Engine breaking...

Not really what is it you want to do with it unless you're on steep serious hills or something like that descending I believe that's what that's all about so maybe you live in those kind of conditions I don't have any of that here I have to drive an hour or so to even get to the hills or mountains If you're driving your car any kind of way it should be charging and discharging and all of that pretty normally just very well But yeah you get up in the hills you won't have any problems with charging You come down to Big Hill and you'll be slam full of juice to the 80% that the car allows I guess

 

It does two things, related but not the same.

First, it increases the total amount of slowing you feel when you lift your foot from the go pedal. That's to make it feel familiar, like downshifting to a lower gear in an older car. Of course in a Prius that amount of slowing is just a number programmed into the HV control ECU saying how much "like a normal car slowing feel" it should give you, and shifting from D to B swaps the usual number for a bit bigger number.

Second, it makes a slight change to the car's strategy for how it produces that car slowing feel. The car is always able to use regen (saving the recovered energy in the battery) and engine braking (twirling the recovered energy away in the engine), whether you are in D or B. In D, the car has a strong preference for regen, and won't start switching to engine twirling until the battery is close to 80% full. At 80%, it will have quit using regen to charge the battery, and finished transitioning over to engine twirling.

When you shift to B, that preference just gets changed. The car is still able to use both regen charging and engine twirling, but in B it will start to bring in engine twirling, and taper off the regen charging, earlier than it does in D.

The whole point of that is to give you a tool you can use to reduce stress on the battery when you're on what you know is a long downhill. A long-enough downhill is going to fill the battery to 80% and end up switching to engine-twirling anyway, so you may as well shift to B if you know that's the terrain. That way, more engine twirling happens earlier, the battery charging is slower, the battery is less heated and stressed, and still ends up well charged by the bottom of the hill anyway.

That's really the only purpose the B mode is good for. On shorter downhills, or just around-town driving, you may as well stay in D and let regen charging recover as much energy as you can; losing it with early engine-twirling would just be a waste.

 

The energy display on the Gen 2 isn't all that great at telling the driver how much regen the car is making at a given moment, and it definitely does not display in any way shape or form when it has gone over to spinning the motor or dragging pads on spinning metal. You either get arrows going towards the battery or you don't. Maybe those change speed or something with more regen, but I don't see it, and certainly cannot be staring at the display for long periods to count how fast they are going by. Retrospectively the display shows little cars inside the MPG bars in the efficiency display, which are not useful at all in the moment for modulating the brake pedal. The 2003 Honda Civic Hybrid we used to own was not as reliable or efficient as the Prius, but it was much better at displaying regen and charging information, with a display showing instantaneously how much was going in or out of the battery.

 

when I was diagnosing the P0011 code on my Prius, it requires me to drive using "B" for at least 10 minutes. I assume this makes the engine run most of the time because in "B" mode the regen braking slows down the car quicker instead of coasting (in "D") a bit further. It just makes you press the gas often after braking. That's why it's mentioned in the manual that "B" mode is not fuel efficient.