Shifting from D to B when slowing down to a stop sign or light?

You won't wear out the transmission, but you'll get worse fuel efficiency for no benefit. B adds some regenerative braking, but the main effect is to increase engine braking, which is just another way to convert motion into heat, like your regular brake pads.
When you are in D and apply the brake pedal, the motor MG2 is used to slow the car down while converting your kinetic energy (forward motion) to electricity to charge the battery. As the car slows down below around 10 mph or if you brake harder than the system is able to handle, it will add in the regular hydraulic brakes, which just throw away that kinetic energy as heat.
When you shift into B, two things happen: The car will increase regenerative braking until the battery is full, but energy will also be sent to the engine, which is hard to turn when it's not actively making power. This slows down the car without recovering any energy, just like the friction brakes. The only reason to use this is when descending a long downhill: Once the battery is full after some downhill driving with the brake pedal applied, the only braking you'll get will be from the friction brakes. Like a conventional car, when all your braking is from friction brakes, you run the risk of overheating the brakes and risking loss of control, as well as simply reducing the lifetime of the brake pads. If you shift into B, or a low gear on a conventional car, you'll be able to control your speed with much less application of the brakes -- making them last longer and reducing the risk of loss of control.

Thus, shifting into B when you wouldn't use engine braking in a conventional car is pointless: You recover less energy than if you were just applying the brake pedal, and you don't gain anything by it -- you can get more regenerative braking (the good kind which recharges the battery) if you just apply the pedal gently.

 

Interesting

 

I agree with sablesm.
You will not wear out the transmission, the extra wear is on the engine.
You should only use B when you are driving down a long hill that would completely charge the HV Battery. (For most Prius, this is about a 600 foot drop, but the PIP can absorb over 2000 foot drop)

 

Hi cablesm, very well explained!

I hope you don't mind too much if I do a little nitpicking:

At low speeds (I don't remember the exact threshold), very light braking, and normal or low SOC, the engine is not braking even in B mode. In this case, the light braking is done in the normal regenerative fashion, with the full normal recharge benefit. However, when coasting with neither accelerator nor brake pedal depressed, there is a noticeably stronger regenerbraking in B mode.

Driving in B mode is sometimes convenient in slow heavy traffic and frequently changing speeds, especially when slightly downhill. Then it often suffices to just lift one's foot off the accelerator pedal, instead of having to press the brake pedal. In these conditions, it is often less tiring to drive in B than to constantly alternate between pressing the brake and accelerator pedals.

I seldom encounter such conditions. But when I do, I am grateful for B mode.
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I have yet to want the drivers behind me to be unaware I am braking, so I use B to avoid speeding up too much, not to slow down.

To each his own.

 

From my experience, I estimate that the Prius slows down coasting in D about as much as a typical automatic transmission car, and when coasting in B mode it slows down about as much as a typical manual transmission car without agressive downshifting. So I think it's still in the range that other drivers are accustomed to without seeing brake lights lighting up.

That said, I do agree that it is safer to have the brake lights come on whenever slowing down.
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Using B mode is not going to hurt anything. I get bored sometimes and like to use B mode when slowing down to make a turn as long is nobody is behind me. The engine drag feels almost like downshifting with a manual tranny.

Other times I use Power mode just to change things up. I know, I know ... not efficient but sometimes I just like to change things up and not feel like driving like a little old lady

 

use when you see a radar gun pointing at you

 

I can't remember the last time I used B. As others have said, it's use is to use engine breaking when the battery's been charged as much as allowable, to take load off the brakes. But: that happens very infrequently, only on a very protracted downgrade, and I've heard even if you do nothing, the car's computer will step in and do some B mode anyways.

For almost all situations, you're not saving your brakes, just reducing charging, and spinning up the engine.

 

I have only used B mode once, going thru Northern Idaho on I-90 over Rocky Mts. on my way to Wisconsin. I do not have any other hills high enough in my area to use it on.

 

I use B often when I need to slow down quickly, say on the freeway and suddenly traffic slows, or any other instance when I need to slow down quickly instead of mashing my brakes. 36k miles on my car so far, my overall mpg is 55 (according to trip meter) and the past few months I'm averaging at least 60 mpg consistently while using B on a regular basis. This effect on mpg is what's really pointless, the hit is very very minimal, minimal enough that it's pretty much incalculable. Either way, brakes rarely need to be replaced on a Prius, and the mpg hit is minimal. Do whatever you prefer, as I do.