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Electric Brake

Discussion in 'Gen 4 Prius Care, Maintenance and Troubleshooting' started by CEnsalata, Nov 26, 2016.

  1. CEnsalata

    CEnsalata New Member

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    Sorry if this exists elsewhere; the search function is difficult to use.

    I have a brand-spanking-new 2016 Prius Four. I've been using the electric brake ("B" shift) often to decelerate when coming to a red light or down an incline when I want to hold the car back. At low speeds the brake engages pretty silently and I feel does a much better job slowing the car than the same feature on my old VW. However, sometimes if the car is building speed while going downhill or when I engage the brake (say 27+ mph) the brake is no longer silently and sounds like it's working hard; not exactly a grinding noise, but the engine has a deep hum.

    1) Is this safe for the car or could I be causing damage? 2) Is it safe to use the electric brake at any speed, or only the lower ones? 2) Since my car has only ~300 mi, is it okay to be using the electric brake, or should I wait until the car is more seasoned?

    Thanks.
     
  2. bisco

    bisco cookie crumbler

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    don't use b mode, this may be the mpg problem in your other thread. have you read the o/m?
     
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  3. ChapmanF

    ChapmanF Senior Member

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    Most importantly, first off: no, you won't hurt the car. Everything about the drivetrain is under computer control at all times, and the computers know all of the relevant limits.

    But next thing: you really don't need to use B mode to get electric braking. The car brakes electrically every time you touch the brake pedal. (It also brakes hydraulically, if it needs to, and it doesn't make a big show of telling you how much of each it is doing, because short of geeky curiosity you don't really need to know. When you press the brake the car slows down. Its computers already calculate how much of that energy to recover electrically and how much to brake off. If you have geeky curiosity you can plug in a ScanGauge or the like, and watch.)

    Now, the B mode you have been using is really only there for a special, rare purpose. Say you know you are at the top of a long, steep descent and you're going to have to bleed off a lot of energy on the way down. Ordinarily, the car would start out by trying to reclaim as much electrically as it can, run the battery up to capacity, and then switch to blowing off the excess energy by using it to spin the engine (which is the sound you're hearing).

    By putting the car in B mode, you're basically giving the computer a hint: hey, this descent is going to go on for a while, don't bother heating up the battery trying to reclaim every possible Wh at the top, just take it easy and start earlier blowing off energy though the engine.

    In other words, with B mode, you're telling the car to reclaim less energy electrically than it normally would if you just used the brake.

    In normal driving there is absolutely no need to fuss with it.

    -Chap
     
  4. mark27lim

    mark27lim Active Member

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    don't use the b! it's NOT an electric brake! Use the brake pedal. that IS the electric brake. you only use b if you need to brake but the battery is already full. b will use the engine to help you brake instead of the electric brake when your battery is too full. that is why you hear the engine is loud. it's perfectly safe for the car don't worry. because if your battery is full and you press the brake pedal the car will use the brake pads to brake instead of the electric brake to prevent overcharging the battery. using b for normal car is correct because normal cars only have brake pads so b will use the engine to help slow down. in the prius you have something better than the brake pads and the engine brake - the electric brake -so use it! only in situations when the electric brake is no longer the best choice then you switch to b the engine brake. brake pads are the last choice except when you have no choice or like an emergency stop


    iPhone ?
     
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  5. CEnsalata

    CEnsalata New Member

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    Ohhhh, okay, learned so much. I was relying on B instead of the brakes because I thought the foot brake would be putting wear and tear on the brake pads. But basically everyone's saying the foot brake defaults first to electric brake and only uses the brake pads if the battery is full. Thus, if the battery is full use B to engine brake instead of having the wear/tear on the brake pads.

    Essentially there are three brakes: electric (foot), brake pads (foot), and engine brake (B mode).

    Did I get that right?

    Thanks.
     
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  6. ChapmanF

    ChapmanF Senior Member

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    You're still trying too hard to outsmart your car.

