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Unclear on regenerative braking

Sat Mar 23, 2013 11:04 pm

I see people here talking about coasting instead of braking to recharge the battery, and people talking about braking instead of coasting to recharge the battery. The way I understand it, regenerative braking occurs when you're coasting while slowing down, like before you have to come to a stop, or going downhill; or it occurs when you're actually stepping on the brake. What I don't get is if one form of regen braking is better for recharging than the other.

And I often read here about avoiding regen braking whenever possible, and I don't understand why you'd want to do that if you want to increase range.

We're averaging only 3.5m/kwh (to me it's "only") in Los Angeles street traffic, all stop and go, no timed signals unless it's by serendipity, so it's just about impossible to drive more than 2-3 blocks without having to come to a complete stop. We're always in Eco. We almost never charge to more than 80-85% and never let it drop to less than 30%. Very little hill driving, no heat/almost never AC (at this time of year). On Carwings we get good marks on regen braking and accessories, not so good on the other scores. We accelerate slowly and gradually. I'm guessing whether we zig-zag through town on side streets to avoid traffic messes is no better or worse than just staying on the miles-long, straight stretches of Wilshire or Beverly Blvds., for example, and stopping and going every 2-3 blocks, because the zig-zagging involves turning into and then accelerating out of a corner.

To recharge the battery more, should we coast as long as possible while slowing for a stop before hitting the brakes? (this is an art, I've discovered, because Eco slows you down so fast that you actually might never even make it to the stop without having to tap the accelerator a bit). Or should we use the brake pedal for as long as we can while coming to a stop? Currently I'm going back and forth on this.

As things stand now, we can leave the house with a predicted 80-mile range, make a 20-mile roundtrip, and get home with a 40-mile predicted range. If possible, I'd like to be able to get home with 60 miles left. Or is that delusional? And I should be happy to be getting 3.5m/kwh?

Another question while I'm at it: From looking at my Carwings data, I'm estimating that the ratio of home charging to regen charging is about 2-2.5:1. Does that sound right? I'm going by the ratio of energy consumed by the traction motor to energy captured by regen braking. The other day, for example, it was 456:186, or about 2.5-1 (much worse if we go on the freeway unless it's slow-and-go). But I'm no techie, no math whiz, no electrical engineer, and frankly a lot of the talk I read here goes zooming right over my little bitty writer's head.

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Re: Unclear on regenerative braking

Sat Mar 23, 2013 11:23 pm

Coasting is using no braking - no mechanical braking, and
no Regen braking. Coasting generally wastes little energy.

Using Regen, either from taking your foot off the accelerator
pedal, or by pressing the brake pedal, wastes some energy,
but does recover some energy, so it is less wasteful than
the mechanical braking, which wastes all the braking energy.

Sometimes it is necessary to apply braking to slow the car.
If possible, use Regen to slow the car, but when that is
unavailable or insufficient, the car applies the mechanical
brakes when you press the brake pedal.
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Re: Unclear on regenerative braking

Sat Mar 23, 2013 11:28 pm

That's a good way to state it. When you're doing regen, there are conversion and resistive losses (in the wiring, battery, circuitry, etc.)

Forget the GOM.

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Re: Unclear on regenerative braking

Sun Mar 24, 2013 12:39 am

To emphasize a bit more what Gary said, coasting is either shifting into Neutral or the rather difficult trick of watching the left pie chart on the center console Energy Usage display and keeping it blank - not even a sliver of power or regen. The only kinetic energy you lose that way is from friction - air against the car, wheels on the road, bearings and gears. Those losses always happen; they can't be avoided. So that is the most efficient way to slow down. That is what the hypermilers do. Frankly, I don't, and I don't get the mileage they get. Personally I consider a moving car in neutral to be dangerous, and keeping my eyes on the center console to be equally dangerous. The closest I come is moderating the accelerator so that only a single dot shows on the dash. That's only a rough approximation to coasting, but I can watch it with my peripheral vision while keeping my eyes on the traffic around me.

Now, as to what cwerdna said about GOM: You didn't use that term, and his reference may be puzzling you. But you did say, "We almost never charge to more than 80-85% and never let it drop to less than 30%." If you got your car in January I have to assume it is a 2012 model, not a 2013 model. The 2012 does not have any gauge that tells you percent of charge. So, like cwerdna, my guess is that you are referring to that big number on the dash that Nissan calls "Distance to Empty" and we call "GuessOMeter" or "GOM". It is only very vaguely related to charge, and it is truly a guess being made by a computer (I would say a badly programmed computer) as to where you are going to drive and how you are going to behave in the future. Please ignore it.

