B Lever on Dashboard

Yes, it is an engine brake. Use it only when the battery is FULLY green.

 

Why? I have used it from purple to green and have seen not much difference at all, the engine brakes work the same on a full battery as on a depleted battery. My Prius has over 12,000 miles in two months and has gone over lots of mountains, I use the B mode a lot. The only thing is the battery will charge a lot slower on B than on D. Please explain.

 

WHAT!
Use the engine "B"rake to help stop your car from going too fast on a downhill . . . regardless of if the battery is fully green or showing only one red bar.

 

I'm know there are PC members more qualified than I am to answer your question. But I'd like to take a stab at it, if only to find out if I understand it too. The Prius uses three methods or combinations of, to accomplish braking, all of which are supposedly transparent to the driver when using the brakes. Two are the conventional brake pads and engine braking that any non-hybrid uses. The third is regenerative braking. This slows the car by using the motor/generators to recharge the batteries, thus slowing the car. The Prius' electronic control units manage all this for the most efficient combination of braking, charging, slowing down, etc, including turning off the engine. However, if you ever drove a car with overdrive, you remember that, in hilly terrain, there was a tendency to free wheel or speed up going downhill, especially with cruise control. If you turned off overdrive or shifted to a lower gear, you did not have to use the brakes as hard or as often. You're using the engine to actually help slow the car down. This is what "B" mode is doing. It's forcing the internal combustion engine(ICE) to be used to help slow the car down which may reduce the need for using the brakes. Another way to look at it is, you don't have drive1 or drive2 or overdrive(on/off), so it's the Prius' answer to using a lower gear. Not for power but for breaking. And, since the ICE is used, regenerative braking isn't helping to charge your batteries as much, ergo, longer time to recharge the batteries in "B" mode.
How'd I do?

 

Sure, you explained why the battery charges slower in B mode than in D mode, something I understood already. I'm still wondering why Bob64 says to use B mode only with a full battery, as if it would somehow damage the car if used on D drive. The only downside to using B mode with a depleted battery is the slower charging times, but depending on the downhill run it might not matter at all.

 

i have heard other people tell me that they use the "b" mode while in traffic and their ac is on, so the ICE will not shut off. I PERSONALLY have not tried this as i live in a a smallish city with less than 5 min of stop and go traffic.

 

This question comes up a lot on this forum. Some of the information in this thread is correct, some not. Here is the real story:

B mode stands for brake mode. As other posters have suggested, it is a form of engine braking, similar to a jake brake on a truck or down shifting a manual transmission. The Prius uses regenerative braking, so this brings up the question of when and why you would you want to use engine braking. The answer lies with the limits of regenerative braking. The regenerative system is limited in how much braking force it can provide. It can do a lot, but there is a limit. The regenerative system is also limited by battery capacity. Once the battery is charged to its high limit, the regenerative system has no place to put the regerated energy, so it has to stop braking.

When the regenerative system maxes out, the Prius begains to use nomal friction brakes. This is the key to B mode. There is almost never a reason to use B mode unless the regenerative system is going to max out on a descent, either by having the hill be too steep, too long, or both. On a mountain descent where you know the regerative system will max out, you can switch into B mode and force the Prius to discard energy, thereby delaying the onset of friction braking. The less you use friction brakes, the longer they will last, and the less chance of brake fade.

There is no need to wait until the battery is fully green before switching to B mode. Ideally you want to time the switch to B mode so that the battery SOC reaches the high limit at the exact instant you get to the bottom of the hill. Switch sooner and you have wasted a small amount of energy that could have been put in the battery. Switch later and you put more wear on the friction brakes.

Unfortunately there is no exact way to know when to use B mode. The size and angle of the hill, and the battery's current state of charge (SOC) will determine when B mode should be used, so only experience will allow you to use it optimally. The good news is that you really don't have to worry about it. If you live in a flat state, then you will never use B mode. If you live in a mountainous state you will need to do some experimenting. If your battery's SOC becomes all green before you get to the bottom of the hill, B mode would have been a good idea. Once the SOC reaches its high limit, the Prius will automatically use engine braking, so to gain anything with B mode you need to use it in an anticipatory fashion.

A couple of other thoughts about B mode and battery SOC: Don't forget to shift back into D, otherwise your mileage is going to suffer. Also, don't be surprised if your engine turns on and off at the bottom of a big hill. The first time you hear this you are going to assume that something is wrong. If your battery SOC gets too high, the Prius will burn off excess battery charge by spinning the engine without burning fuel. You usually experience this at a stop at the bottom of a big hill. You are sitting there and the engine races: "vroooom", then stops, then races again "vroooom", then stops. The cycle continues until the Prius is happy with its SOC.

Tom

 

This is not correct. First, it won't prevent the ICE from shutting off, and second, why would you not want the ICE to shut off? The air conditioning in a Prius is driven by an electric motor - it works just fine when the ICE is off.

Tom

 

i guess they are paranoid that itll cycle the battery too much/ dont knwo what they are talking about

 

Thanks qbee42

 

If your foot is not on the brake pedal, the battery is not warm, and the state of charge is not too high, then in b mode you are slowing down quite a bit quicker. And that is a combination of extra engine drag and of higher regenerative current. On my favorite hill, which I descend at about 50 mph with a battery that is cool because of recent startup, the b mode regenerative current is about 30 amperes compared to about 20 amperes for d mode.

