Stoaty's Guide to Energy Efficient Driving of the Leaf

davewill said:

You post this a lot, and I have tried this several times but it never works. The car soon slows down 1 mph and starts using power at much the same rate as before.

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I agree any benefit from utilizing coast on the CC is temporary. It's gonna take the same amount of energy to move you forward at say 55mph regardless how you got there.

i think we are all overestimating the benefit of regen here.

the amount of power to slow to zero is much less than the power it took to get it there.

accelerating means wind friction and gravity are overcome. both of which ADDS to the effort to slow down which is power not available for regen

after we calculate that, then we can start with the losses associated with regen. one posted 66% efficiency, ok. i can buy that. now remove the energy from gravity and wind resistance and what is left?

30-40% maybe? either way, i think that is still pretty good although i doubt its that high.

DaveinOlyWA said:

i think we are all overestimating the benefit of regen here.

the amount of power to slow to zero is much less than the power it took to get it there.

accelerating means wind friction and gravity are overcome. both of which ADDS to the effort to slow down which is power not available for regen

after we calculate that, then we can start with the losses associated with regen. one posted 66% efficiency, ok. i can buy that. now remove the energy from gravity and wind resistance and what is left?

30-40% maybe? either way, i think that is still pretty good although i doubt its that high.

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The wind is there on both ends as a fixed constant, like a brake dragging. Regen losses are easily calculated in the system components. Everyone seems to miss the large losses due to the software limited LOW power of the LEAF regen under many circumstances which goes to brakes.

DaveinOlyWA said:

the amount of power to slow to zero is much less than the power it took to get it there.

accelerating means wind friction and gravity are overcome. both of which ADDS to the effort to slow down which is power not available for regen

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Sorry, Dave, but I think you are wrong on this one. Yes, it is true that if we run a hypothetical test on a frictionless system in a vacuum we should in theory be able to recover all energy when slowing that was expended while speeding up; and that is certainly not true with air in the system. But the losses due to air resistance are part of the cost of getting from A to B in less than infinite time. They are related to your instantaneous speed, and their instantaneous values have nothing to do with whether you are accelerating or decelerating. You are still making forward progress while slowing, and the air drag is a penalty against that progress, not against the deceleration.

Ray

DaveinOlyWA said:

30-40% maybe? either way, i think that is still pretty good although i doubt its that high.

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Yes, rolling resistance and drag and drive train losses (don't forget them, the Leaf's motor isn't very efficient at higher speeds) are dead weight losses. No way to recover those. The energy you can recover through regen is deceleration, either from slowing down or from coming down hills. As an overall number I'd think 30% would be on the high side but attainable.

Regen itself though might be 70% efficient in the best case (one way return, 55% round trip).

This is what I meant by saying that people use different numbers for regen efficiency so you have to understand what number they're talking about.

Regen is very effective if used at moderate speeds in heavy stop and go traffic, since it cant be 100% efficient then its always better to drive as if you had no brakes.. but it does take longer to get anywhere.

A modern motor/inverter should be as efficient accelerating or decelerating, and lithium ion batteries are already very efficient at moderate SOC usages so I would not be surprised if 85% or higher regen efficiencies (for a one way electrical roundtrip) are achieved... at least during some portions of the braking cycle.

planet4ever said:

Sorry, Dave, but I think you are wrong on this one. Yes, it is true that if we run a hypothetical test on a frictionless system in a vacuum we should in theory be able to recover all energy when slowing that was expended while speeding up; and that is certainly not true with air in the system. But the losses due to air resistance are part of the cost of getting from A to B in less than infinite time.

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I think he's agreeing with this though I may have misunderstood what he was trying to say. I find it easier to think in terms of what regen can do. Regen can turn kinetic energy into chemical energy. That's it. It can't turn heat into chemical energy. Hence anything that creates heat, such as drag which heats the air, or rolling resistance which heats the tires and road, can't be recovered by regen.

Herm said:

A modern motor/inverter should be as efficient accelerating or decelerating, and lithium ion batteries are already very efficient at moderate SOC usages so I would not be surprised if 85% or higher regen efficiencies (for a one way electrical roundtrip) are achieved... at least during some portions of the braking cycle.

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I can't see how 85% would be possible. Since the drive train is something like 80% efficient and you need 10% overvoltage for charging, something around 70% would seem to be the max best case for a one way trip from wheels to battery. Round trip, battery to battery, you're looking at something more like 55% (.8 X .8 X .9).

davewill said:

LEAFfan said:

7. To stay in the neutral bubble and to keep your speed constant once up to speed, you can use the CC by going one mph over what you want, then push down quickly on the button and it will put you in the neutral bubble.

