Regen question

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I'd like a lever that is along side the gear selector, that I can pull back to provide drag (regen).

The lever to the left of the #1 engine thrust lever, label "SPD BRK" (Speed Brake):


1929052.jpg
 
Is it safe to assume that as long as the regen indicator doesn't hit the 30KW max that you never use the mechanical brake?

I must say that the few times I exceed 30KW regen, I can't really tell at all whether the system started activating the mechanical brakes or not. It's indistinguishable to me.

You know, like how on a hybrid you can tell when the enginer turns on or shuts off, even ever so slightly? Well, on the other hand, the switch from regen to mechanical braking can't be detected at all.

And as I continue to apply pressure and the car slows down further, the regen bars retract away from 30KW back down to 20KW or 10KW before coming to a complete stop. I wonder if the mechanical brakes, once activated after 30KW, stay on until the car reaches a complete stop or not? Or whether there is truly regen going on when the indicator goes back down from 30KW to 0 KW.

If the brakes get deactivated and regen takes over again on the way back down from 30KW to 20 to 10 to 0, I sure as hell can't tell. I'd also think it can't be that kind of switching back and forth between mechanical and regen that quickly and seamlessly. So I gotta assume that once the mechanical braking takes over, it stays on until the car stops. Unless you release the brakes before that happens, like riding downhill. In which case regen kicks right back in.

Then, at a complete stop, I must assume that the mechanical brakes stay on to keep the car in place, right? Surely, you can't rely on regen to keep a car stationary in place.
 
Volusiano said:
Is it safe to assume that as long as the regen indicator doesn't hit the 30KW max that you never use the mechanical brake?
No. Depending on conditions friction brakes may engage at any time. There's a lot of things which may cause friction brakes to kick in under 30 kW. Primary culprits that I've noticed.

1. Battery over 80% full.
2. Quick application of the brakes.
3. Hard braking.
4. Bumpy road.
5. Low vehicle speed.

There's probably a few more.

Volusiano said:
I must say that the few times I exceed 30KW regen, I can't really tell at all whether the system started activating the mechanical brakes or not. It's indistinguishable to me.
It's not always easy to tell when the friction brakes engage. Sometimes you can hear/feel them.
 
One way to tell how well you are using regen is to look for brake pad dust on the front wheels when you wash the car. I washed ours yesterday and found very little and the car only gets washed about once every other month.
 
That's not necessarily an accurate indicator. Our Acura never produces any brake pad dust on the wheels even after months of not being washed...

ERG4ALL said:
One way to tell how well you are using regen is to look for brake pad dust on the front wheels when you wash the car. I washed ours yesterday and found very little and the car only gets washed about once every other month.
 
You could rig up an indicator if you monitor the hydraulic pressure in the brake lines.

Its just easier to drive like you had no brakes at all.
 
Volusiano said:
Then, at a complete stop, I must assume that the mechanical brakes stay on to keep the car in place, right? Surely, you can't rely on regen to keep a car stationary in place.
This is correct. The friction brakes are needed to bring the car to a complete stop and hold it there.

Herm said:
You could rig up an indicator if you monitor the hydraulic pressure in the brake lines.
There are two approaches that I use to determine whether the friction brakes are being applied or not:
1. Monitor the Energy Info screen. If you increase pressure on the brake pedal and the amount of regen (kW) doesn't increase (go further below zero on the circular, graphical gauge), then you know that the additional braking is being done by friction.
2. Look at the efficiency meter above the tree growing area. The indicated efficiency will drop whenever the friction brakes are applied. Of course, that's not the only thing that affects the indicated efficiency.

planet4ever said:
Nissan seems to be concerned about doing too much QC, so the regen level might also be about as high as they feel comfortable having it. Of course most people don't regen for 30 minutes straight, but if you're dropping 5000' feet like abasile does routinely ...
For most of this descent, we are doing in the range of 10-20 kW of regen. I believe I remember reading that QC can do 0%-80% (usable) SOC in 26 minutes, and that it maxes out at 50 kW, with a taper as the SOC approaches 80%. So I don't think we are hitting the battery quite as hard as QC.

