Understanding regenerative braking

[TickTock]

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).

This is a little misleading. The round trip efficiency had been shown to be near 85% by multiple people. So if you break using regen, you will be able to accelerate back to 85% of the starting energy. The fact that regen is only 1/2 the max acceleration only means you cannot brake as fast as you accelerate without engaging the fiction brakes. It doesn't limit how much of the energy you recover, though. Just the rate at which you recover it.

 

[LEAFfan]

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.

The VW e-Motion is supposed to have exactly that...an adjustable regen lever.

 

[TonyWilliams]

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.

This is a little misleading. The round trip efficiency had been shown to be near 85% by multiple people. So if you break using regen, you will be able to accelerate back to 85% of the starting energy.

So, what is misleading? We're comparing regen to coasting.

The fact that you claim 85% doesn't equal coasting.

Nor does 50% or less in the example I gave.

 

[SanDust]

LOL, where did you come up with that stat?

I assumed this would be obvious but apparently not. It's not a stat actually, it's a fact. However, my bad, I missed by a few MPG when doing the math in my head. Perhaps that is what threw you off so much.

If not here are the facts: The EPA rates the Leaf as having a range of 73 miles. Using the very same tests it rates the Prius as delivering an 50 MPG. As a consequence, any drive cycle that delivered 100 miles of range for the Leaf would have to deliver 68 MPG in a Prius. (100/73 X 50). So claiming a 100 mile range for the Leaf is exactly as valid as claiming 68 MPG for the Prius. FWIW if you believe that a Leaf has a range of 100 miles then you believe that the Volt has an EV range of 48 miles. No doubt some people on the right day will be able to go 100 miles in a Leaf or 48 miles in a Volt or go 68 miles on a gallon of gas in a Prius. But that's going to be the exception not the rule.

 

[SanDust]

The fact that you claim 85% doesn't equal coasting.

No way. Obviously converting kinetic energy to magnetic energy to electrical energy to chemical energy would be more efficient than eliminating all conversions.

 

[planet4ever]

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.

I agree with you, but LEAFfan clearly seems to be saying something different.

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.

Please explain to us, LEAFfan, what this mysterious speed-dependent energy draw is, and what, electrically or mechanically, is different between neutral and feathering the pedal perfectly. True, when you shift to N the shift control system must send the inverter a message to ignore the pedal messages. But what's the difference between that and the pedal not sending any messages that trigger either power or regen? Apart from those messages, are you claiming that there is something different in the mechanical power train, or in phase or amperage of power being sent to the motor? I don't think there is any power going to the motor in either case.

By the way, I have certainly seen the bright slice disappear completely on the Electric Motor pie chart when feathering the pedal at fairly high speeds, even freeway speeds. Admittedly it is difficult to keep it completely dark for very long.

Ray

 

[LEAFfan]

I'm going by the energy screen meter and it always shows energy being used in the neutral bubble in ECO. I haven't tried the feathering/neutral bubble in 'D', which someone said is where the needle disappears. I didn't realize that it would make a difference. I'm going to try 'D' tomorrow and watch the energy screen. Why would it (the pointer) disappear in 'D', and not in ECO when both are in the neutral bubble?

 

[Volusiano]

I'm going by the energy screen meter and it always shows energy being used in the neutral bubble in ECO. I haven't tried the feathering/neutral bubble in 'D', which someone said is where the needle disappears. I didn't realize that it would make a difference. I'm going to try 'D' tomorrow and watch the energy screen. Why would it (the pointer) disappear in 'D', and not in ECO when both are in the neutral bubble?

The neutral bubble is a lot coarser than the indicator on the energy screen. You can be in the neutral bubble but still using up a little bit of energy. However, if you pay attention to the energy screen, you can feather the pedal just a little bit more and get the needle in the energy screen disappear in either D or ECO. I've been able to do it and hold it there easily (disappearing needle).

The bottom line is that you can treat the neutral bubble as your coarse indicator, and the electric motor energy meter as your fine indicator.

 

[edatoakrun]

LOL, where did you come up with that stat?

I assumed this would be obvious but apparently not. It's not a stat actually, it's a fact. However, my bad, I missed by a few MPG when doing the math in my head. Perhaps that is what threw you off so much.

If not here are the facts: The EPA rates the Leaf as having a range of 73 miles. Using the very same tests it rates the Prius as delivering an 50 MPG. As a consequence, any drive cycle that delivered 100 miles of range for the Leaf would have to deliver 68 MPG in a Prius. (100/73 X 50). So claiming a 100 mile range for the Leaf is exactly as valid as claiming 68 MPG for the Prius. FWIW if you believe that a Leaf has a range of 100 miles then you believe that the Volt has an EV range of 48 miles. No doubt some people on the right day will be able to go 100 miles in a Leaf or 48 miles in a Volt or go 68 miles on a gallon of gas in a Prius. But that's going to be the exception not the rule.

There is only one factual statement in your post above, that "...The EPA rates the Leaf as having a range of 73 miles."

 

[TickTock]

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.

This is a little misleading. The round trip efficiency had been shown to be near 85% by multiple people. So if you break using regen, you will be able to accelerate back to 85% of the starting energy.

So, what is misleading? We're comparing regen to coasting.

The fact that you claim 85% doesn't equal coasting.

Nor does 50% or less in the example I gave.

I wasn't disagreeing with the statement the regen doesn't equal coasting. Clearly coasting will always get you there with less energy then maintaining speed via traction/regen. I was just objecting to the language that suggested that using regen + traction will suffer more than 50% loss. It may have not been what you meant to say but that's how it read to me. The max power of acceleration and max power of regen has nothing to do with the efficiency.

 

[camasleaf]

But what's the difference between that and the pedal not sending any messages that trigger either power or regen? Apart from those messages, are you claiming that there is something different in the mechanical power train, or in phase or amperage of power being sent to the motor? I don't think there is any power going to the motor in either case.

Maybe not in the motor, but is the inverter still "gating" in D or ECO? Is the inverter not "gating" in neutral? See http://www.mynissanleaf.com/viewtopic.php?f=31&t=6541" onclick="window.open(this.href);return false;

turbo2ltr wrote "While gating but not applying torque, IIRC, our 42kW inverters draw about 450W." A 80kW inverter could draw 850W. The SOC meter might be only reading the output of the inverter. For the mechanical power train I do not see much differences, it is allways coupled to the motor.

 

[planet4ever]

He later retracted that statement because he was thinking of motors he works with rather than the LEAF motor:

I guess my brain was turned off when I replied. The motors we use aren't PM motors so we always have a draw for the magnetizing current even when not moving. Apples and oranges.

Ray

 

[LEAFfan]

I'm going by the energy screen meter and it always shows energy being used in the neutral bubble in ECO. I haven't tried the feathering/neutral bubble in 'D', which someone said is where the needle disappears. I didn't realize that it would make a difference. I'm going to try 'D' tomorrow and watch the energy screen. Why would it (the pointer) disappear in 'D', and not in ECO when both are in the neutral bubble?

The neutral bubble is a lot coarser than the indicator on the energy screen. You can be in the neutral bubble but still using up a little bit of energy. However, if you pay attention to the energy screen, you can feather the pedal just a little bit more and get the needle in the energy screen disappear in either D or ECO. I've been able to do it and hold it there easily (disappearing needle).
The bottom line is that you can treat the neutral bubble as your coarse indicator, and the electric motor energy meter as your fine indicator.

Ah, I've always been putting it into the neutral bubble with the CC and it always shows a little energy being used. Now, what you are saying makes sense. I'll have to try that.