Understanding regenerative braking

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Scowbay

Member
Joined
Sep 9, 2011
Messages
12
I haven't been able to find a comprehensive summary of how regenerative braking works on the leaf (mine is coming in December so haven't been able to experiment.)

Here's what I get so far, but not sure if it's correct:

1. regenerative braking happens when you take your foot on the "gas" and coast - e.g. up to a light or down a hill.
2. the amount of regeneration increases if you go into eco mode
3. what I believe is, pressing lightly on the brake pedal increases regeneration

If 3 is true, does the Leaf electronically decide between physical and regenerative braking? In other words, does light pressure increase stopping power solely through regenerative resistance? Or is there a combination of physical braking and regeneration, or does that only happen with pressing down harder?

I got to think about descending a long hill - e.g. coming down out of the mountains. If 3 is correct, I'd be lightly stepping on the brake the whole time, signaling everyone with by brake lights, even though I might be going fast. Seems a little odd...
 
My experience has been just that, a light foot on the brake will invoke
regen, but a heavier foot reduces regen - per the power meter anyway.

For "engine braking" you can just take your foot off the accelerator; if
you use the brake you are probably slowing enough so the brake lights
are appropriate.
 
Scowbay said:
I got to think about descending a long hill - e.g. coming down out of the mountains. If 3 is correct, I'd be lightly stepping on the brake the whole time, signaling everyone with by brake lights, even though I might be going fast. Seems a little odd...
If you use ECO coming down the hill you will be using the brake pedal much less than ICE cars that are coming down the hill in Drive; in fact it may be less than most cars in L3, or whatever their first low gear is called. (Well, unless you try this with the battery fully charged.) So, no, it won't feel odd to you, or look silly to the cars behind you.

Ray
 
Scowbay said:
I got to think about descending a long hill - e.g. coming down out of the mountains. If 3 is correct, I'd be lightly stepping on the brake the whole time, signaling everyone with by brake lights, even though I might be going fast. Seems a little odd...
We live in the mountains and make a 5000 foot descent in our LEAF approximately once per week. As long as we start down the mountain with no more than roughly 65% state of charge, and use ECO mode, we don't have to use the brake pedal very much. There is usually more than enough regenerative braking by reducing pressure on the accelerator. (This is called "A-pedal regen".)

On the other hand, we sometimes descend several hundred feet on a steeper canyon road where we do tend to hold down the brake pedal gently and continuously. While the A-pedal regen isn't enough for this descent, we are still able to maximize regen as indicated by the ECO/tree meter and the Energy Info screen. Other drivers will just have to get used to the fact that it sometimes makes sense for hybrid and electric car drivers to "ride their brakes".
 
So is there a distinct point where pressing harder on the pedal reduces regeneration? And does anybody know if when pressing lightly, the total braking effect is regeneration and the brake pads are not touching the rotors? Just curious about whether they have set up a progressive system that actually switches from one type of braking to the other.
 
Typically the LEAF blends both types of braking, based on a formula involving battery state of charge and temperature, speed, pressure on the brake pedal, surface conditions, etc. There is no distinct point at which there is always a transition between regenerative and friction braking. However, with light, continuous pressure on the brake pedal, almost all of the braking will usually be regenerative.

The best way to avoid the brake pads touching the rotors is to use ECO drive mode for deceleration whenever possible. A-pedal regen never involves friction braking. Of course, in many cases where traffic and terrain permit, it is even better (with respect to efficiency/range) to drive gently and coast to a stop, thereby avoiding the drivetrain and battery charge/discharge losses inherent to regenerative braking. "Coasting" requires either carefully holding the accelerator in the position where motor power is at zero, or shifting to Neutral.
 
abasile said:
Typically the LEAF blends both types of braking, based on a formula involving battery state of charge and temperature, speed, pressure on the brake pedal, surface conditions, etc. There is no distinct point at which there is always a transition between regenerative and friction braking. However, with light, continuous pressure on the brake pedal, almost all of the braking will usually be regenerative.

The best way to avoid the brake pads touching the rotors is to use ECO drive mode for deceleration whenever possible. A-pedal regen never involves friction braking. Of course, in many cases where traffic and terrain permit, it is even better (with respect to efficiency/range) to drive gently and coast to a stop, thereby avoiding the drivetrain and battery charge/discharge losses inherent to regenerative braking. "Coasting" requires either carefully holding the accelerator in the position where motor power is at zero, or shifting to Neutral.

try telling all this to that guy at the Jewish Journal who trashed the car. Actually, he trashed himself, but most ICE drivers wont get that.
 
abasile said:
"Coasting" requires either carefully holding the accelerator in the position where motor power is at zero, or shifting to Neutral.

