Using the Engine Brake....

[angelajf07]

When I purchased my 2010 yesterday, the dealer mentioned using "b" mode when in steep grades but also mentioned using it when coming to a stop. He said it would help charge the battery at a higher rate and would "save" the brakes. DID I GET SOME BAD ADVICE?

[nparker13]

Yes, I believe its actually worse for fuel economy to use it when its not needed, it uses the engine/transmission more than regen motor. Someone who knows better should verify this though. Note that going down a steep grade is what B is made for (as riding the brakes is not good either).

[rachaelseven]

Yup. Bad advice. The B mode wastes energy by using the compression of the engine to stop the car. You're much better off leaving it in D, unless it is a VERY LONG downhill grade... we're talking mountain descent here. The B mode is just to save you riding the brakes after the battery has already fully charged. Plenty of posts on the subject here if you use the search function, so lot's more information available.

[JimboK]

Technically the dealer is correct about B mode both saving the brakes on a long downgrade and charging the battery at a higher rate. The latter should be considered a byproduct of it and not a goal. The advice of the others is sound.

See this for an in-depth discussion of B mode.

[qbee42]

Use B mode only on steep downgrades. Otherwise it simply wastes energy.

Tom

[bwilson4web]

Sometimes you want to waste that energy instead of heat stressing the traction battery. The reason is battery charging is a heat generating, chemical reaction. For example, here is chart that shows what happened to my traction battery temperature when I force charged it:


Ordinary driving around town was warming up the battery on a cool night but I stopped at the bottom of the hill to put on a maximum battery charge. A 9C temperature rise came from the forced charge.

This is a well established characteristic of NiMH battery chemistry as this chart from a paper titled "Thermal Behavior of Small Nickel/Metal Hydride Battery during Rapid Charge and Discharge Cycles" Takuto Araki, Masato Nakayama, Kenichi Fukuda, and Kazuo Onda" has figure 10 charts showing temperature response of an AA sized battery under charge and discharge shows:


So just to make sure, I did a hill descent test with and without "B" and though there was not a dramatic increase in temperature, it did go up by 1C for this 525 ft. hill:


Now the reason for concern is 'heat is the enemy' of our NiMH batteries. If someone were going up and down a bunch of hills where the car was cycling between charge and discharge ... say foothills near mountains ... they could 'heat pump' their traction batteries to some impressive temperatures. If you'd like more details including what happens when NiMH batteries are overcharged, here is my report: Prius Battery Photos

Ultimately, the choice is yours but my rule of thumb is to use "B" when I'm looking at say a mile or more of downgrade at high speed. But I'm trying to get maximum life out of my 2003 Prius with 121,000 miles.

Bob Wilson

[angelajf07]

Thanks! Great info!! I LOVE MY NEW PRIUS!

[a priori]

You really don't need mountain passes to take advantage of B mode, though I will suggest to you there are not many places in Minnesota where the B mode would be advantageous. The city streets in Duluth may be steep enough, but not long enough. There are sections of roads in the river bluff country and far SE MN where B would work well, but it really wouldn't be necessary unless you were already fully charged (HV battery) going into a long descent.

One nice thing about the B mode, though, is it will keep your car at a fairly steady speed. So, if you start a long, steep descent, you could put it in B mode while moving at 60 mph, and the car should maintain about 60 mph so long as gravity allows.

If you really want to learn more, please follow the earlier link to a great description of B mode.

(AND: Welcome to PriusChat and Congrats on your new 2010!!!)

[yardman 49]

Originally Posted by a priori

You really don't need mountain passes to take advantage of B mode........

Hello Angela:

I live in a hilly, but not mountanous, area. On some of the steeper local hills, I love using B mode when descending. It helps me from not having to ride the brakes going down these hills.

The Prius is constantly monitoring the traction battery temperature, and in most cases won't allow the car to overcharge (or overheat) the battery. There's even a cooling system for the traction battery, with a fan that can run at three speeds (four if you count "off" as a speed). So either manual regenerative braking or using B mode for going down hills won't usually cause a problem.

However, you did get bad advice about using B mode for stopping. I can see where some may like the feel that a manual transmission gives them from "downshifting". and how they might like using B mode for this purpose, as it makes it feel like they are "doing something useful", akin to what downshifting would do. However, this will actually limit your braking regeneration somewhat on level areas, so it should not be done.

[MaggieMay]

Originally Posted by bwilson4web

... Now the reason for concern is 'heat is the enemy' of our NiMH batteries. If someone were going up and down a bunch of hills where the car was cycling between charge and discharge ... say foothills near mountains ... they could 'heat pump' their traction batteries to some impressive temperatures. If you'd like more details including what happens when NiMH batteries are overcharged, here is my report: Prius Battery Photos

Ultimately, the choice is yours but my rule of thumb is to use "B" when I'm looking at say a mile or more of downgrade at high speed. But I'm trying to get maximum life out of my 2003 Prius with 121,000 miles.

Bob Wilson

Bob - this makes me wonder if there's a relationship between the temp of the battery and the mysterious red top line of the the battery indicator. (see All Bars On Battery Indicator) only a few of us have seen this, but Red usually means Warning or Hot - like - "Be careful you dope, the battery's overheating!!"

Could that be possible?