It seems to me that Nissan is more conservative with continuous regenerative braking than with quick charges, probably because the former could potentially be done with much greater frequency. Even when the car has a little under nine bars of charge, we notice that it starts to limit continuous regen to ~ 10 kW after we've descended a few thousand feet. At 10 bars after a few thousand feet of descending, we might only have ~ 3-5 KW of regen. This is why we like to start our descents with 7-8 bars of charge or less. We've been making that 5000 foot descent once or twice per week on average.
 
abasile said:
planet4ever said:
Nissan seems to be concerned about doing too much QC, so the regen level might also be about as high as they feel comfortable having it. Of course most people don't regen for 30 minutes straight, but if you're dropping 5000' feet like abasile does routinely ...
For most of this descent, we are doing in the range of 10-20 kW of regen. I believe I remember reading that QC can do 0%-80% (usable) SOC in 26 minutes, and that it maxes out at 50 kW, with a taper as the SOC approaches 80%. So I don't think we are hitting the battery quite as hard as QC.
Yeah, I kinda guessed you wouldn't be using 30 kW regen all the way down, which is why I cheated by not finishing the sentence. ;)

On the QC charge, page CH-6 of the Owner's Manual shows "approximately 0.5 hours," so, yes, the time could be 26 minutes, but I expect it is really variable. More significantly, the chart shows the half hour QC time as being from Low Battery warning to 80%. Tony's chart shows bar 1 disappearing at 17% SOC and has a footnote that LBW can be before or after that. My experience is that LBW comes quite a while before the last bar disappears, so I thought I was being generous by calling it 15%. It might be more like 20%. Next, I don't remember seeing any figures on taper below 80%, and assumed that only happened above 80%, but I could be wrong. To back up my calculation, I do remember seeing reports that actual QC draw was running at about 30kW even though theoretically it could go up to 50kW.

Ray
 
Tony's chart shows bar 1 disappearing at 17% SOC and has a footnote that LBW can be before or after that. My experience is that LBW comes quite a while before the last bar disappears, so I thought I was being generous by calling it 15%. It might be more like 20%.


I think the note says that LBW may happen before Bar 1 disappears. The 17% (48 of 281 raw data) is when LBW comes on, regardless of Bar 1. The chart is not clear as to when Bar 1 will disappear, since the range planning would be based on LBW from that moment on.

I actually don't know when the number 1 Bar is supposed to disappear (by SOC data). It's not important to range.
 
planet4ever said:
So our 30kW regen is already hitting the battery about as hard as QC does.
I'm with Ray on this, in fact that was my first thought when I saw the requests for more regen. I believe that this video has been posted on the forum before, but it's worth watching is you haven't seen it yet. It's in Dutch with English subtitles ;-)


Click to play video
 
tps said:
Yep, the motor continues to spin the same direction. The direction of current flow, however, reverses. Current always flows from higher voltage to lower voltage. When charging a battery, the charger has to put out a higher voltage than the battery's voltage to get current to flow into the battery. The inverter is where this magic happens when the car is moving. You can think of it almost as having a continuously variable voltage conversion ratio between motor and battery. To provide power to the motor, it must adjust the ratio so that to motor will draw the desired amount of current from the battery. To regenerate it adjusts the ratio so the battery will draw current from the motor (acting as a generator). This is not really so much different from down-shifting the transmission in a car to get a bit of engine braking effect when going down a hill, except that an ICE engine just wastes all the braking it's doing as more heat exhausted through the radiator, whereas the inverter puts the energy (minus the conversion and charging loss which is exhausted as heat from the LEAF') back into the battery.

