B Lever on Dashboard

[HeidiHo]

I have ready my Owners Manual and cannot find anything that explains the lever on the dashboard with B on it. I have a feeling it is some sort of brake? Maybe an engine brake? If so, would someone please explain it's use for me.
We are going on a trip to the mountains and if it is an engine brake, it would sure be handy, instead of using the brakes via the pedal.
Thanking you in advance,
HeidiHo
mccord11@windstream.net

[Bob64]

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

[rpiereck]

Originally Posted by Bob64

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.

[Sufferin' Prius Envy]

Originally Posted by Bob64

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

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.

[dwreed3rd]

Originally Posted by rpiereck

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.

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?

[rpiereck]

Originally Posted by dwreed3rd

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.

[redhandeddenial]

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.

[qbee42]

Originally Posted by rpiereck

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.

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

[qbee42]

Originally Posted by redhandeddenial

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 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

[redhandeddenial]

Originally Posted by qbee42

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