. If you can see it, it's going to rain. :LOL:
     
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  16. Salamander_King

    Salamander_King Senior Member

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  17. SeattleBebop1

    SeattleBebop1 Member

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    Ah, useful experiment. So you had 0% battery left, just shifted into B mode going downhill and that alone was enough to start charging the battery. I thought maybe I had to add a little electric battery back first by manually getting it to "charge" (pressing the EV/HV button for 3 seconds).
     
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  18. SeattleBebop1

    SeattleBebop1 Member

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    I've only done a bit of hiking in Colorado! Flew into Grand Junction, rented a car, stayed at the Colorado National Monument, then drove down to Canyonlands/Arches for the grand event (March of last year, it was as we were returning home that the authorities started clanging the alarm about COVID-19 in the US).

    Southerners are a lot easier to converse with. Just made some new friends in my neighborhood during our last Snowmaggedon (I offered to let them borrow my sled) and we hit it off. I was not surprised to learn two of them are from LA and TX, and the other is from Southern Illinois (southern hospitality culture there too). The Seattle Freeze is real.

    Hahah, 35F-50F here.
     
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  19. Salamander_King

    Salamander_King Senior Member

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    You don't have to put in B. Just coasting in D will regenerate, although I think B mode would give you more braking and more regen. The trick is that until SoC is ~3% (at least 1mile of EV range), you will not see it on the dash, and it will not go back to EV mode until SoC is 6%. That is a lot of regens if you do not have very steep and/or long downhill. For normal terrain, it would be much faster to use CHG mode to get up to 80% SoC.
     
    #19 Salamander_King, Mar 29, 2021
    Last edited: Mar 29, 2021
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  20. jerrymildred

    jerrymildred Senior Member

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    Right. B mode, if you are not in EV, will use engine braking and regen, rather than just regen, depending on brake pedal pressure. On my rare mountain drives, I found it best to leave it in drive till it charged enough to show EV range and then put it in EV and possible B mode.

    True in my experience too, although I think I'd say southerners and country folk are easier to converse with than northern city folk.
     
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