'B' When slowing at stop lights.

No, the Prius will not 'hurt' itself in normal driving.* B is to help preserve your brakes on long downhills, you are using it as it was meant to be used.

* Continuing to drive once you are out of gas can kill the battery.

 

I think with the advent of 3rd gen, after running out of gas the engine shuts down much more promptly, to avoid hybrid battery damage. Just from reading here.

Also, regarding to B or not to B: with a short downhill (1200* feet would qualify) your best tact is likely to do nothing. In other words, leave it in Drive, drive down the hill braking as needed. If it was a mile (5280 feet?), that'd be borderline. But with a relatively short downhill run, you will maybe just top-out the battery charge. And the brakes will be fine; it'll be mostly regen braking, all the way down. And a bit of regular braking won't break them, it's not Pike's Peak.

* Guess I should have asked, is that 1200 foot road length, or 1200 foot elevation drop. If the latter, yeah, B is warranted.

 

Nope. Every bit of energy from the descent that the system can harvest through the drivetrain is that much energy you don't have to brake off, and heat up your brakes.

Near the top of the descent, almost all of the harvested energy can be sent to the battery and stored. As the battery nears full charge, the system will begin sending more of the energy being harvested over to spin the engine, which just turns it into noise, mostly. Once the battery hits full charge, the system will send all the harvested energy into spinning the engine (with a vengeance), and none to the battery.

Unlike a conventional car, where you might downshift to do engine braking, but it would be up to you to avoid overrevving the engine, the Prius knows a top safe engine speed for braking. Once it is harvesting that much energy from your descent, it just keeps harvesting at that level, not more, so it won't overspeed the engine.

If gravity is giving you energy faster than the engine can spin it off at that level, all it means is that the engine braking won't be enough to hold your speed steady, and the car will be gradually picking up speed (as you saw by shifting to D, but it can happen in B too if the grade is steep enough). You'll have to make some use of the friction brakes to keep the speed controlled. But at least you'll be letting the drivetrain do as much as it can, keeping your friction braking to a minimum.

There was a thread on here somewhere about, on a grade steep enough to need the brakes, is it better to apply them rather steadily, or to let the speed pick up for a bit, then firmly snub it down with the brakes, then let them cool while the car picks some speed up again, then snub again, etc.

There was a variety of opinion, but eventually also a report was linked that had studied the question, and accorded with intuition that snub ... cool ... snub ... cool did preserve more braking ability.

Using B to handle more of the load lets you have longer ... cool ... periods and less snubbing, even if you still need some.

 

Mendel, it's 1,200' verticle, so about 4-5 times that as the prius drives(?); next time i'll try to remember to watch the odo from bottom to peak.

 

Living in the mountains, anecdotally, snub-cool-snub is the way to ward-off brake fade.

 

... and his engine will also be fine.

 

I have understood this snub-and-cool approach, or pulse braking method, to be based on the presence and prevalence of mis-matched brakes: nearly everything in the old days, and still essentially all tow vehicles even today. A continuous light braking down the hills would put a lot of heat into the lightest-touch brakes or axles, and little or nothing into the heavier-touch brake hubs. The braking load and heat is not evenly distributed, so one position overheats and smokes prematurely. Heavier pulsed braking puts them all to work, evening out the load better.

This warning still applies to combination vehicles, as there is not reason to expect the tractor and trailer brakes to be matched. And plenty of not-so-well-maintained trailers out there.

But modern cars with EBD (electronic brake force distribution) should be far better matched. The front brakes are not paired up with a different set of rear brakes from day to day, or ever, so this potential mismatch should be far less of an issue. Even cars of in-between vintage, before EBD, had hydraulic proportioning valves to help balance out braking forces. With these, a continuous light brake touch should be far less of a problem, if any at all.

If every brake is taking its correctly proportioned load, then there should be not real difference between the methods. The total heat to be dissipated is the same, regardless of method.

Snub-and-cool also diminishes the usefullness of regen braking, at least on hills short enough to not fill the battery. But this method was ingrained in my spouse long before I met her, and my attempts to re-train her to make better use of regen braking have been utterly useless.

 

B is OK but probably totally unnecessary to select it manually.

A LIGHT press of the brake pedal should initiate regen braking.....only.
If you turn on the cruise control, it will automatically initiate B mode if needed.

No need to make it more complicated than it IS.

 

When you manually select B, you're giving a hint to the car.

There might be some capacity in the battery to receive charge.

If you are in D and press the brake lightly, the car will send the battery as much juice as it can take. Once it can't take any more, the car (my Gen 3 anyway) will divert the power to engine braking instead.

If you are in B, then you get some additional control of descent speed before you have to touch the brake, and then even when you do press the brake lightly, the car will send a smaller amount of harvested power to the battery. You've essentially told it that there's going to be plenty of descent ahead to fully charge the battery, even without being super opportunistic about it up front. More of the harvest will be sent to the engine to spin off.

You end up at the bottom of the hill with a still-fully-charged but less-heated battery, and a little more engine noise given off, and a bit less use of the brakes.

If the descent won't be that long, then yeah, no particular advantage.

 

Ah, that's a good case for B mode. We take a route sometimes, 1.3 miles with around 500 foot climb, coming back down I use B off and on, depending on grade. The battery still displays full by the time we get to the bottom.

 

Thank you all: this thread helped me understand my new awd Prius better.
Knowing how B vs D works now has helped mileage.

 

Oh I forgot your location: that eastern approach into Port Alberni, very long and steep. We might be passing through on the way to Long Beach this summer, if we can wangle a reservation.

 

Get in touch if you're heading this way, and we can have a prius chat on your way through town