Pulsing brakes at the bottom of a long steep hill

Are you using B mode for the descent?

The battery won't overcharge, the computers make sure of that.

Never coast down steep or long hills, keep it in gear. When the battery is full, some energy is diverted to spinning the engine faster, as normal engine compression braking. D mode does only a limited amount of this. B mode starts doing engine braking immediately and can go full bore. (It will get loud, but it won't overspeed or be damaged.)

While this does delay battery filling, it does so by heating up the brakes. This is the wrong way to go down long or steep hills, because it can overheat the brakes, causing failure. Or warp them, causing brake pulsing.

Use B mode to scrub most of your downhill energy, then the brakes to handle any excess. This will prevent them from overheating.

 

The only normal operating pulsing I understand happens when ABS kicks in, as in wheel slip. (It sounds like that as you mention allowing the car to accelerate and then hitting the brakes)...

moto g(7) power ?

 

Is this to Cypress Mountain? If so, @Mendel Leisk is quite familiar with that hill, and D mode doesn't sound appropriate from what I remember of his descriptions.

 

More familiar with Seymour, but for either yes,,that’s a time to use B. When it flattens out enough that I need to use gas I’ll shift back to D, but mostly it’s unrelenting steep, and I keep it in B. B reduces regen btw.

We don’t go up those hills regularly, in winter I can’t take the cold, and don’t ski, so it’s just to sightsee. And I believe it’s tough on the car.

 

I haven't actually been to Seymour, Cypress, or Grouse, so am not keeping them straight. A visit to each would greatly help my mental confusion among them.

 

I’ve never seen regen quit, even coming down Seymour. It does get really full, seems to run EV even past the median (as displayed on Hybrid System Indicator), like it’s trying to get charge down more aggressively, once we’re down, driving on the flat.

 

This is exactly the situation B mode is intended for. The revving won't hurt the engine. It's just acting as an air pump to slow the car. You have a good idea in keeping it in EV on the flat at the top to make room in the battery, but once you start down that steep slope, just put it in B and let the car do its thing while you modulate the speed with the brake or accelerator. Don't forget to put it back in D at the bottom.

 

I would verify the rotors are not warped.

 

You can DIY the check. Dial indicator with magnetic base (check for trueness), and micrometer (check thickness) are what’s needed. Both items about $50. I can post repair manual info in a bit, if there’s interest.

 

USE B. That is what it is MADE for.
OR set the cruise for a reasonable speed and let it try to maintain it.
The result will be about the same.

The pulsating might indicate that your rotors have been overheated in the past and they are a bit warped.
Might.

More likely it is as you suspect and the system is switching from regen to friction and back.
BUT......if the engine never speeds up much, you are using too much friction brake.

 

Repair Manual excerpt attached with brake rotor specs and inspection instruction. Note, this is for regular 3rd gen Prius, but maybe "close enough for government work". I think the spec's would be very close, or the same. I believe rear brakes are a little different on the v, have an integrated drum brake (for parking brake). Sadly there doesn't seem to be a PDF version of the v (or c) RM.

FWIW: I think way too many rotors are needlessly replaced, and/or remachined. Mechanics can't be bothered to check them, and customers are impressed by shiny/new.

 

I have skied there, both downhill and nordic, but that was a very long time ago.

No, the computer will never apply the friction brakes. Only the driver can do that.

Are they warping from excessive heat part way down, then at least partially un-warping when cooling down later?

Put the car in B mode at the very beginning of the descent, before much or any battery charge has accumulated. The presence of brake problems down the hill is a very strong indicator that D mode is the wrong mode. N is even worse.

Mt. Washington's base elevation is 3558 feet. Courtenay is at sea level. That elevation change is more than enough to get very hot brakes.

Cruise control is not a good idea on windy twisty mountain roads, or on winter roads. And the results won't be exactly the same, B mode will spin up engine braking earlier, well before the battery is full. Cruise control will tend to fill up the battery first, before engaging substantial engine braking.

If the road isn't straight and bare and dry, then stick with B mode.

 

I sometimes start out on windy twisty roads by setting the cruise control at its minimum allowed speed (25 mph), and then using my right foot to drive the car. It will eagerly slow the car when backing off the pedal, sort of like B mode. It does use the battery harder than B mode, so I ought to remember to just shift to B when a long descent is coming up, but I don't always.

 

It's a balance between regen and friction, calculated by computer, constantly recalculated during your use of the brakes, and a more complicated dance than most people guess.

This illustration is from the 2001 New Car Features manual, introducing the first generation of Prius:



Starting with Gen 2, they were able to eliminate the little sliver of hydraulic running down the lightest-braking edge of the graph:



That illustration comes from the 2nd gen NCF manual.

If you are looking for a deeper level of detail than "it is computed moment to moment, it depends on a lot of things, charge is one of them, regen is limited both at higher and lower speeds and most prominent in between, and the system will (try to) seamlessly blend between regen and friction at the transitions", then the best place to look is in the NCF manuals themselves.

 

Chapman has provided a lot of core detail. I'll try to fill in a bit more around the sides.

This depends a lot on the driver and style. Regeneration is limited by the maximum battery charging rate, which is sometimes reduced for temperature or other reasons. It is easy to exceed this limit, and many people's natural braking style easily and commonly goes higher. For many people, it requires some training or practice to stay under the limit when practical, which also requires starting braking sooner.

A regular Prius has a smaller battery than any plug-in car, thus a smaller regeneration braking limit too. A Prime can do a bit better, a Tesla can do a lot better.

It doesn't reduce total braking, just transitions to friction-only to make up for the loss of regeneration. Though the transition is not always seamless, sometimes I feel it.

This descent vastly exceeds the battery charge capacity, so a large amount of energy must to thrown away somewhere. Assuming the distance is too short and steep for air drag and wheel rolling resistance to take much, then the only remaining choices are brake heating, and engine compression braking.

You have been mostly avoiding engine compression, so that leaves mostly brake heating. Over short time periods, it doesn't matter much if one rides the brake gently or pulses the brakes harder with gaps in between, the total heat is the same. Aside for stopping at pullouts to give the brakes time too cool, the only practical alternative is send some of that energy into the engine instead. Wind it up to high RPM as a vacuum pump or air compressor, heating up some air getting pumped out through the exhaust system. It won't be anywhere near as warm as regular fuel-burning engine exhaust.