B under the D in the Drive gears?

The B is "regenerative braking mode" meaning when the car costs and it also engages a mild charge without having to apply the brake pads. I'm a PC noob, so please elaborate if somebody else knows more than I.

 

Low gear, used to slow the car down in a steep decline in the road. It also charges the battery.

 

Stands for Braking mode and operates in the same way as it does on other Prii so there's a fair bit out there both on PriusChat and beyond. I find this one quite extensive:

http://techno-fandom.org/~hobbit/cars/b-mode.html

As far as I know the manual only has a one line mention of "Position for engine braking".

The vast majority of the time you shouldn't use it. Most people generally summarise it as best used on long descents where you know you are going to fill your HV battery charge as far as it will go (with 8 of 8 bars showing on the display). It's technically not full at this point but the Prius won't charge it any higher.

It mimics low gear on a regular car - but read the link above for a fuller description of what's going on.

Edited to add: I must say the one thought I haven't found a clear answer for is the situation where you don't think the battery will fill but you are on a descent that is sufficiently steep that you need to brake so that the blue indication on the HSI on the ECO Score display completely fills up... and then some. At this point, in D, you will be using your friction brakes as well as the regen braking. I'm not sure if it is best to switch to B mode in this instance or not.

 

The Prius will use regenerative braking mode anyway in D mode - even when you take your foot off the accelerator it will apply a bit to mimic the feel of a normal car. In B mode it will bump up this default no-pedal input level of regen braking, but it will also do more to throw away energy in addition to the regen system, to reduce the need to throw away energy through your friction brakes, which is more of an issue on longer downhills.

Normally you don't want to throw away energy, so normally you leave it in D.

 

I think the best use for B is on extremely long downhill runs, In normal driving, even with fairly long hills, you can ignore it and just touch the brakes as needed.

The last time I can recall using it was coming into Whistler from the mountains to the north. It was warranted, but even so I switched back to D after a a minute or so (it worked out), because engine rpm seemed alarmingly high.

B mode increases engine braking (engine rpm goes up), uses the engine to slow you down, and reduces charging amount. I believe that's actually the object: to avoid overcharging, or delay fully charged state. The latter would likely mean the car would switch over to using the physical brakes more, increase the possibility of overheating brakes.

Again, it's only needed for very long, sustained downgrades.

 

Let’s start with your Prius brakes and get to B mode as we happen to.

Motor/Generator2 is directly linked to the front wheels, if it gets power, it can drive the front wheels, if it absorbs power, it can slow the front wheels. It has a fairly low limit of how much rotational power it can regenerate back to electricity, but when you are gently braking above 7 MPH, it stores at least some of the power you would waste as heat in a normal car as battery charge. (This is why your HV Battery resists being full, you want room to regenerate electricity from braking)

Given long enough continuous gentle braking, your HV Battery will fill to the limit the computers feel is wise, so they begin using the engine as an air pump. No fuel is used, but mechanical friction and compressing air absorb power from the front wheels so you are not relying solely on the friction brakes. This keeps your friction brakes cooler, but does not save power for later use. (You will notice the counter intuitive 'engine gets louder and busier the harder I brake' effect)

You have friction brakes, Discs in front and either discs or drums in back, that are always used below 7 MPH and used above 7 MPH when you need more deceleration than Motor/Generator2 can provide. In the case of a panic stop they completely replace the above brakes to reduce changing brake feel. (Sadly, in some 2010 Prius, normal braking changing to panic stops causes a noticeable change in brake feel)

Friction brakes convert the rotation of the wheels to heat, but two effects can cause brake failure: Very rarely, you can get the rotor so hot it can absorb no more heat and your brakes fail. Mostly a racing phenomenon, larger, better ventilated, rotors and drums can help

More commonly, your brakes get so hot that the brake fluid boils, and so cannot press on the brake pads until it cools. Toyota does all they can to keep your friction brakes cool, and I have heard no tales of them overheating.

Just for completeness, you have a parking brake that pulls a mechanical cable to brake using the friction brakes at the back wheels. In theory, it is never used above 0 MPH, but if all your other brakes fail, it is worth a try.

Now we can see what B mode does. It adds engine braking even before the HV Battery is ‘full’.

This can be handy when you know the road ahead will completely fill the HV Battery and shut down regenerative braking. By putting your car in B at the beginning of long gentle braking, and throwing away energy early, you keep the friction brakes cooler farther down the hill. The computers will not do this as they have no idea how long braking is needed ahead.

So why did Toyota give you a B mode? It is a means for you the driver to keep your friction brakes cool. You use it any time you would use L in a conventional automatic transmission. (Someone at Toyota was too honest to label it L, as it does not use lower gearing) It is also required by law in the US.

The various Prius models have different HV Batteries, but in general I would be in B on any hill with more than a 600 foot drop. The Prius PHV has a much larger battery, so can you drop 2000 feet and still not fill the HV Battery.

 

I have a related question, can you use it at any speed or should you be at low speed like downshifting to low? Would you cause damage at higher speeds?

 

You may use it at any speed, there is no downshifting. At least in my Gen 2 you cannot use Cruise Control with B.

(If there WERE speeds that would damage the car, Toyota would have locked them out. P and R will not work above about 5 MPH as an example, they go to N)

 

B can be engaged at any speed. It does not cause damage.

 

I noticed that the way the selector is set up, passing thru N to return to D from B is the only path. In another thread here I watched a video, over an hour long, about how to drive the Prius to maximize fuel savings and he warned against shifting into N at any speeds above 62 mph or keeping the car in N above that speed could cause damage. He didn't mention how long it would take for that damage to occur but it does make me nervous about shifting thru N to return to D when my speed is above 62 mph. Are my fears just new car nervous phobia? Don't want to hurt my Prius out of ignorance.

 

The only 'warning' I know of about N, is not to speed up from under 42 MPH to over 42 MPH in N.*

If it was unsafe to shift to B, Toyota would not let you shift to B, they pay for any warranty work.

* It is unwise to be in N at any speed including 0, for long periods of time just because the Prius never generates electricity in N and you can run down your battery. It should warn you before this is unsafe.

 

I think we may be getting confused here. This thread is about the Prius c with the floor mounted gear selector with the traditional linear P - R - N - D - B travel. From B there is only one place you can shift to and that's D.

I'm not familiar with the liftback's joystick style selector, but I would find it hard to imagine you can't directly select D while in B.

 

That is the arrangement in the liftback like mine.

 

Like this.

 

That diagram should not imply that you have to go through N to get to B, only that the dhift knob returns to a fixed location when you aren't moving it.

 

I have an '06, gen 2, and it's been my understanding with that mode, "B" is for slowing the car down long down hill stretches, using only engine compression to slow the car. It has nothing to do with regenerating the batteries - hence no danger of over charging etc. That could be different with later models, but Toyota has built in many safe guards in to the hybrid system that I would doubt there would be any use that would endanger that system.

That's what I understand about your question.

 

In my liftback, one must press the handle toward N and hold a second or two to get it into N. It seems to me to go directly from B to D and D to B with normal shifting.

 

It actually doesn't. It shows you pass thru N when returning to D from a previous B selection.

 

I think you are right. The car is smart enough to not move into N, if you don't pause there long enough, while returning to D from B.