Coasting in 'N' is better than coasting with the neutral bubble because zero energy is used in 'N', but with the neutral bubble, there is ALWAYS some energy used (less than 5kW), unless you are traveling a very low speed...10mph or less.
 
thankyouOB said:
try telling all this to that guy at the Jewish Journal who trashed the car. Actually, he trashed himself, but most ICE drivers wont get that.
He didn't trash the car. He was fine with the car. He trashed Nissan for making what he concluded were unfounded claims about how far the car would go on a charge.

Regen is a great thing but if you're driving on freeways it won't increase your range. The only thing which would be of substantial help in this case would be a better Cd and a smaller frontal area and a more efficient drive train As a driver, he seems plunk in the middle. Keep in mind that if you run the numbers the only people that would get a 100 mile range in the Leaf are those who get 72 MPG in current model Prius. There aren't that many of those people.

As a data point, the friends I have who own one or more Prius and who are sensible drivers report their MPG for the Prius is in the low 40s. That translates into about a 67 mile range for the Leaf. So a 58 mile range is not unreasonable, especially, given how inefficient the Leaf is at higher speeds, if you have to do a lot of freeway driving.
 
SanDust said:
thankyouOB said:
try telling all this to that guy at the Jewish Journal who trashed the car. Actually, he trashed himself, but most ICE drivers wont get that.
He didn't trash the car. He was fine with the car. He trashed Nissan for making what he concluded were unfounded claims about how far the car would go on a charge.

.
Uh, NO!
He said he was going to get RID of it and made up a story that the dealer''s have lots on their lots.
Leafs — which the company had expected to sell out — are piling up on dealer lots like, well, fallen leaves.

You think that is praise?
get real, brother.
maybe you read the below as more praise. I didnt. He is getting rid of it.

Here’s what I tell them: I am ready to turn over a new Leaf — my own.


http://www.jewishjournal.com/rob_eshman/article/my_2011_nissan_solyndra_20111026/" onclick="window.open(this.href);return false;
 
LEAFfan said:
abasile said:
"Coasting" requires either carefully holding the accelerator in the position where motor power is at zero, or shifting to Neutral.

Coasting in 'N' is better than coasting with the neutral bubble because zero energy is used in 'N', but with the neutral bubble, there is ALWAYS some energy used (less than 5kW), unless you are traveling a very low speed...10mph or less.



That is incorrect, there is always a neutral point with the pedal while in D or ECO, where did you come up with this arbitrary number of 5kw? That assumes there is always consumption between acceleration and regen which is false. Abasile is correct.
 
Scowbay said:
So is there a distinct point where pressing harder on the pedal reduces regeneration?
Yes. This is readily observable by watching the "use/regen" dial in the "Energy Usage" display.

When you press the brake pedal harder, sensors (brake stroke sensor and master cylinder pressure sensor) inform the brake control module, which then knows you want to stop harder and increases pressure in the brake master cylinder and uses less regenerative braking. [Edit:Only the brake stroke sensor tells the Brake Control Module (BCM) how hard you are wanting to decelerate. The master cylinder pressure sensor tells the BCM the... pressure in the master cylinder, which the BCM compares with a"target pressure" to see if it should increase brake boost or not.] From the service manual, page BR-11: "[the brake booster] contains a motor and generates boost force according to the amount that the brake pedal is depressed and the amount of cooperative regenerative brake control".

Scowbay said:
And does anybody know if when pressing lightly, the total braking effect is regeneration and the brake pads are not touching the rotors?
I don't know and I don't see how anybody can tell whether the pads are or are not touching the rotors at any moment.
 
EVDRIVER said:
LEAFfan said:
abasile said:
"Coasting" requires either carefully holding the accelerator in the position where motor power is at zero, or shifting to Neutral.

Coasting in 'N' is better than coasting with the neutral bubble because zero energy is used in 'N', but with the neutral bubble, there is ALWAYS some energy used (less than 5kW), unless you are traveling a very low speed...10mph or less.



That is incorrect, there is always a neutral point with the pedal while in D or ECO, where did you come up with this arbitrary number of 5kw? That assumes there is always consumption between acceleration and regen which is false. Abasile is correct.
I tend to agree with EVDRIVER and abasile. I think the neutral point with the pedal is where the electric motor energy indicator shows nothing at all, not even a sliver of a line above or below to 0 mark. Now you can't always perfectly hold the pedal such that the motor will always at neutral point, as compared to being in N shift mode. However, apply power is only a quick tap of the pedal forward, while applying power from Neutral shift mode requires you to shift out of N back into D or ECO. I prefer to feather the pedal to achieve Neutral. It may not be as perfect as shifting to Neutral, but I think it's close enough and it's less hassle.
 