You do have a "gas gauge"; it's those 12 blue and white bars surrounding the GOM. When all 12 of those are gone, you are on "empty". Like a gas car, the gauge is calibrated so that "empty" gives you an early warning. But the LEAF gives you three much more specific warnings.
  • The first is "low battery" which we call LBW. The GOM starts flashing when you get down to this point. It's nothing to worry about - you are really about as far from empty as charging to 80% is from full.
  • The second is "very low battery" which we call VLBW. At this point the GOM "gives up" and just shows three dashes. You really are getting fairly close to empty at this point, and pushing much further might reduce the life of the battery.
  • The third is "Turtle". A reduced power message pops up on the dash and a picture of a turtle shows up there. This, my friend, is just about the end of the road. Find the closest safe place to stop, and call for help. Hopefully you will never see the turtle, but even if you do, know that Nissan has programmed the car to protect the battery. It will stop the car, likely within half a mile, and there will still be a few percentage points of charge remaining.

So the next time the GOM says 30 just grin at your wife and tell her this little beauty hasn't begun to show you what it can do. You haven't lived until you hear the mellifluous voice of the lady who tells you how low your battery is.

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Re: Unclear on regenerative braking

Sun Mar 24, 2013 7:45 am

Seems to me that 3.5 m/kW is a bit low (unless it's colder in LA than I think); I consistently get 4+. Since you obviously are thinking about what you can/can't do, I suggest trying the following:
1) try "D" instead of "ECO" for a few days and just limit your acceleration. I only use ECO in low battery situations as I don't like the drag it places on the car (as you've mentioned)
2) if by "coasting" you're actually shifting into "N", I would definitely avoid that. I brake much less than I did in an ICE car simply because the natural regen of an EV (even in "D") makes for smooth deceleration.

Bottom line: acceleration and (top) speed are the real killers of EV range (OK, and cold temps) 8-)
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Re: Unclear on regenerative braking

Sun Mar 24, 2013 8:05 am

Well the good news for OP is they have much room to increase their efficiency. I average 5.7 m/kwh and I never shift into N. I stay in ECO and coast in anticipation of lights, congestion etc. As one adjusts to the amount of drag from regen coasting one learns when to remove foot from accelerator to come to a stop without using the brake.
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Re: Unclear on regenerative braking

Sun Mar 24, 2013 8:14 am

To improve energy economy. Whenever possible prioritize in this way:
1. Coast as long as possible. I use the A-pedal to zero the power. This provides the best control. I tried using "N" but it takes time to reengage "D" or "ECO" in the event you suddenly need power.
2. When you need to slow, lift slowly from the A-pedal and use regen. If you do not need to slow as much as the regen provides then tip in slightly into the A-pedal to modulate the regen.
3. Finally when braking is necessary, do it as smooth and gently as possible.

When accelerating I try to slowly raise the power level to between 20 and 40kW. This is a more efficient zone than higher levels, also more efficient than very low levels.
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Re: Unclear on regenerative braking

Sun Mar 24, 2013 10:17 am

garygid wrote:Coasting is using no braking - no mechanical braking, and
no Regen braking. Coasting generally wastes little energy. garygid

I see I created a semantic issue by using "coasting." By that I meant simply taking my foot off the accelerator and using the regen braking that supplies, as opposed to stepping on the brake pedal. I never and never would put the car in neutral to coast. I'm still learning here - both terminology and gauges. Thanks for your help.

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Re: Unclear on regenerative braking

Sun Mar 24, 2013 10:43 am

wlegro wrote: I never and never would put the car in neutral to coast.
Why not? Yes, I know that conventional wisdom said don't coast, as you will lose control. But with the LEAF it is such a simple thing to get back "in gear" - just a flick of the wrist. I think it is neat "working the gears", flipping between N, D, and ECO. Just wish I had B.
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Re: Unclear on regenerative braking

Sun Mar 24, 2013 10:56 am

i live in la and the best range is off the freeway, unless the freeway is slowed to 35-50.

i routinely got 4.5-5 on my 50-mile rdtrp to work and back with 80% freeway, but then changed tires and added some rolling resistance with michelins and now get 4-4.5, with occasional forays under 4 on the part of any trip that is freeway heavy.

you can do better with practice.
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