"B" Range...What's Really Going On...

Regardless of hill length, I've never seen our regular Prius cease regen braking, at least I don't think so. The battery charge indicator will show full, but the charging indicator persists when I'm on the brakes. The worst case for me is our local ski hills, even there, continuing regen, all the way down. Once in a while I'd try B, but the rev's would freak me and I'd quickly switch back to D, seemed just fine.

With our previous Civic Hybrid, it was dead-obvious when regen shut down. Any significantly long hill would do the trick: the regen indicator would drop, then disappear, and at the same time you really had to lean on the brakes, the required effort increased about 50%. Almost a safety isue, imho.

 

The HSI confirms your test.

Good Job!

 

I am more confused than before I read this thread. Everyday going to work and back I have a hill that is 1 mile going up and 1 mile coming down. Do I or do I not use the "B" going down? I have no idea what all this technical jargon means, I just want to save money with more MPG. I know I need to do things on my part and I am, but this confuses me.

 

B mode is never an MPG saver, it is a way to slow the car using engine braking. If you have no trouble slowing without B, you do not need it, ever. It is just adding wear to your car.

Toyota is required by law to have a mode with more engine braking, but you are not required to use it. Donner Summit (and most passes between Nevada and California), Cabbage Hill (in Oregon) and many passes in the Rockies need B as they exceed a 1000 foot vertical drop.

The tallest point in MN is only 550 feet above the surrounding landscape, you never need B in MN.
Eagle Mountain (Minnesota) - Wikipedia, the free encyclopedia

 

Thank you for the easy explanation. That summed it up quite nicely.

 

Actually I did. It is much harder to force Prius to refuse regen braking but quite easy in Yaris Hybrid (= Prius C). You need some speed, some hill at 6 bars in battery (it's hard to tell Prius that there will be long steep hill and discharge battery just before it starts, especially driving uphill with high energy demand).

So - when you drive down the hill in D mode you need to keep feet on the brake pedal. It will highly regen, heating battery quite fast and easly switching to regular brakes. As the battery overheats or overfills the ICE will kick in and spin to quite high RPMs (I think my regular Prius spins higher than Yaris or the mountain was too small). Then you are left with only regular + engine braking. Not the best solution for long hills.

When you start the same hill with B mode you problably don't need to keep feet on the brake. Braking energy will be split across two channels. Part of the energy will be stored in the battery (but slower than in normal braking), rest will be dissipated (wasted) in engine.
At the end of the hill you will end up with fully charged battery (but slower, less hot) with cold brakes ready to take an action when really needed.

So if the hill is quite short or the speed is low B mode is not essential. But if the hill is very long or you drive on the highway B mode is essential to keep car in good condition during hill braking.

Back to graphs from #1 post - braking on the highway hill with 60+ mph speed full regen at 100 AMP is almost nothing... Same force applied in the city is enough to stop the car without using regular brakes.

 

How hot does the battery actually get?

 

That was an unfortunate comment.
The system will not LET the battery overheat.
You should not worry about it.

 

Unfortunate? How so, just because I asked a curious question?

And I didn't ask if the battery would overheat.

I asked: HOW HOT does the battery get?

100 degrees? 200 degrees? What temperature?

geez...!

 

I'm sorry. I did NOT mean your question was unfortunate but the comment about heat that caused you to ask WAS.

Batteries heat a bit when charging or discharging. The normal operation of the vehicle will not cause the battery to heat enough to be a problem.
That is why it is unfortunate that he mentioned the heat at all. It causes people to worry unnecessarily.

 

Yes I know that batteries heat up when current is drawn from it. I just wanted to get a sense of like if I could hook up a thermometer to it... has anyone made a graph or whatever showing temperature vs. time/usage, or is there a gauge anyone has hooked up to it? That would be neat and I'd like to know... just saying. Like how hot before switching to regular brakes?

 

On a regular automatic on a two wheel drive car you usually have on the transmission nuetral, reverse, 3rd gear usualy makred D or D with a circle for drive. 2 gear nad 3 gear. This is usually true for no matter how many speeds/gears it has. 2 and 3 are your low gears. Most people never use them. I sometimes use 2 in snow.