SanDust said:
Keep in mind that if you run the numbers the only (my underline) people that would get a 100 mile range in the Leaf are those who get 72 MPG in current model Prius. There aren't that many of those people.

LOL, where did you come up with that stat? That's just too hilarious! Seems you are pulling this out of your b***. Do you really believe that only Prius drivers know how to hypermile? I've been hypermiling since 1980 and recorded 82mpg on a 43 mile economy run with a VW Rabbit! There are many hypermilers out there that have never had a Prius.
 
EVDRIVER said:
LEAFfan said:
abasile said:
"Coasting" requires either carefully holding the accelerator in the position where motor power is at zero, or shifting to Neutral.

Coasting in 'N' is better than coasting with the neutral bubble because zero energy is used in 'N', but with the neutral bubble, there is ALWAYS some energy used (less than 5kW), unless you are traveling a very low speed...10mph or less.

That is incorrect, there is always a neutral point with the pedal while in D or ECO, where did you come up with this arbitrary number of 5kw? That assumes there is always consumption between acceleration and regen which is false. Abasile is correct.

Oh really? Maybe you misunderstood. I didn't say there was no neutral point. I said ENERGY/kW is being used with the neutral bubble, but not in 'N'. So according to your post, you are saying that you use NO energy with the neutral bubble/pedal? The ONLY way you don't use any kW from the motor is in 'N', NOT the neutral bubble. And I didn't say 5kW...read more carefully...I said LESS than 5kW, which it is. It varies by what speed you go. I'm surprised with all your EV knowledge, you would make that erroneous statement. To verify, use your 'neutral bubble/pedal' and then read your energy screen. It isn't ZERO, therefore energy IS being used by the motor. Like I said, maybe you just misunderstood what I was explaining.
 
I think the disagreement is really a matter of theory versus practical application. Yes, you can get zero power to traction using the throttle, but it is asymptotically more difficult the closer to zero you try to get. It is fairly easy to get uder 0.2kW (using the traction power display on the SOC meter) but nearly impossible to maintain true zero. In this way Leaffan is correct, shifting to N is the only real practical way to get to true zero. However, you gotta ask how much is that <0.2kW worth. There is a certain safety aspect associated with having to shift to D before engaging traction (some accidents are avoided by accellerating rather than braking). The answer, of course, varies driver to driver.
 
The energy "bubbles" are too coarse to know if you are really at zero motor power. Yes, you could be in the "zero" bubble and still be drawing 5 kW. To get as close as possible to Neutral, you have to monitor the more precise Energy Info screen while driving, or buy an SOC/power/voltage meter from garygid.

If I am going to be coasting for a good while, I will sometimes shift into Neutral so as not to have to feather the A-pedal. And given my inability to feather the pedal with absolute perfection, I believe 'N' to be more a little more efficient for long coasts. But for short coasts, I don't find shifting to be worth the hassle. Also, the transition from 'N' to 'ECO' or 'D' can be a little jerky, and potentially lossy, due to the sudden jump to a different power level.
 
Newbie question: is regen disengaged when in neutral? Is the idea that it's more efficient to coast at high speed than to use regen which slows you down and makes it necessary to apply power sooner?
 
Scowbay said:
Newbie question: is regen disengaged when in neutral? Is the idea that it's more efficient to coast at high speed than to use regen which slows you down and makes it necessary to apply power sooner?

Yes, no regen or power in Neutral. No regen at 100% battery.

Coasting uses no power and gains no power (beyond the static loads of climate control, radio, lights, horn, etc).

Regen takes energy to produce (that you could be freely rolling) of which there are losses to put that power in the battery, and losses to extract it. It also means you used too much energy prior to the stop sign to need brakes/regen.

If I accellerate at 80kW (max available), I only have about 40 kW max of regen available to slow down. 50%, and that doesn't include all the losses mentioned above. Obviously, that's not normal driving, but even the best case scenario doesn't equal coasting.

Ideally, you would coast to every stop sign, meaning you only used enough power to get to the stop. Naturally, this won't work on a down hill portion before the stop, which will require braking (either friction or regen, or both).
 
Ok - I get it. Thanks.

Seems to me that if I were designing an electric vehicle, I'd put a paddle control on the steering wheel that would allow me to dial in strength of regen, from 0 to some safe max. Not unlike downshifting in current cars.
 
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