Is there a legitimate use for charge mode in the Prime?

Oh No! Paul, not going to talk to me anymore.

My wife and I've driven 4 Toyota hybrids (and PHEV) for a total of 425,000 miles.

Neither of us use the B mode when going down a long downhill stretch of freeway, not even an extended 6% grade. As pointed out by fuzzy1, you almost never find yourself at the top of a steep grade with a depleted traction battery. By the same token, none of our hybrids have ever needed work on the brake systems.

Bye Bye Paul

 

You're not intended to put it into B mode just because you're going down hill. It's intended to provide some braking when regenerative braking is unavailable, and friction braking is inadvisable.

I've been on some long downslopes in British Columbia, and I'd have destroyed my brakes if I'd kept them applied for the 15 minutes or so it took to descend the slope. As for draining your hybrid battery beforehand, why? Use the B engine brake instead. That's what it's for.

N.B. in a conventional non-hybrid car, the engine provides some natural braking, but when that's insufficient, you downshift to a lower gear. Pumping the brakes for short periods to allow them to cool between applications is also advised.

 

In fifteen years of driving a regular Prius, I regularly used B to avoid friction brake use on tall hills. Several of my regular hills had 2000 foot drops at highway speed, so I'd engage B at the top, knowing the battery would fill regardless. For shorter hills, B was entered later to ensure the battery still filled before the hill ended, maximizing recapture.

On those same hills, I now use the RAV4's B-equivalent for regeneration without engine braking. Though sometimes engine braking still triggers towards the end of the hill, I'm still learning.

But it seems that numerous other drivers do not downshift their traditional gassers, and get away with it. I don't really know the point where it becomes a problem with passenger cars. (Commercial trucks get into trouble very quickly.)

The only car cases I can point to directly, are a vehicle (Florida plates) that descended Pikes Peak not far behind me, which was smoking and stinking to high heaven at the mandatory brake checkpoint and ordered to take a time-out in the cooling-off area; and a Hawaiian member here who followed the faulty advice of a couple other members and misunderstood D as being adequate, only to have to rebuild his overheated brakes twice before figuring out he was reading the wrong 'experts' here, and really needed B. He had a 3000 foot descent, and I suspect at significantly less than mainland highway speed, thus less energy dissipated by air drag.

In a non-plug-in hybrid, B is also intended to mix engine braking with regenerative braking for as long as the later remains available.

In a PHEV, B can also be used to control speed with regeneration alone, for a much greater distance than a regular hybrid can regenerate.

Why? Because, in a PHEV, all the engine braking represents wasted energy. Wasted energy from either gasoline unnecessarily burnt for the previous climb, or unnecessary kWhs drawn from a charging station near the top. Wasted dollars for that wasted fuel. You could have achieved higher MPGs and/or MPGe's by choosing a no-engine-braking strategy.

Use the PHEV's big battery to capture more regeneration. To displace fuel use. That is one of the things it is there for, one of its advantages over a tiny-battery hybrid.

 

Yes I'm just making stuff up as I go. But that darn manual keeps getting in the way. Because you see, I actually own a Gen 5 Prime. If your manual says something different, it's not a Gen 5 manual. It's probably the manual from the car you actually own.

 

I think what we have here, is someone who is willing to go beyond the absurd to chase the illusion of winning the argument. Even though he is a long way off track.

 

And which poster would that be?

 

I guess there's no forum rule against trying to assert stupid arguments.

 

It's not that hard, but seeing how you struggled with the battery type, I don't think you'll ever get this one.

The car will engine brake, yes. But B mode is neither necessary nor sufficient for it to do that.

Think a bit about what needs to happen in your situation if the driver doesn't select B. Do you think it will stop regenerating, thus stop the normal drag and just go into 'coasting'?

No, it will engine brake in either B or D, to provide whichever drag is required, and only when necessary. B may make it decide it's necessary sooner, but doesn't force it.

B's meaning is just 'more drag, please'. And that may come from engine braking if the battery is full enough, same as D.

 

B mode is inefficient as it requires power to mg1 to drive an unfueled engine for engine braking when the traction battery is full. Normal braking creates variable regen based on the driver’s requirements.

Only when braking hard or under -6 mph are the hydraulic brakes invoked.

Certainly B mode will apply more retarding force than no brake pedal on a steep descent. But so will the brake pedal used as normal by my wife who has no clue about B mode.

Proof is in the pudding - 310,000 miles of hill country driving including a steep private road grade every day to and from the house. Original rotors and front pads. Rear pads were changed at 250k while we were in there for a noisy hub.

 

I hate to repeat myself for about the fifth time, but don't you realize that there's nothing holding your car back when you are coasting down a long hill? It's not like a conventional car where there is some engine drag, and you can increase that drag by downshifting. The Prius is different; it will keep picking up speed once the hybrid battery is at full charge. When the hybrid battery is at full charge, it will not absorb the energy that needs to be dissipated. The engine is stopped, so it will not create any drag.
OK, so you prefer using friction brakes. But I don't like to overheat them and burn them out on long hills. That's the very reason Toyota designed in an engine brake. And it is that. It may not be the same make and model as that on an 18-wheeler, but the working principle is the same. The valve timing is changed so that the pistons create drag by compressing air, thus dissipating energy as heat. The alternatives are dissipating that energy as heat from friction braking, or capturing that energy if your hybrid battery can still take it.

If you live in a fairly level area, you may never need to use the engine braking, good for you, but there is a place for it where you can descend hills for long distances. I see people complaining about premature brake wear on a Prius, and I can understand why, if they are ignorant about the engine braking feature.

 

So now, am I a troll again, for simply having all my facts in order?

 

after the battery is full, the prius uses engine compression without gas to slow down

 

Are you saying the engine brake is automatically engaged without the driver activating B mode?

 

it has to, that's the only way to prevent overcharging the battery.

 

If it automatically applies the engine brake when the battery is at full charge, why is there a B position?
Don't you think the driver may want to choose when to apply the engine brake?

 

you can use b to slow the car down before the battery is full.

 

I don't get what you're trying to prove. I'd want to slow the car down with regen instead of using engine braking, and wasting energy by producing hot air.
Then if the battery is full, it makes sense to use the B to save the brake pads.
I don't think most people would want to do it your way.

 

that's a possibility

 

This is a strange situation. Several people have challenged what I'm saying, but they are doing so without substance. Hard for me to figure.

 

There is nothing strange about this situation. Take for instance, that a person posting as Paul Gregory made the statement "I'd want to slow the car down with regen instead of using engine braking, and wasting energy by producing hot air."

This statement totally ignores the fact that the car's regen function is not 100% efficient, and some of the energy is wasted as... You guessed it, HEAT!.

That same person has ignored the fact that there is rolling resistance that comes into play when coasting down a long incline. Unfortunately that results in heat accumulating in the running gear as it coasts.

I suspect that the B mode that has so enthralled Paul Gregory is a function of the braking system that is only really NEEDED in rare circumstances. It might even be a placebo that makes insecure drivers feel better about having something that seems like it could be an emergency brake mode.

TLDR: most Prius Prime drivers don't use the B mode when going down hill at a controlled rate. The brakes regen do an adequate job of slowing the car as needed even with extended steep inclines.