This is an excellent description of how a brushed DC motor controller works. The Leaf, however, uses a brushless motor which is far better for three main reasons 1) no brushes to wear out and 2) the coils are on the *outside* housing (stator) instead of on the inner rotating part (rotor) so you can get the heat out much easier and 3) you get regen for free. Brushless motors use permanent magnets on the rotor. The controller monitors the position of the rotor and simply activates the coils on the outside in sequence. When coasting, it will align the coils exactly with the rotor field. When accellerating it just adjusts the timing of the coil activation to lead the rotor (drawing power from the battery and pulling it along). For braking it changes the timing to lag the rotor causing the rotor to induce voltage in the coil - sending charge back to the battery. There's extra detail to deal with back EMF and limit regen but it's actually a quite elegant solution. 80% efficiency is very doable with good fast switching, low resistance switches - in fact almost required. If you are less efficient then that you have to figure out how to get that heat *out* of the controller.

I have a lot of respect for the engineering in the controller. Some buddies and I designed a smaller (18hp) brushless motor controller for a battlebot a few years ago and we literally blew up one prototype (and others just caught on fire) before we worked out all the kinks - there's a lot of energy moving around in that thing!
 
TonyWilliams said:
The 17% (48 of 281 raw data) is when LBW comes on, regardless of Bar 1. The chart is not clear as to when Bar 1 will disappear, since the range planning would be based on LBW from that moment on.
Ah! Thanks for the clarification, Tony. I can now see that is what you were saying, but I was just too dense to catch the subtle point. I agree, the bars no longer matter once you get the Low Battery Warning.

So, to rephrase my point just slightly, Nissan's definition of a QC in "about" half an hour is for 80% - 17% = 63% of the usable battery capacity [to which the lawyers would want to add: with a new battery, standard temperature , standard voltage, maybe some other factors].

Ray
 
planet4ever said:
Nekota said:
The 80% efficiency is the product of the motor acting as a generator (about 90%) and the inverter box capturing the regen AC power back into DC to charge the battery (another 90% operation resulting in 0.9 x 0.9 = 0.81).
Ah, but you forgot half the story. You then have to convert the stored energy back into mechanical energy, perhaps at 81% efficiency again. 0.81 * 0.81 = ~0.66. I personally doubt the round-trip efficiency is much higher than 70%, though I could buy 85% in each direction, which would get it up to 72%.

Ray

Well the question was the efficiency of regeneration back into the battery which is a one way trip. Of course the system loses energy when going from the battery to the traction motor and your description is what I would expect on slowly climbing a hill with the traction motor and then recharging the battery on descent which would result in your ~70% 'cycle' efficiency. The KW loss going from the battery to the wheels is described by Nissan to have an efficiency range of 87% to 95%



Which leads to the question of what the 80KW and 30KW values represent? Are they the energy at the input of the motor or inverter or battery?
 
TickTock said:
tps said:
Yep, the motor continues to spin the same direction. The direction of current flow, however, reverses. Current always flows from higher voltage to lower voltage. When charging a battery, the charger has to put out a higher voltage than the battery's voltage to get current to flow into the battery. The inverter is where this magic happens when the car is moving. You can think of it almost as having a continuously variable voltage conversion ratio between motor and battery. To provide power to the motor, it must adjust the ratio so that to motor will draw the desired amount of current from the battery. To regenerate it adjusts the ratio so the battery will draw current from the motor (acting as a generator). This is not really so much different from down-shifting the transmission in a car to get a bit of engine braking effect when going down a hill, except that an ICE engine just wastes all the braking it's doing as more heat exhausted through the radiator, whereas the inverter puts the energy (minus the conversion and charging loss which is exhausted as heat from the LEAF') back into the battery.

This is an excellent description of how a brushed DC motor controller works. The Leaf, however, uses a brushless motor which is far better for three main reasons 1) no brushes to wear out and 2) the coils are on the *outside* housing (stator) instead of on the inner rotating part (rotor) so you can get the heat out much easier and 3) you get regen for free. Brushless motors use permanent magnets on the rotor. The controller monitors the position of the rotor and simply activates the coils on the outside in sequence. When coasting, it will align the coils exactly with the rotor field. When accellerating it just adjusts the timing of the coil activation to lead the rotor (drawing power from the battery and pulling it along). For braking it changes the timing to lag the rotor causing the rotor to induce voltage in the coil - sending charge back to the battery. There's extra detail to deal with back EMF and limit regen but it's actually a quite elegant solution. 80% efficiency is very doable with good fast switching, low resistance switches - in fact almost required. If you are less efficient then that you have to figure out how to get that heat *out* of the controller.

