"B" mode and the Prius Prime

Only in HV mode. In EV mode it enhances regeneration.


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If there is sufficient headroom in the battery and you are in EV mode, then B mode increases the regeneration without running the ICE. If there is insufficient headroom or you are in HV mode, then the ICE runs.

You will find that the Prime can bank a LOT of energy in the hills. When I enter the mountains, I typically like to have the HV battery at about mid-point, with some power to go up but more importantly capacity to absorb regeneration. Since the ICE is normally running anyway, I use that to climb hills and then regenerate on the way down into the battery. When the battery gets too full, I shift to EV to discharge it for more headroom and continue that process.

If your HV battery indicates zero percent, the car won't let you go into EV mode, so you sometimes have to cheat a bit and go into charge mode to put a percent in, then you can go to EV and fill the battery from regeneration.

 

On long downhills in EV mode, B mode and lightly pressing the brake pedal are pretty much equivalent (at least until the battery is full): somewhat enhanced regeneration (no engine braking, no friction brakes). In conventional cars, one uses B mode in long downhills to prevent the brakes from overheating. And with a regular Prius, the battery does not have that much capacity, so the system has to revert to engine braking pretty much immediately in B mode.

And my experience has been that if I am in HV mode due to to low battery, and am going down a long downhill, I will use the brakes very lightly if necessary to maintain speed, and regeneration will recharge the battery, and I will be automatically put into EV mode when the charge level is high enough.

 

On a long downhill, if you have no EV range to start with, just drag the brakes a bit and it will regen enough to get you some range. Then B mode will work if you want it. I did it lots of time on our vacation driving in the Rockies. Just don't brake so hard that you engage the friction brakes and waste energy.

I also found that in EV mode, B didn't do anything that I couldn't also do with my right foot in D. The same inertia reduction with result in the same amount of charge no matter which "gear" you use. In fact, B often did too much braking so it was simpler to just keep a light pressure on the brake pedal.

 

Interesting. See, I think I could have definitely made good use of B on my 200 mile drive back from the dealer where I bought mine. Had I known how it worked. My battery was deader than a doornail. Almost the entire ride home it read "--%" On one downhill, which was several miles long, I was using the light pressure method. I was trying to keep the regeneration bar from going to the max, but I was struggling due to the steep grade. By the bottom, I hit 90 mph. If I used any brake pad, it had to be very little...but, it would have been nice to get some more aggressive regen (if B would have made that possible). Would B have kept me at a lower speed? I travel that roadway from time to time, so it would be nice to know before next time.

 

You need to have some % battery, even if it is 1%, to regen going down hills. For B to regen, you need to be in EV mode. B in HV mode uses the ICE. B will slow the car down, how much depends on the grade. If it is steep, you may need to use the brakes a little.

 

Got it. While I started that hill at "--%" I got up to 7% by the bottom. So, next time, I will wait until 1% using the light braking method, switch to EV mode, then shift to B and see what kind of deceleration that gives me. If it is not enough, use light braking method while in B for additional battery recharging. Sounds good. I appreciate the help with that. I may also experiment with B if I ever leave my house below 100% because I have a very very steep downhill less than a mile from my house. It's only about 0.5 miles long, but it is so steep it costs me 7% battery to climb it at the speed limit in EV mode. It's also a heavy traffic area, so climbing more slowly would not be acceptable to other drivers. I love that Hilltop Reserve setting the Chevy Bolt has. I could use that if Toyota had a way to stop charging at 90 or 95%.

 

You got it that once you get even just 1% EV range, you can put it in EV and use B. However, the MG's will only regenerate so much before the hydraulic brakes have to jump in to help. It won't matter whether it's the mode or your foot telling them to regenerate, the limit is the same. If B happens to provide the right amount of rolling resistance, you're golden. If it's too much or too little, you're still going to need to use that right foot. If B won't keep your speed low enough, you'll be using friction braking. Go ahead and play with it. I did. It's the best way to learn how it works.

 

You're right. I just did a little playing running errands. Started out in EV mode out of my garage and throughout the neighborhood (about 1.0 mile). I don't lose even 1% to that point, so I guess maybe I am driving too efficiently? It's a 25 zone and there are a lot of kids in the neighborhood. What can I say? So, then I make my turn to go down the very steep ~0.5 mile hill. I'm still in EV mode, but I decide to switch from D to B. The braking / regen meter goes down to about 50% with no pedal. That's not even close to enough to slow me down, so I apply additional brake pedal. I'm doing my best to keep it from being pegged, but I still gain speed. Now, here's the weird thing. I get in a line of traffic at the bottom of the hill where there is a stop light. The car decides it no longer wants to be in EV mode and runs the ICE while I am dead stopped in traffic. Maybe 45 seconds later (light still red), the ICE kicks off and goes back to EV mode. The entire rest of my trip, including multiple stops on errands, the car stays in EV the rest of the way. Was the car just "warming up" the ICE? Or did I make it mad because I started to go down a hill with 100% battery and tried to use B when I shouldn't have because I didn't have any space available for recharging? Because this is why I wish I had a "Hilltop reserve" mode like the Bolt. Outside temp was 72 degrees with light rain. Climate control was OFF the whole trip.