In a CVT two wheel drive, (never been in a 4x4 or awd with a CVT). You usually have neutral, reverse drive and B. In a CVT you have unlimited number of bands (instead of gears), do not ask me what this means or is. The B that I have no idea what it means, is similar to the 2 and 3 low gears on a regular automatic. Only time on my prius C I really use the B is in snow or the one time I drove up a mountain. If I am wrong please let me know.

 

Downhill or when slowing, B is similar to Low or 3 or 2 in ordinary automatics. But at steady speeds on flat ground, or climbing mountains, B does nothing except possibly disable ICE-Auto-Off in situations where it would shut off in D, thereby dinging MPG a bit.

Snow on flat ground? No different than D.

Driving down a mountain is where B is different and useful.

 

B is for braking, as in, engine braking. As mentioned previously, same as low gear in normal transmission (1st or 2nd gear). I use it in stop and go traffic where go is less than 20mph; uses less brakes especially in all the highway bridge construction in my daily commute.

 

i have never used b in 12 years.

 

The purpose of "B" is to decelerate (i.e., "B" for "brake") in much the same way as selecting a lower gear does on a standard or automatic transmission. It's typically used for descending hills where you want to slow the car down without over-using the brake pads. This is less of an issue in the Prius since the brake pedal first engages regenerative braking and so there's less wear on the brake pads to begin with, but there are some circumstances where you might want to use it.

In "D" (drive) mode, you can descend a hill with the brake pedal pressed and the car will use regenerative braking to slow the car down, unless the hill is too steep to keep the speed down using regenerative braking alone. The amount of regeneration available at slow speeds is also quite limited, so if you're descending a steep grade slowly then it may not be able to hold your speed. In those cases pressing the brake pedal down hard enough to keep the car's speed steady will require the friction brakes to be applied in addition to whatever regen is available, and this is where "B" mode is of benefit.

"B" mode engages the engine at zero throttle (i.e., no fuel consumption) as well as applying more regenerative braking than you get in "D" mode with your foot off the throttle. This gives you braking via engine compression as well as via regenerative energy recovery.

If the battery reaches its maximum capacity while in "B" mode, then the car stops using regeneration and it moves the CVT into the max RPM band for the engine - this applies more compression braking to make up for the missing drag of regeneration.

If you press the throttle pedal while in "B" mode then the car will use less regeneration and/or lower the engine speed by selecting a different CVT range, this reducing the overall decelleration. If you press the throttle even harder then it will start applying power via the electric motor and/or gas engine just as it does in "D" mode.

Because the car uses a smart engine control system, "B" mode doesn't keep the car in a "low gear" the same way that conventional cars do when a low gear is selected. You can actually drive the car normally in "B" mode, with the caveat that the "dead point" of the throttle pedal (the point at which you are applying neither power nor drag) requires more pedal depression, and the point above that (when your foot is off the pedal or you are applying just a very small amount of pressure) will engage regen braking more aggressively and idle the engine at higher RPMs in order to provide more braking force than the standard "D" mode applies.

 

Sean, I think you've missed the mark. Leaving it in "D" is better for virtually every situation. "B" is only warranted on protracted downhill runs, where there's a danger of the battery being completely topped up, and braking then switches to friction braking only and, a good stretch of downhill run still ahead of you. Basically: Mount Seymour.

 

I have 'B' to be useful on a long downgrade in order to keep the speed down without needing to constantly brake. I live in East San Francisco Bay and find it useful when descending twisty 10% to 15% grades in the local hills.

 

Of course. I'm just pointing out that it's not really a "low gear" in the sense of a traditional car - it's really just an instruction to the engine control system to behave differently when the throttle is lifted above the "don't accelerate or decelerate" point. For example you can't hurt the car by driving fast in "B" mode, although I agree that there really isn't any good reason to do so.

 

Wow I really learn from these posts....thanks guys for all your knowledge......