I have never used the b in my shifter

It's not commonly needed, but if used properly on mountain roads it will help save your brakes. I never use it here in Michigan, but I have used B mode a fair amount on climbing trips out west.

Whether you know it or not, your Prius will automatically switch to a form of B mode when needed. The reason for having it on the shifter is to allow the driver to shift in advance if he knows it will be needed, thereby saving a little wear on the friction brakes. Since the mode is already built into the control system, it costs almost nothing to include it on the shifter.

Tom

 

The B is a mystery for some. I know a guy who was told that it meant "Battery mode". He was told so by a salesman...

 

I used it last week on a short road trip over some small mountains. The descents weren't huge, but still large enough to push it to 8 green bars even in "B" mode. Switching to "B" mode just delays the onset of full friction braking, in this case from about 1/2 to 2/3 way down the mountain in "D" mode to almost right at the bottom in "B" mode.

Yes totally optional, but in my opinion worthwhile doing on large descents. You just have to know the terrain ahead and remember to do it.

BTW. Came off one of the mountain descents and onto a long flat at 8 green bars, popped it back into "D" and managed to keep it going at 85 to 90 km/h for about 3km in full EV mode. That's the longest flatland EV glide ever for me.

 

You must be a straight A student...

 

me neither, no mountains round here. but then, i've never used any of the low gears in any of the automatics i've had over the years.

 

You have a very unique shifter layout...

 

Use it everyday. Lots of hills leaving the house, so a way to save some brake pad. It's amazing how long it takes the Prius wheels to get dirty from brake dust. The stories of huge mile intervals before the first brake job must be true.

 

I live in the Netherlands. And it is called that for a reason.
Biggest hill we have is the dyke around my village. Our countries highest hill is 200 miles away at a whopping 1059ft.

 

I wonder if GIII drivers tend to use B mode more than GII drivers due to its more traditional shift/throw location.

 

Never use "B" in Edmonton, but use it every time I go back to B.C. It's wonderful on mountain descents. The Prius has so little drive line drag and aero drag it takes off down mountain passes. "B" helps a lot to reduce that. I could let it go but if caught doing 130+ km/hr in an 80 zone it's kind of expensive. I know just from the books. Hasn't happened to me yet.

 

Living on the Manitoba praries, I had never needed the B either; until my recent ski trip to Montana & British Columbia. Coming down some of those "hills" my battery filled up very quickly and I then shifted between D and B as necessary rather than use my friction brakes.

If my cruise was on and my speed exceeded the set speed while going downhill, Prius was smart enough to first use regen to slow my speed and when the battery was full, automatically use engine braking. Saw this happen several times. However, if cruise was not on, had to switch to B manually to get engine braking.

Learned the hard way a number of years back to use engine braking whenever possible. Relying only on brakes, I smoked a set (and I mean literally had dense smoke rolling out of them) coming down out of a pass on my way to Yellowstone. God knows how close they might have been to complete failure...

 

True, if you don't need to use B mode. But not if you routinely drive mountain roads where the descent exceeds the battery's capacity to absorb regenerated energy. That's just it: if you don't need it, you don't need it. If you do need it, it helps save the brakes.

There isn't anything magical about this. It isn't hard to understand, and it isn't a philosophical point of discussion. Either you drive these sorts of roads or you don't.

Tom

 

Having smelled a lot of burning brakes on mountain roads, I can testify that lots of people don't get it.

 

I'm confused. The idea of B mode is that it saves your brakes by using friction braking, and yet you also use it to stop it charging the batt on long decents? Isn't it going to be charging on friction braking?

 

I wouldn't define "B" as a gadget.

As others have pointed out, their are legitimate driving conditions inwhich having "B" mode is a real benefit. If you owned a Prius and lived in a mountanious region? I don't think you'd call it a gadget.

Of course if you are in the city, suburbs or seldom drive in the mountains? You aren't going to use it much, that doesn't make it a gadget, it just makes it a tool you don't need to use much given your circumstance.

 

Let me try:
No matter how empty your HV Battery is, you can fill it on a descent that has 600 foot of vertical drop. Now you have no more regenerative braking until you empty the HV battery some.

In a somewhat similar fashion, your friction brakes have a capacity to absorb (and later dissipate) heat, should you exceed that limit you lose your friction brakes. (This would be bad)

Toyota has engine braking that automatically starts when the regenerative braking is full, but you can start it early by choosing B mode at the top of long descents.

By using all 3 braking systems, you reduce the electrical load on regenerative braking and the heat load on the friction brakes, so they do not exceed the braking needed to get down the hill.

Both engine braking and friction braking throw 100% of the energy away, while regenerative braking captures about 30% of the energy in the HV Battery, so you want to use regenerative braking everywhere EXCEPT long descents.

This PDF (aimed at truckers) is about an example hill I would use B mode on from the top. http://www.oregon.gov/ODOT/MCT/docs/EmigrantHill.pdf?ga=t

One anomaly of engine braking, is that the harder you brake, the faster the engine goes, and the less you brake the slower it gets, this may surprise you.

 

You have that wrong, so it's no wonder that you are confused. B mode helps reduce the need for friction braking - just the opposite of what you said.

Friction brakes are the normal disks or drums that we all know from ordinary cars. The Prius has these, but tries not to use them. When possible, the Prius runs MG2 as a generator to slow down, and takes the electricity generated by MG2 and charges the HV battery. This works up to a point, and that point is when the battery hits the high charge limit. At that point the Prius switches over to standard friction brakes like every other car.

B mode uses MG2 as a generator just like standard regenerative braking, but instead of sending all of the charge to the HV battery, it sends some of it to MG1, which is used as a motor to spin the engine. The engine, being a big air pump, dissipates a fair amount of power spinning around and around. This is wasteful, but that's what we want. With the Prius this is called "engine braking" and is similar to downshifting in a normal car or the jake brake on a semi.

Because B mode throws away some of the regenerated power, it can brake for a longer time without charging the HV battery to the high limit. This means we can brake longer without switching to friction braking.

Note that normal regenerative braking also will use engine braking, but only after the HV battery gets close to the high limit. B mode starts when you tell it to.

Tom