Mountain Driving - "B" mode or braking?

While this is true at low speeds, at higher speeds or with a warm battery it is surprisingly easy to get into mechanical contribution. If your mountain driving involves frequent alternation between up and down hill you can quite easily warm up the battery enough to lower the transition at which mechanical braking contributes. (I've seen it limit regeneration to 50 Amperes, which really is not a whole lot of braking at highway speeds)

It depends. If you understand the terrain well enough to project whether regeneration current going into the battery at the moment will be productively used on the next uphill, then by all means regenerate. But if the downhill is of some length and your feel for it and your current state of charge suggests strongly that you will reach maximum well before the bottom of the hill, then engaging the B mode early, and thus deliberately "wasting" some of the potential energy in turning the engine instead of heating the brakes or charging the (soon to be fully charged) battery is a good idea. I think the completely ideal case is probably reached if you hit full state of charge right at the bottom of each hill. (somewhat beyond 8 bars, but not much). Sooner than that and you have stressed the electric system more than needed, later and you have wasted energy.

The Prius battery is actually a pretty limited device. It is nowhere near being able to provide or accept the energy change represented by even 2000 feet of altitude change.

I live in Albuquerque, almost all of which is within a 1000 foot altitude range. In consequence I hardly ever use B mode. Real mountain driving is quite different. I hope some Prius mountain owners will comment.

 

First you want to keep your speed reasonable for the conditions regardless of mechanism, "B" or brakes. However, my data suggests "B" is a better choice:

Using "B" appears to keep the battery cooler, a good thing, compared to using the brake that appears to spike the charging current.

Charging our NiMH battery is endothermic, it generates heat, and heat is the primary enemy of our traction battery. For example, this is what happened when I 'force charged' my battery:

The temperature spiked, distinctly.

Bob Wilson

 

I find if I can predict I will be descending for more than two minutes using "B" is better (spreads the regeneration over a longer period, thus heating up the battery less - same energy but over a longer time so the temp of the battery doesn't spike as much). If just going up and down on short hills I use "D".

In other words, descending a mountain pass is "B" territory. Otherwise use "D".

 

There is also an issue of convenience. I've used B mode before just because it was easy. On a long mountain downhill, you can shift into B mode and control your speed entirely with the accelerator pedal. Press a little harder and the car uses less engine braking; ease up and you get more. If you find yourself using too much accelerator, shift back to D.

Tom

 

It makes it very easy. Since it is also easy to switch between modes, try both and see what you think. I found myself shifting back and forth just to compare, since I don't get to use B mode where I live.

Tom

 

I drive in the mountains a fair amount and I've been wondering the same thing. Often I find that my battery is up to max green long before I've descended fully off a mountain pass - what happens to the systems when the bars get up to max green and your still descending? Does it keep charging.

 

The battery can take a little more charge even at full green, but there is a high limit.

This has been described many times on this forum, but here is a quick review of what happens down a big hill:

1) As you brake or coast in D mode, the front wheels are used for regenerative braking, converting the car's potential energy into electricity, which is stored in the battery.

If more braking is needed than can be done by regeneration, the friction brakes are engaged.

2) When the battery approaches its high limit, the Prius engages engine braking, converting some or all of the braking energy into heat by electrically spinning the engine without fuel.

3) At the high limit of charge, if more braking is needed than can be supplied by engine braking, the Prius also engages the friction brakes.

With B mode, it works exactly the same way, except engine braking is done from the start, helping to postpone the onset of friction braking. There are also some minor parameter differences in the control for B mode, but that is a minor detail not noticeable to your average driver.

What does it all mean? If you know or suspect the downhill will exceed your battery capacity, engage B mode. That's all there is too it. You may elect to use B mode just for ease and comfort. It won't hurt anything, but using it when unneeded will rob you of a few mpg.

A note to the physics police: I used the term potential energy in #1 above, assuming there is no change in vehicle speed. If the vehicle speed varies, kinetic energy comes into the equation. If you know this, then you can inserted it as needed (both lexically and physically ).

Tom

 

Tom: On this point there seems to be at least some disagreement on the forum in various posts. Does the Prius ever engage a B like more on its own? It certainly starts the ICE to spin MG1 with resistance in an effort to burn off extra electricity, but does it ever do so without fuel to the ICE effectively creating its own like B mode (beyond the split second it takes to get the ICE up to running RPM)? I have not had my ScanGauge long enough to track RPMs and fuel usage on a downhill run or complete charge situation long enough to ever observe this. Of course I also live in flat land affording me no real opportunity to ever see it if it does happen. Just courious if you or others have ever "observed" the car effectively creating its own B mode without the driver shifting to it?

Bob

 

Yes - I see it. During downhill runs when the battery gets to 8 greens the engine will increase from the normal 960RPM up to 2000-3000 RPM. The engine speed is increased by applying pressure to the brake although not as high as using B mode - I don't see the engine speed going above about 3500RPM.

kevin

 

Several of us have. See this for more.

 

Yes, the car will switch to B mode on its own. I have witnessed this on several occasions. It's really an "almost B mode" type of mode. It acts like B mode, but is a little less aggressive.

Tom

 

Thanks all for the information. I get that same impression from studying the data Jim provided.

Thanks Jim for making me even more jealous of CAN view owners than I was before. Do you have a similar spreadsheet where you have had the car in a state where the ICE was not running at all and then started spinning without fuel? In the data you provided the ICE was always spinning as your speed (+60 MPH) certainly required it. Likely you don't go down many steep hills which results in a high SOC with speeds less than 40 MPH to see if the ICE starts spinning just to burn off excess energy...

I'm just curious as this is the kind of stuff I find so fascinating about the car. Again, thanks for sharing the data

Bob

 

I have witnessed the ICE start spinning without fuel at low driving speeds. We were camped 2,000 feet above the floor of a basin, and each time we drove into town the battery would go full green and the ICE would start engine braking. The vehicle speed was about 12 mph and the ICE was off. The transition to engine braking was very obvious.

Tom