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You post this a lot, and I have tried this several times but it never works. The car soon slows down 1 mph and starts using power at much the same rate as before.

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So you are saying that you use the same power from the energy screen with one bubble as I do with the neutral bubble? Interesting, but not with my LEAF. I can see a slight difference between the two, no matter what speed I'm going in the neutral bubble. In fact, at low speeds, I have seen ZERO for more than 30 secs (no needle) on the energy screen while it's in the neutral bubble. How do you think I can get 6.5m/kW h (dash) on a 22 mile city trip (no impeding traffic) and 9.1m/kW h on the 15mph loop otherwise? I always use the neutral bubble with CC.

LEAFfan said:

So you are saying that you use the same power from the energy screen with one bubble as I do with the neutral bubble? Interesting, but not with my LEAF. I can see a slight difference between the two, no matter what speed I'm going in the neutral bubble.

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No. I'm saying that the CC always goes back to using the same number of bubbles shortly after I try your "trick" of clicking the CC down 1 mph.

All you are likely seeing is the hysteresis in the power bubble display. The amount of power you are using is exactly the same in either case. In fact, you have likely wasted a bit by going above the desired speed and then slowing down...

davewill said:

LEAFfan said:

So you are saying that you use the same power from the energy screen with one bubble as I do with the neutral bubble? Interesting, but not with my LEAF. I can see a slight difference between the two, no matter what speed I'm going in the neutral bubble.

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No. I'm saying that the CC always goes back to using the same number of bubbles shortly after I try your "trick" of clicking the CC down 1 mph.

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well, in all things its still relative. if you can power up to 80kW and regen up to 30 kW that would be regen at 37.5% right?

ah, if only it were that easy!! but lets face it. power used is power used. there is no free lunch...BUT there is "wasted" lunch because the power that could be regenerated was equal to 35 kW, then we are wasting that 5 kW in heat from brakes.

problem is. even when driving flat, windless and efficiently. regen never puts it all back. it does not put half back and i challenge anyone to post #s that says they got ¼ back. i say the car is built to be efficient. so friction, bearing, wind drag, etc. i believe to not be more than X%... ahh, cant have an exact number since it changes with speed.

remember wind has direction, etc. so its always pushing back. even when in regen and anything else. so it is velocity lost and therefore energy from momentum not recaptured.

DaveinOlyWA said:

well, in all things its still relative. if you can power up to 80kW and regen up to 30 kW that would be regen at 37.5% right?

ah, if only it were that easy!! but lets face it. power used is power used. there is no free lunch...BUT there is "wasted" lunch because the power that could be regenerated was equal to 35 kW, then we are wasting that 5 kW in heat from brakes.

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I think you may know this, Dave, but the way you have stated things here is rather misleading. What we have in our universe is a law of conservation of energy; we do not have a law of conservation of power. Regen deals not with recovering power (kW), but with recovering energy (kWh).

There are two kinds of energy that regen can be useful in recovering: kinetic energy (based on velocity) and potential energy (based on altitude). It may well be that the efficiency of regen varies with speed, which takes us into calculus if we are trying to understand regen of kinetic energy. 50+ years ago I majored in math, but these days calculus makes my head spin, so let's keep things simple and look at regen of potential energy instead.

Here's a simple experiment that someone might be willing to do, though it is of questionable safety. You would need to live close to a freeway with a significant elevation change, such as close to the base of a thousand foot or more climb up to a mountain pass, and you would need to be able to measure charging energy at the wall:

• Charge to 100% at home and drive the freeway to the pass at a steady speed, say 65 mph.
• Come back down the hill at the same speed, using neutral whenever possible, and never using regen. Apply the brake in neutral as needed to keep your speed constant.
• Charge to 100% at home again, and repeat the test.
• This time come back down the hill in ECO mode, again holding the same constant speed, but never using neutral.
• Charge to 100% at home a third time.

The difference in kWh between the second and third charge is the amount of regen energy you recovered. Since the speeds were held constant and the same at all times, that was all recovered from the potential energy you had at the top of the hill. That potential energy can be calculated easily based on the elevation difference and the weight of the car (including driver).

I'm sure someone is going to post and say this is crazy, that for a fair test you need to allow the car to speed up going downhill. That is true if you are trying to compare the efficiency of regen with the efficiency of coasting, and certainly speeding up will give better numbers than holding your speed and using the brake. That could be a useful test for a third run, to see whether regen or freewheeling is more efficient at a particular starting speed. But the answer will vary based on the speed, and it will not tell you the efficiency of the regen itself.

