When to use the "B" position?

The B position will cause the gasoline engine to rev up. The energy used to spin the gasoline motor becomes frictional heat loss and cannot be recovered.

I usually leave the gear selector in D until the traction battery SOC gauge on the MFD shows eight green bars. I would then shift to B.

Once the gauge shows eight bars, even if you don't shift to B you'll note the engine speed increase. This is because the hybrid vehicle ECU does not want to overcharge the traction battery.

 

Patrick,

I could use some additional clarification.

Does "B" only just spin up the ICE?

I've somehow come to believe that when in "B", regen is also enhanced.

Yes/No?

Further, my experience is that if you forget to drop out of "B" at the
bottom of the hill, you can happily drive all the rest of the day,
unawares that staying in "B" restricts the HSD's use of electric only
drive -- not really EV because it's the computer's choice not the driver's --
and results in significantly reduced FE/MPGs.

Again, Yes/No?

 

Regen is enhanced over using the Disc brakes, but not over D mode.

You can't use Cruise Control in B, so you can realize you are still in B that way.

 

I would agree with both of the above statements as edited. When in B, regen is enhanced compared to being in D and coasting. However regen is best when in D and braking moderately.

Remaining in B while driving around is the logical equivalent of leaving your normal car's automatic transmission in Low gear and driving around (except there's no regen with a normal car, of course.) The result in both cases is poorer mpg compared to leaving the gear selector in D.

When you are in D and apply the brakes moderately, most of the braking force will be regen, not friction. The braking will be friction when your speed drops down to a single-digit number, or you are doing a panic stop.

 

Regenerative braking = converting forward momentum into electrical energy (using MG2) and thereby slowing the car down.

Friction braking = using the brake pads on the rotors to slow the car by dissipating the forward momentum as heat energy.

The Prius software attempts to use regenerative braking as much as possible, automatically blending regen braking and friction braking when it needs to, and switching completely into friction braking in the event of a panic stop (ABS only functions with friction braking) or when the braking force from MG2 is no longer enough to slow the car (under approx 7 mph).

 

In D mode, when you take your foot off the gas, you are using MG1 to give the illusion of engine braking, this does mild regen. Once you put your foot on the brake, it uses MG2 as more serious regen.

B mode goes straight to MG2 regen, but because it also makes the engine an air pump, it does less regen than just hitting the brakes in D mode.

On a long down hill, the only place I recommend B mode, you can over charge the HV battery such that all you have left is Disc Brakes, using B mode adds engine braking to delay that.

 

Or if the battery is full, which is when you want the B position so you don't need to ride the friction brakes until you burn them out on a long downhill.

 

http://www.techno-fandom.org/~hobbit/cars/b-mode.html

 

Don't over-think this. The purpose of "B" is to help avoid burning up the brakes. Don't try to use it to manually manage the battery state-of-charge because you will almost certainly do it worse than the car itself does, and fuel economy will be reduced.

 

Thanks, Richard. I was wondering about that. I do regularly drive into the mountains and coast down 8-10 miles of 4-6% grades. The SOC always gets to fully green in these situations. Does the battery NOT have a charge controller? I have a solar panel and battery for a trailer, and that had a charge controller so the battery wouldn't over charge. I would hate to overcharge the battery and fry it with this kind of driving pattern, but moving to B seems like an awfully low tech way to protect the battery on such a high tech car. I figured I would use B like I use 3 on my Silverado--namely, when I don't want to wear down by brake pads by braking for 10 miles. Usually, however, just coasting and anticipating is enough to avoid friction braking.

So, can the HV battery be hurt by constant charging and little depletion, or does it just get full and disregard the excess?

 

it won't overcharge but it will switch to friction brakes to slow after that and they will then fade if the hill is long enough... I think of B as usefull when I would use a low gear on a downhill in a regular automatic except that if I'm not at a full battery I may delay B until it is full.

 

The computers will always protect the battery, but when they quit charging to do so, you will lose the regenerative braking.

For most drivers, B is a 'frill,' but if you drive down long slopes, it is a safety feature.

 

A minor addition to the above: when the battery SOC gets to the high limit, the controller will first switch to engine braking. Think of it as an automatic shift into B mode. If that still isn't enough, only then will friction brake be applied.

The advantage of switching to B mode early is to forestall getting to the high SOC limit. I've never had this be a problem on a 6% grade at highway speeds. At lower speeds it can be an issue due to lower aerodynamic drag.

At no point will the battery controller *ever* allow the SOC to get dangerously high or low.

Tom

 

I'm hoping this was just a typo. :madgrin:

 

Perhaps more of a Freudian Slip. I knew eventually my true feelings about B mode threads would color my response.

Tom

 

You're most welcome!