    You don't need to shift to B when the battery is full to tell the car to behave differently then. It will behave differently on its own - it knows when the battery is full.

    The car has three ways of slowing you down: electric regen, brake pad friction, and spinning the engine. It will make its own smart choices among all three of those methods, every time you use the brake pedal. (You won't generally notice it going straight to engine-spinning unless the battery is already at capacity, but then you will, and you don't have to do anything differently. You press the pedal when you want to slow down. The car makes the choices.)

    The only thing B mode does is hint to the car to make its choices a little differently. The car doesn't (yet!) use anything like a GPS-linked terrain map to be able to tell when you're descending a mountain versus, say, stopping at a level traffic light. So if you're in a situation where you know there's going to be a lot of energy to reclaim coming up over the next 6 miles of 7% downgrade, you can shift to B to let the car know that. It will still make its own choices among all three methods, but it will be less concerned with reclaiming every possible watthour, and more willing to throw energy away.

    That's the only sort of situation the B mode is there for.

    -Chap
     
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  7. Coast Cruiser

    Coast Cruiser Senior Member

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    First thing I learned about my new Prius: if you keep looking at all that stuff (the indicators and gauges) and thinking about all that stuff... you're going to drive yourself nuts!

    Best advice I got was, "just drive it like a normal car."
    Press the gas pedal to go, and press the brake pedal to stop, and try not to pay so much attention to all those screens and hybrid things. They can actually become distracting.

    Read the Owner's Manual. It's very boring and way too long, but you gotta read it. And when you're done? Just get in your Prius, and drive it like a regular car. :LOL:

    And the gas mileage you mentioned in your other thread, don't be concerned with short little trips! They mean nothing. Only check your gas mileage at the end of a empty full tank. You should get 55 to 60 MPG in the city, without even trying... Just drive normally. Fill your tank all the way up, and then just look at the "Trip A" little screen. (Reset it to zero at each fill up.) At the end of 600 miles, I average 60 miles per gallon or more. And I don't worry about the battery, or the braking, or the hybrid screen indicator... nothing! Most of the time, I turn all those screens off.
    (Your mileage will vary. I live in very mild weather.)

    Don't try and overthink it. Don't try and analyze it. Just drive it. Pretend it's not a hybrid. :D (but all the strange sounds will let you know it is. Just ignore them.)




     
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  8. MrMischief

    MrMischief Active Member

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    I disagree with this. It really bothers me not knowing when the friction brakes are on. In a traditional car when you're coming down a long mountain pass you can gauge how often you're using the brake to estimate if you might be overheating them or not. I don't feel I have that same sense in the Prius and would love if the ECO screen on the entertainment system had an indicator to show when the friction brakes are in use or a temperature sensor on the front friction brakes.
     
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  9. bhtooefr

    bhtooefr Senior Member

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    I can generally feel when it transitions to friction brakes - the braking reduces a bit for a moment, then it brakes harder, with no change in pedal force. And, it'll automatically bring the engine in on sustained descents, too, I find.
     
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  10. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    While the Engine Braking is counter-intuitive, (the harder you press the pedal, the louder the engine gets) the car will not hurt itself.

    On a long enough, steep enough downhill, the normal electric brakes will completely fill the big battery. (more than a 600 foot vertical hill) Rapid charging heats the battery, and subsequently, friction brake parts, so using B for engine braking from the top of the hill can save some overheating.

    Since B gets worse mileage than D, do not use it when you want best mileage. For many owners, there is never a time when B is appropriate, but Vermont may have some steep, sustained grades. Appalachian Gap - Knowledge Encyclopedia

    Since there is no gearing change, there are no limits to when you can use B, but it is not able to completely arrest acceleration, it is just an additional tool.
     
