Controversial B mode thread :-)

I feel somewhat awkward posting this. World is no longer flat sort of thing.

F8L, from his own experience and intuition, had suggested B mode recharges the battery better on downhills, and I said I'd look into it. Of course, we all post all the time in newbie question areas that this is incorrect, it's actually less efficient, energy thrown away as heat, yada, yada. I've learned everything I know about the car on these forums, and didn't question it, since everything else seemed to be dead on.

So off I go today. I'm just driving once a week for my 12v battery in the summer, so I took off some on some unknown route here in Vermont. Luckily I picked a nice mountainous one. Mileage was in the high 30s (that's right, I was getting in the 30s like a common peasant) fifteen minutes into the drive, it was was all uphill basically.

On the way back down, I remembered to test out F8L's idea. I'm going downhill, not too steep yet, and put it into B mode with BTA showing about -20 amps at that point. After a quick flash of of -40 or so (or was it +40? Uh oh, someone else go out and check, or else it'll be another week :-D) it settled right in at -25!! So B mode was 25% more efficient in packing in the amps.

My first thought was if this had something to do with more lost momentum, so you're getting more energy because you're slowing down more, but still less than if you slowed down that much, in terms of mph, while in drive. Someone will have to explain that to me if it's the case. Nonetheless, there are times when braking is a function of time and not speed, so putting it in B mode over the same period of time as drive gives more regen.

I tried it a bunch of times to make sure. Same result over and over down this 10 minute mountain. It was nice to see the 100 mpg bars after a couple down near or below 25 mpg from the uphill. Got to the bottom, switched SG back to SOC, and discovered I was already at 83% even though I had more to go. Managed 62 mpg roundtrip even with those huge hills, thanks to the regen. It helped me go a long way, including an 80 mpg bar with acceleration from 0 to 50 mph.

I also verified that in B mode, even though the engine comes on quite hard, at like 2400 RPM for what I was doing, gallons per hour still showed 0.00.

I wish I had pictures of all this, but with SKS turned off for the summer, my key was stuck in the ignition, couldn't mechanically lock the car to run and get my camera.

So - no gas use, but more energy regenerated in B. F8L was right. I feel like I'm either missing something somewhat big, or else the conventional wisdom just sort of got passed down like folklore without testing.

Thoughts? Comments? Insults?

 

You're right, just did it many times in downhill portions, not over same one. I'll be happy to try it out next week like you say, starting with same SOC at same point a couple times and descending. I would then look at SOC at a specified distance after I start, correct (as opposed to specified time)?

There are just some times when you want to be losing momentum the whole time (stop sign, lower speed limit coming, etc.), and if you can pass that entire distance with more amps into the battery, I'm confused how that theoretically charges the battery less.

 

dtuite,

I'm guessing so because just from observation, SOC seems to fill very quickly at higher percentages, like 70 to 80, than from 60 to 70. Even if not, if I'm going to go use the gas to check it out I figure it should be controlled enough so that it will mean something either way.

 

Lucky for you guys I left my windows open overnight. And it rained. After sopping up water from the seats with probably hundreds of Kleenex, I drove around to air out the car some more.

I happened to find a perfect hill tucked away, apparently created for regen braking testing, since it was newly paved, had a speed limit of 35 mph, and had literally no traffic on it...perfect. Quite steep, I might add. From the bottom heading up I was getting between 11 and 18 mpg instant. I sacrifice for PriusChatters

Conditions: overcast, 68°F, no AC or any other accessory use in car, all four windows wide open for my poor, wet seats. Length of hill 0.4 miles. Started from same location at top of hill (trucks use low gear sign) and stopped at same location at bottom each time (stop sign). Started each trial when SOC got down to 58.0% exactly. Accelerated gently to 7 mph each time (next telephone pole basically) and let gravity do the rest. I would brake naturally when I felt I needed to, so I did not brake in the same manner, at the same place, everytime, which represents what I was looking to find out - how D regen differs from B regen in "normal" driving. Numbers below are SOC at stop sign at bottom of hill once fully stopped.

D mode - from 58.0% SOC
1: 65.0%
2: 65.0%
3: 66.0%

B mode - from 58.0% SOC
1: 65.5%
2: 65.5%
3. 65.5%

So B mode works better or just as good as D most of the time. But there's one more critical thing. In Drive I would easily hurtle down the hill, and if I hadn't braked, I imagine I would've been going 60 mph by the bottom. I never went over 40 mph to make sure engine stayed off. In B mode, I was going much slower the whole time, since it was better at slowing the car (by the way, like uart says, engine does not start spinning in B mode until 20 mph, so that extra drag I felt slowing the car down comes from more regen). So B mode produced as much energy, and usually more, than D, but at much slower speeds. If I had been going the same speed, B mode would have produced more.

One question I have, which may reveal another advantage of B mode, is whether traction control is easily tripped in it as well. The road was so smooth it never happened, but would B limit or eliminate the switch to friction brakes over a pothole unlike D mode, thus keeping you in regen longer? If you're not aware, if you go over a pothole while braking, even letting up on the brake for a second or two and reapplying still will have you stop in "friction mode". Thus one bump while braking while down a steep eliminates almost all of regen, excepting about 10 amps or so. If B mode avoids this, it would be a huge plus.

I guess what I don't understand is where our ideas about B mode NEVER equalling D come from, since it does. B basically beat D on this one little test, and would thus seem to do it even moreso if travelling the same speed in both "gears".

Anyway, I would invite others to do more testing, but I'm pretty goshdarn convinced of using B on any hill of decent length. I won't be using it in daily driving to stop signs, but the idea that is completely worthless and will harm your fuel economy seems to be unfounded, and it may actually help.

 

Hey Tom, thanks. I know I have some limitations in terms of technically describing things in which I have no expertise. I guess regardless of how I'm trying to say it, if braking in B can give equal or better increase in SOC compared to D, overall the use of that stored energy during the trip will help give better mpg.

 

uart, that means that B mode got me to the same SOC without having to go as fast as in D.

Let's say you're coming over the crest of a bridge. You're going 35 mph. You let your foot off the pedal, or brake somewhat, and come to the stop at the bottom to pay a toll.

Now let's say same thing, but you use B mode once over the crest, but you're starting downhill at the same 35 mph. You would expect it to do better in that situation in terms of SOC boost given what I've seen, no?

 

I see what you mean now. That experiment is beyond an individual, we'd need a team to try to pass the same point at the exact speed and exact SOC everytime, as well as maintain the same speed profile over 0.4 miles (or get a self-driven Google Prius to do it!).

I think the point about bumps triggering traction control is important though. One bump and your entire hill in D is wasted. There is also this graphic:



It shows us how easy it is to mix into friction braking by pushing enough on the brake. If you're going down a steep hill, B mode is supposed to avoid friction brakes. So I'm starting to understand the theoretical the more everyone explains, but in the practical, it could very well help avoid riding your friction brakes down a hill without realizing it.

Also in actual driving, I wouldn't be able to control my speed profile in a precise manner. I get that for a real study of this, we would need that. I'm just not as convinced of the dogma that B doesn't work as well *practically* in terms of regen, or even better since it would eliminate some of the unnecessary brake pad wear/heat with more brake pedal depression, and potentially avoid traction control circumstances.