Prius B Mode

Important to understand the way regenerative braking works... B-mode uses your gas engine to slow you down rather than the electric motor. So the energy in B-mode is lost energy, whereas energy in braking above 6mph for the most part is stored in your batteries. I never use B-mode and I too have original brake pads even though my car has 1/4 million miles.

While not much use these days, the origins of B-mode was from long ago when cars had less refined braking technology and you would downshift to help slow you down going down steep hills so your brakes wouldn't overheat and fail.

 

The video isn't bad, but there are much better discussions of B mode that can be found right here on PriusChat.

He exaggerates when he says B mode gives you 100% of available regen braking force as soon as you let off the go pedal.

You can see this for yourself if you have a scan tool plugged in to the diagnostic port and watch the regen braking torque. When you let off the go pedal in B mode, you do feel more braking torque than when you do the same thing in D. But it is still less braking torque than when you use the brake pedal even moderately.

The split of how much recovered power gets stored in the battery and how much gets spun off in the engine is not fixed, whether in D or in B. The electrical energy that's recovered by the MG can always be sent to both of those destinations, and the computer gets to pick how much goes where. Shifting to B changes the bias somewhat, so more of the energy gets spun off in the engine and comparatively less is sent to the battery. That's in keeping with the purpose of B mode, to be a little gentler on the battery when you know you're heading down a long hill. But it doesn't always mean everything to the engine and nothing to the battery; it only reaches that split when the battery hits 80% SoC or greater. The same thing will happen in D when the battery reaches that state of charge, and any remaining excess power will be spun out the engine. It's just that, before that point, D will be sending more of the power to the battery, so on a long downhill, you may reach the no-more-for-the-battery-please point sooner in D than in B.

 

Pretty good explanation. Because he was focusing on overuse of B mode, he left out another factor. With a regular Prius, B mode us actually meant to do two things. Not just reduce wear and heat on the hydraulic brakes. The other goal is to prevent filling the traction battery too rapidly on big steep hills. He talked about the motor spinning the engine to use compression braking. The motor gets it energy from the regeneration taking place, which reduces the amount going to the battery. If you're on a long, steep hill and don't put it in B, all that regeneration from braking will got to the battery. Once the battery hits its limit, B mode braking will begin automatically to protect the battery from getting too much charge. I don't think it'll say "B" on the dash, but that's what it's doing.

Bottom line is that on long steep hills, it's a great feature. In day to day driving, it's very counterproductive, as the video says.

 

Another little quibbly point: in a gasoline engine, the energy-dissipating action of the engine is not really "compression" very much, because the throttle is closed so there is very little air going in to be compressed. The engine in B mode is really more of a vacuum pump, and the work of trying to suck air through that closed throttle is where the bulk of the energy is going.

It's a whole different picture in a diesel, where there isn't a throttle, so engine braking in a diesel really does pump a lot of air. But the diesel also needs an extra trick: if it were doing nothing but spinning and pumping air, it wouldn't be able to shed much energy, because the air would just be like a spring: it gets compressed on the compression stroke, and then gives that energy right back to the crankshaft on the power stroke. Even though no fuel is burned and there's no extra power for the power stroke, still the energy of compressing the air comes springing right back to the crankshaft and hasn't been gotten rid of.

So the diesel adds the trick of tweaking the valve timing so right at the top of the compression stroke the air gets released (with a loud pop for each cylinder in turn) rather than staying around to give the energy back ... giving the familiar "Jacobs brake" sound when a big truck uses engine braking.