'B' Gear / Regen Level

A selection marked B exists both in Prius HEVs and Prius PHEVs, but not with identical behavior.

In the HEV, which has a relatively small battery, it is provided chiefly so you can hint to the car that a long downhill is coming and it should start diverting some energy to engine braking earlier than it normally would, and charge the battery less aggressively.

"Less aggressively" is a comparison to the same slowing rate in D. The same slowing rate isn't exactly what you see if you shift from D to B, because B is also programmed to change the slowing rate (and so to feel the same as downshifting in another car).

So if you are simply coasting in D and you shift to B, you may notice the charge rate increase (along with possibly hearing the start of the telltale engine RPM increase of engine braking).

However, if you compare coasting in B to the same slowing rate in D (achieved by light use of the brake pedal, or when cruise is set for lower than the current speed), you will see that B uses less charging and more engine braking to achieve the same overall slowing.

That's all for the HEV. A PHEV, with its larger battery, has less use for a mode that limits charging. In PHEVs, the owners' manual makes a little pun on B and describes a "Regeneration Boost" feature. That's not in the HEV manual at all.

 

I thank you for your explanation sir. It has become quite clear for me now.

 

haha ok so I read this entire thread and convinced myself I should not really be using B for regular vehicle slowing.

but I own a ‘25 phev and your last sentence implies that in the plug-ins case it IS worth considering to maximize regen? Or can I still achieve basically the same thing managing the charge metre with the brake pedal in D?

Additionally in the car settings i can set a regen level. Is this an phev only option? it’s a bit unclear to me if it applies to B or D or both? The additional drag from adjusting this setting seems to be noticeable only in B, and personally I mostly prefer a more gradual brake over a longer distances anyway (when conditions allow)

 

I think that's a PHEV-only setting (goes with the PHEV-only "Regeneration Boost" feature).

Even in a PHEV, "maximize regen" shouldn't really feel like a goal in itself. Conversion losses are paid when you take energy from the battery to move the car, and paid again when you put energy from slowing the car back into the battery, and paid again when you take that energy back out.

To the best of your ability, the goal should be to drive steadily enough that you're just taking energy out of the battery once and using it to move the car.

When you have to slow, the PHEV will generally capture as much as it can back into the battery regardless of the mode.

 

agreed 100% on the ideal being to coast to a stop when possible, i just live in a big city where its a traffic light every 300m and a lot of angry, impatient drivers. so the expectation is to hop off the mark on a green and then race to the next red as fast as you can. basically the worst driving habits rule the day and if you don't at least SOMEWHAT accommodate it you have road rage to deal with on top of things.

so basically i'm just always looking for strategies to make the best of an imperfect situation!

 

You might try using dynamic range cruise control to slow down. I have started using this and noticed that on the energy flow meter it puts the car into a regenerative brake state. For example when decelerating from a highway speed to a stop if you slowly lower the speed setting down to its minimum of 19 mph the car will slow primarily with regenerative braking as far as I can tell from the energy flow meter. I then switch into to B to slow from 19 mph to zero. I can't quote any change in efficiency numbers but I would imagine it is saving the brake pads and rotors. I figure that since this is how the car decides to do it in dynamic range cruise mode it must not be too stressful on the electric motor which I understand is being run in reverse when shifted into 'B.'

 

Do not use the B mode for regular driving, as it is bad for fuel economy. It tends to use engine braking, which is dissipative.

In terms of regenerative braking, the B mode and regenerative-boost level only modify the brake-pedal and accelerator-pedal modulation. So, you press the brake pedal less or release the accelerator pedal less to achieve a given amount of deceleration. They do not increase the maximum available regenerative power (maximum regenerative deceleration before friction brakes are applied). Therefore, they shouldn’t be used for normal driving. If you really want one-pedal driving at the cost of substantially worse fuel economy, you can use the B mode and regenerative boost.