My definition of the efficiency of regen, for my proposed test, is (energy recovered) / (potential energy difference at the pass). I am personally guessing that number will be somewhere between 50% and 70%, but that is only a guess.

Ray

SanDust said:

Since the drive train is something like 80% efficient and you need 10% overvoltage for charging, something around 70% would seem to be the max best case for a one way trip from wheels to battery. Round trip, battery to battery, you're looking at something more like 55% (.8 X .8 X .9).

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I've seen you, or others, mention this 80% drive train number several times. What is it based on? The SAE report includes a graph which seems to say it is much higher than that: LEAF electric powertrain efficiency

By the way, I mentioned to DaveinOlyWA above that power is not energy. Voltage is also not energy, so a 10% overvoltage does not imply a 10% energy loss.

Ray

P.S. Just so we have our terminology straight, here are the definitions from the Nissan LEAF Warranty Information Booklet:

• Powertrain coverage applies to components listed below, under the headings Drivetrain, Restraint System, and Electric Vehicle System
• DRIVETRAIN: Drive shafts, final drive housing and all internal parts, universal joints, bearings, seals and gaskets.

OK, here is one more piece of data on regen: my return commute involves a 1,000 foot descent on the 405 from the top of the Sepulveda pass. The elevation is lost over 4-5 miles. I usually coast in neutral at speeds up to 68 MPH, with very little use of regen. A couple of days ago, I decided to limit my speed to 55 MPH by setting the cruise control to that speed; car was in ECO mode. Thought I was getting a fair amount of regen over the next 5 minutes. When I got off the freeway, the instantaneous miles per KWh on the dash stayed at 8, indicating that I had some regen energy to use up. I was able to travel about 1.5 miles on fairly level road until the dash meter went back to normal. Since I normally get 5.8 miles per KWh, I estimated that I got about 0.26 KWh of extra energy to use for driving, or about 1/6 of a fuel bar. Looked at another way, my 47 mile round trip commute normally takes 8.1 KWh of battery (47/5.8). If I saved 0.26 KWh, then I only used 96.8% of the energy I would normally use for the trip, a savings of 3.2%.

Conclusion: regen in this situation saved a small amount of energy, and should increase my miles per KWh from 5.8 to 6.0 (47/(8.1-0.26)). I am going to test this out shortly, by using the nav miles per KWh (which reads 6.0 on a trip) to see if it increases to 6.2.

planet4ever said:

The difference in kWh between the second and third charge is the amount of regen energy you recovered. Since the speeds were held constant and the same at all times, that was all recovered from the potential energy you had at the top of the hill. That potential energy can be calculated easily based on the elevation difference and the weight of the car (including driver).
...
My definition of the efficiency of regen, for my proposed test, is (energy recovered) / (potential energy difference at the pass). I am personally guessing that number will be somewhere between 50% and 70%, but that is only a guess.

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I predict that number will be lower than 50%. For a non-steep descent at 65 mph, it should be far lower than 50%. Most of the potential energy will be used to overcome wind resistance.

Ray; ok i see where you are going and you are negating the "constants" so regen efficiency would be high. but the amount of power that is used to get up the hill is what i am getting at.

i tried something similar as far as trying to see what regen did. but controlling speed did not work out so well. even at max regen, i had to use friction brakes. tried this on the Puyallup Hill. problem is there is a very sharp turn at the bottom and even knowing it was there , the hill was too steep so pretty sure friction brakes played a pretty big part in controlling speed.

abasile said:

I predict that number will be lower than 50%. For a non-steep descent at 65 mph, it should be far lower than 50%. Most of the potential energy will be used to overcome wind resistance.

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Of course most of the potential energy will be used to fight the wind. Do you think you can travel at 65 mph on flat land without using any energy? But the test I proposed very appropriately cancels out the effect of the air resistance, so I still predict a number above 50%.

DaveinOlyWA said:

problem is there is a very sharp turn at the bottom and even knowing it was there , the hill was too steep so pretty sure friction brakes played a pretty big part in controlling speed.

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That is why I specified a freeway, which I assume would not have any sharp turns. You might still have to use friction brakes, depending on the grade, and that would bias the results.

Ray

it is a freeway. around here we dont reshape the land.

they did that once. brought in a gazillion yards of fill for the Hwy 101/I-5 interchange outside Olympia. it took 5 years, cost several hundred million and people still complain about the money that was spent and it was completed like 15 years ago

Stoaty, I'm very impressed with your efficiency and agree with your reasons, etc. Just curious, what's the pressure in your tires?