    #10 JimboPalmer, Nov 26, 2016
    Last edited: Nov 26, 2016
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  11. ChapmanF

    ChapmanF Senior Member

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    You can get some of what you're looking for with a ScanGauge, but I like the sensor idea. I don't know how good I'd be at estimating my brake temperature just by how often I applied them, but little non-contact thermometers in the brake backing plates would be pretty nifty, and could also tell you about dragging brakes on the flat and other things you might not expect. I can't imagine that would cost too much in this day and age, with Walmart selling $20 non-contact temp guns. I guess ruggedizing the sensor and correcting for the wide range of ambient temps might be a challenge.

    -Chap
     
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  12. Mendel Leisk

    Mendel Leisk Patron saint of newly poured sidewalks

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    Around here, 99% of the time, I put it in D and fugedaboutit. The only exception is coming a down a local ski mountain, which my wife and kids cajole me into going up at least once yearly. There I use B, and switch back to D on any flatish portions of the descent. It sounds ominous, more so as speed climbs, and the more you use the brakes. I try to keep the speed down.

    By the bottom state of charge is pretty much full to the gills, but I've yet to sense it switched to friction brakes only. So yeah, save B for rare occasions like that.
     
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  13. drysider

    drysider Active Member

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    I have a ScanGauge and you can tell when the hydraulics are braking by watching the battery amperage. Even light braking will be recharging at 20 amps or so. When the battery gets full and the hydraulics take over, it goes to low single digits.
     
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  14. kithmo

    kithmo Couch Potato

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    How to tell if the friction brakes are off, drive over a manhole cover, the sucker will let go of the electric brake on one wheel for a brief moment, scary at first until you get used to it. IIRC it's something to do with the ABS only working on the friction brakes and the differential.
     
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  15. bhtooefr

    bhtooefr Senior Member

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    Technically, it's correct that it's only letting go on one wheel, but that's because the Prius has an open differential, and therefore all torque is delivered through one wheel - the one that's easiest to turn (either for the motor and engine to turn when propelling the vehicle, or for kinetic energy to turn when braking). This usually means that a loss of traction on one wheel would result in a loss of all regenerative braking, so it'll switch to friction brakes. And, it can control the left to right distribution of braking with friction brakes, it can't with only MG2 (although friction brakes can be used to make the differential respond in the desired way).
     
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  16. mark27lim

    mark27lim Active Member

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    i think u can tell when the friction brakes are being used. look at the regen meter. when you brake it will start to fill out backwards. when it maxes out backwards, any furthur braking pedal press from that point means the car will start using the friction brakes. if your battery is already full, i reckon just minimal touch on the brake pedal will max out the regen meter immediately?


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  17. ChapmanF

    ChapmanF Senior Member

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    As you noticed, there's guesswork involved in interpreting the dash driving gauge. It's not a well-defined data display so much as a kind of rough guide to the driver. The amount by which it 'fills' backward, AIUI, represents the rate of regen, not the battery capacity.

    But with a ScanGauge, or an OBD app on a phone, you can independently watch the regen torque and power, and also perhaps the amount of friction braking applied, separately. (That latter is an XGAUGE shown as 'FRI' on the Gen 3 XGAUGE list, I've never tried it myself, and don't know if it also exists for gen 4).

    -Chap
     
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  18. bhtooefr

    bhtooefr Senior Member

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    You can get friction and/or engine braking with the CHG region barely filled, depending on circumstances - consider it a guide for how hard you're braking roughly, not how much power is actually being regenerated and going to the battery.
     
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  19. alanclarkeau

    alanclarkeau Senior Member

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    In 7000km, I haven't used B Mode at all.
     
  20. alanclarkeau

    alanclarkeau Senior Member

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    Yes - my understanding is that if this blue area -> upload_2016-11-27_14-45-16.png isn't completely full (assuming the battery isn't FULL), then it is ONLY using regenerative braking. As you've indicated earlier, when the CHG area gets filled with blue, you know that it is regenerating and possibly using friction brakes. It would be handy to have a little RED light at the bottom of the CHG zone when the friction brakes are applied. Maybe the 2021 version.
     
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