If you are on a long downhill which in your experience or judgment is going to lead to a full state of charge at the bottom in either case, then using b mode to get more of the drag from the engine and less of a drag from the combination of regenerative and friction brakes spares your friction brakes and your electrical system without actually wasting any potentially available regenerative capture energy at the end.

The entire range of state of charge used in the Prius is only equivalent to a surprisingly small altitude change when reckoned as the potential energy of the automobile. So hills that are much less impressive than coming down from an Alpine mountain pass meet this condition. Almost any hill that requires steady application of the foot brake and has a total descent of more than 800 feet or so of altitude is quite possibly a candidate.

If there is lots of downhill still to go and you are steadily using the foot brake, there is no virtue in waiting to start use of B mode until all green shows.

Regeneration is limited, even with a cool battery in moderate state of charge, to something like 90 to 100 Amperes. Using B mode on steeper hills you descend at a fair rate of speed can spare the friction brakes some wear--the same maximum regeneration rate is still available. I do not need to do this on a 4% grade at 50 mph, but suspect that a 6% grade at the same speed with even a few hundred feet of total descent would be a reasonable place for this option. A Scangauge monitoring traction battery current is very helpful in judging this. While at 25 mph 90 Amperes is pretty strong slowing which is easy to stay under, at 50 mph on a downhill it is really easy to get past it into friction braking if you slow for traffic conflict or a stop.

 

Actually... I have read somewhere that this might be true. If the engine is running and your in B, it won't turn off. I'll have to test this out today on my commute to see if it is true or not. I'll post back with the results of my test.

 

Thanks archae86:
There's a numer of stretches of several miles each on my wifes drive to work, best route for time, distance and traffic. Speed limit is 35mph, well enforced, mostly residentiaal, up and down multiple fairly long hills, probably at least 6% grades, several hundred yards, some stop and go traffic. We shift into 2nd in our 2000 Avalon XLS, sometimes even 1st in downhill traffic, to avoid riding the brakes and having the tranny continously shifting up and down, especially trying to avoid those annoying times when you go to give it a little gas just as it decides it needs to shift. I figured these stretches would be ideal to experiment with "B" mode.

 

I understand the comparison to jake brakes. I have considered the B mode more like downshifting on a grade to increase engine compression. Both allow a safer descent and less reliance on physical brakes. The result, of course, appears to be the same. But you raise a question in my mind about the use of friction brakes in normal driving, not on grades. When I approach a red light or stop sign and apply the brake pedal, am I actuating the physical disks/drums or is a form of regenerative braking/B mode being used to slow the forward motion, as a sort of stop-by-wire?

 

To compare: B mode spins the engine in addition to regular "D" Mode, which wastes energy, but also increases regeneration by a bit... Keep in mind that you can achieve the exact same regeneration "increase" by pressingly lightly on the brakes in "D" mode.

Regular "D" mode avoids engine spin, and regenerates as much as it can until it hits below 7mph, which it WILL transition to friction brakes.

 

Ah, I remember where I read it from now:
http://www.techno-fandom.org/~hobbit/cars/b-mode.html

 

Yes. . . ., and it depends.

In a panic stop where you suddenly slam on the brake pedal, the friction brakes are immediately applied, and nearly all the braking is done by generating heat via friction with the pads on the disks/drums. There is still a bit of regen going on at the same time, but not much.

In a gentle stop on the older model Prius, there is a gradual change with most of the braking done regenerative at higher speeds, and the pads being applied more firmly as the vehicle slows down. It isn't a linear transition, I've seen a graph of it around here somewhere and there is a definite curve to the percent friction/regen as a function of speed.

In the more modern Prius (2004+), from what I've read around here, it is nearly all regen until you get down around 7 MPH at which point there is a sudden transition to almost all disks/drums. During especially gradual braking on an especially smooth road you can feel this transition. Some describe it as feeling like the brakes "release" momentarily, others have described it as feeling like the brakes suddenly grab a bit.

If you press the brake pedal hard enough your braking desires eventually exceed the ability of the regen system. At this point the regen system is assisted with the disks/drums to provide the additional braking force required.

Now in the more modern (2004+) Prius, if you traverse a sizeable bump, hole, or very rough patch of road (such as a railroad crossing) while braking, the Traction Control/Anti-lock brake system may cut off the regen braking and switch over to disks/drums just like in a panic stop.

In the unlikely event of a failure of the braking control system, solenoids that are typically held closed fall open and provide a direct hydraulic path between the brake pedal and the disks/drums to revert to traditional friction braking.

The brake fluid pressure is maintained by an electric pump. In the unlikely event of an accident or other emergency situation whereby the battery is unable to provide the power needed to run the pump to regain brake pressure, there is a bank of capacitors that are maintained at a constant full charge. These the pump can draw power from these capacitors for long enough to bring the vehicle to a stop.

Somewhere around here there was a document posted that presented much of this in a great graphical form. If I find it, I'll post the link (or perhaps someone who knows where it is will post the link before I get to it.

 

please no trash in this forum