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Discussion Starter · #1 ·
Is it Ok to stay in "B" after the Batteries are fully charged ?? Typical would be on long downhill grade.
The additional noise is why I ask.
Thanx,
Carl
 

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I don't believe "B" has anything to do with charging the battery. It uses the ICE as an engine brake, similar to downshifting in a conventional car. Applying the brakes however, does charge the battery, but you can't overcharge it. I very rarely use "B".
 

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I think the original poster is confused because the same conditions that might cause you to use "B" are the same conditions that tend to lead to very high state of battery charge which is what leads to the noise. The noise is the car NOT charging the battery any more.

Someone who recalls this better than I, please correct me, but I think you actually charge the battery LESS efficiently in "B". So I'd say, yes, go on with it!
 

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There is some additional noise in B position in comparison with D because of the engine spinning to provide engine braking complementing electric motor braking.

There is some additional noise in B with "full" battery in comparison with B with charging battery because the engine is spinning faster to compensate for the loss of electric motor braking. It's perfectly normal and corresponds to what would happen on a priustoric car with manual transmission.

Staying in B on a long downhill grade after battery reaching "full" prevents brakes overheating, so I would say it's safer than coming back to D.

And yes, charging is less efficient in B than in D. :)
 

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Depending on grade, it can sound like a jet landing. If the battery is full, the computers will get rid of the excess charge
 

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Tideland Prius said:
Depending on grade, it can sound like a jet landing. If the battery is full, the computers will get rid of the excess charge
Ummm,
You've never actually heard a jet landing, have you?
 

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Question about "B"......


I live in Florida (no hill or steep grades), should I use "B" to help break when just coming to a red light? Or is this putting un-due stress on the car?

-dexter
 

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My rule of thumb is that if I am tempted to ride the brakes on an extended downhill, I go to "B". I can't imagine ever using it in FL. Coming down from Idylwild this weekend, I didn't wind up using it either and that's a pretty steep decline. On the Grapevine, I used it.
 

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hyperion said:
If you read your manual that explains "B" you probably will never have use for it on Florida roads. (Page 137)
Not even comming down from the top of Mount Dora ???
 

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hyperion said:
Is Mount Dora a cool place to visit in the heat of a Florida summer?
Never been there, but occasionally I see the inevitable bumber sticker... "I climbed Mount Dora" It's kind of refreshing to know the town doesn't take itself seriously.
 

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frenchie said:
There is some additional noise in B position in comparison with D because of the engine spinning to provide engine braking complementing electric motor braking.

There is some additional noise in B with "full" battery in comparison with B with charging battery because the engine is spinning faster to compensate for the loss of electric motor braking. It's perfectly normal and corresponds to what would happen on a priustoric car with manual transmission."


This makes sense, except... I have questioned my Toyota dealer about this myself as I go over a mountain in my commute. By the time I get about 3/4 the way down in "B", the engine starts reving higher. I don't want to overrev so at that point I put it in "D". The Toyota person said that he had another Prius owner express this concern and said the manufacturer said the reving is to prevent overcharging of the battery. He thought it was best to go into "D" when this occurs.

Bottom line, I would not want to leave it in "B" when it begins to rev/make noise.
 

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What do you mean by overrev? The engine will not ever spin faster than 5000 RPM (give or take a few). That's slow compared to most modern engines.
 

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RSnyder said:
What do you mean by overrev? The engine will not ever spin faster than 5000 RPM (give or take a few). That's slow compared to most modern engines.
The engine suddenly starts to make noise like it's reving high. It could be only 3,000-3,500 RPM (can't tell since no tachometer). It never sounds like that in normal operation. It's alarming to me even though it might not actually be a damaging situation. And the head of Toyota service recommends going to D, so that's what I do, even though I still have more of a descent.
 

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Well, brake pads are cheaper than engines. But watch out for the extreme case. Crashing due to brake fade is most expensive.
 

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Someone asked "is it okay to stay in B mode even after the battery is full?"

I would note that while descending a hill, it is only AFTER the battery is full that you actually need B mode at all, in order to save your brakes. Until then, you're on regen braking, so using B mode is not saving your brake pads.

The reason the engine speeds up after the battery is full is that there is no more room to store the energy, and it has to be thrown away (by using the engine as an air pump).


Someone else asked about using B routinely to coast to a stop.

Bad idea, unless you are careful not to engage B until you are under 18 MPH. Above that speed, it will use at least some engine braking, which wastes energy. Below 18 it will use all regen braking, but then so will your brake pedal so, best case, you're gaining nothing.

Generally speaking, using B mode for any other purpose than steep downgrades is a bad idea. You should only consider B mode if you have NO other choice but to step on the brake AND the battery full, so that regen braking is impossible.

If that scenario never happens to you, just forget B is even there.
 
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