 

Darn Prime is so efficient. Yes you hit the 100% mark. And it will hit it sooner when the weather turns colder. Then it will keep the ICE running until it hits operating temp.

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Yep you probably should have used the A/C. Might as well cool the battery and you down before going down the hill. That’s the only “Hilltop reserve” option you have in the Prime. Then turn it off when you get to the bottom.




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Excellent suggestion. That is exactly what I will do. And in the winter, I will run the heater aggressively that first mile in the neighborhood. That should definitely get it off that pesky 100% that just won't go away fast enough. I also did my first longer drive on EV only tonight. That range is GREAT for my needs! I left with a full battery and came home at 12% (5 hour charge estimate at plug in). I'll post stats on efficiency if I find how to do that. I didn't start or reset any trip meters at the beginning, so I'm not sure if that's needed to do that. The first half of the trip, I drove in D. The ride home, I drove those same steep hills in B. I think B is going to be my go-to for everyday city driving. It's not quite 1 pedal driving because I still accelerate down the hills and have to apply additional regeneration to keep from speeding, but what it does do is make sure I'm remembering to get on the regen more aggressively at the tops of those hills. I believe it is going to make me more efficient, overall, and save me some on brakes. One person told me they lived in San Francisco for a couple of years and wound up getting their first brake job at 60,000 miles (with about 30,000 miles of it being on those steep hills). It gives me something to shoot for...I would be happy getting 60k on the original pads.

I also need to customize my displays. I don't have the GOM to tell me my estimated electric miles remaining. I was going to do it while I was plugged in, but I don't know that you can do very much from Accessory mode only. Another thing I wanted to try to figure out is if there is a way to lock the car while it is in Accessory mode. I know you have that option to run the A/C while you are away, but what about the radio? I obviously have a lot of reading yet to do.

 

Good idea.

Noooooooo!!!!!! The car will probably shut itself off since that's what the manual says will happen, but you still might kill your 12V battery.

 

People keep saying that when the battery is 100%, the engine will come on in B mode. But that is not the only limit.

Sometimes if the amount of regen charges it too fast or for too long, the engine will come on. It seems to come on a little later if you use the brake pedal instead of B mode, but eventually it will come on either way.

For example last time I drove down Freemont Pass the engine came on pretty quickly even though the battery was nearly empty. It's a 7% grade with a 65mph speed limit for several miles, so the amount of regen needed to slow the car down is quite high. The engine came on in the first mile or so, and the amount of energy recovered into the battery was disappointing considering the size of the hill.

On another trip I drove up Mt. Evans (14,000 feet), and on the way down I was able to get the battery from completely empty to 80% full using mostly B mode. The speed limit is 25-35, and the grade is a little gentler. That trip was interesting because I started with the battery discharged at the top of the mountain, and from there I drove 60 miles all in EV mode. Of course it took a lot of gas to get up there.

I think the battery temperature also affects whether the engine comes on. If it's too cold or too hot the engine is likely to come on. And I've heard a rumor that if you come to a stop in B mode in EV mode the engine will come on, but I don't think I've seen it myself.

If the engine comes on in B mode in EV mode, it will use very little gas, and it will complete the warm-up cycle when you get to the bottom of the hill. If you anticipate the engine coming on, it would probably be better to let it warm up first so that the oil is the proper viscosity.

 

I agree about the battery charge rate heating up the battery and forcing the engine on. It happened to me coming down the north side if Grand Mesa. I was almost to the bottom and the battery percentage was at 98%. I thought I was going to make it, but that's as far as I got when the ICE started up. I don't know what would have caused it other than it was getting warm from the long, continuous charging. Still, I made it to I-70 and something like 20 more miles up I-70 in EV.

 

Thanks for that section of the manual. The 20 minutes auto-off would be PLENTY of time. I was referring to a situation where maybe I need to run into a gas station or convenience store for a little bit and wanted to entertain my guests in the car outside while they waited. It seems my only option at this point is to do it with the doors unlocked. I would much prefer to lock them, with the A/C and radio playing...but only for like 10 minutes on average. If they have to sit in silence (with A/C), so be it. Most cars don't even have the capability to run the A/C while the car is parked...I just thought maybe there was some way to use the radio, too.

 

You can leave the FOB in the car with the people, leave the car in Ready mode (Park with parking brake applied) and they can lock the doors. When you come back they unlock and you get in.

 

Brilliantly simple. Why didn't I think of that? This car has a bit of a learning curve. That is the solution I was after all along.