I have a lot of respect for the engineering in the controller. Some buddies and I designed a smaller (18hp) brushless motor controller for a battlebot a few years ago and we literally blew up one prototype (and others just caught on fire) before we worked out all the kinks - there's a lot of energy moving around in that thing!
Thank you for this enlightening insight into how it really works. It makes a lot of sense now.
 
After searching this site using Google, I found the DC Quick Charge plot and associated post, dated March 5, 2011, that I was basing some of my assumptions on. From http://www.mynissanleaf.com/viewtopic.php?f=9&t=2919#p66586" onclick="window.open(this.href);return false;:

DarkStar said:
Sounds like you're asking about "DC Quick Charging" since there is no such thing as Level 3 (L3) charging yet...

From what I've been told, here is a typical DC Quick Charge profile:

DCQuickChargeProfile.jpg


This is a Nissan LEAF with over a 50% SOC. It starts out at 48 KWh and 5 minutes later starts decreasing.

Hopefully once I get my LEAF, I'll be able to get a charging profile graph from our DC Quick Charge station here in Portland and we'll find out how quick it goes for sure. My understanding is that from 0% SOC it's fairly linear to 80% SOC and that takes just about 26 minutes. This jives given a rate of 48 KWh. The last 20% is what you see decreasing there on the graph, taking about 20 minutes. So in theory, the first 80% is charge at a rate of 2.46 miles per minute of charge, then it decreases to about an average of 1.25 miles per charge per minute.

Now, I am going to question whether the subject vehicle actually had over 50% SOC at the start of Quick Charging; perhaps DarkStar was making a faulty assumption. If this is "a typical DC Quick Charge profile", then perhaps it actually reflects charging from roughly 25% SOC up to 80%. If that is the case, then at 80% SOC the maximum continuous charge rate would be about 12 kW. That could explain why I am finding continuous regenerative braking to be so limited as the SOC approaches 80%. In the latter portion of each of our 5000 foot descents, perhaps we actually are hitting the battery as hard as the tail end of a "typical" QC.
 
Did the SOC value for the "low battery" warning increase with the newer firmware revisions (e.g. to provide extra buffer)? If so, then maybe the 0-80% quick charge in 1/2 hour (as documented in the original LEAF manual) actually gets you a bit more charge than you would get if the manual reflected the current firmware revision.
 
abasile said:
you have to subtract aerodynamic and friction losses before arriving at a figure for the amount of kinetic energy available to be captured via regen. If you do this, then it is not unreasonable to expect 80% efficiency. Coming down 5000' from my home in the San Bernardino Mountains, I think I am getting close to this, but it is hard to get a precise number without fine-grained SOC information.
Since I completed my SOC meter last week, I have done one quick regen test up and down a steep street here in Claremont. For those who know Claremont, it was the North end of Mountain Ave. Google Earth shows an altitude rise of 508' over distance of 4175' = 12% grade. I was in ECO mode with no A/C.

I set the SOC meter to display what I call "clicks" (the integer units the SOC meter converts to % by doing a division), since these are the finest-grain measurement units available. Going up I expended 12 clicks (what I call the units the SOC meter converts to %) driving up, and 5 clicks of regen returned coming back down. I was never going over 30 mph, so aerodynamic losses were low. <<Edited - added new data below>>

To measure the loss due to motion, I did a separate test on near-level ground at similar slow speed, on a circular route so that gravity effects would average-out. After two complete trips around the course, I had lost 7 clicks over 2.2 miles;
2.2/7 = .31 miles/click motion loss
BTW I use a tire pressure of 39 lbs/in^2.

Applying this result to the regen test course,
(4175'*2/5280 = 1.58mi)/(.31miles/click) = 5.1 clicks loss due to motion (rolling + aero).
Energy available for regen recovery: 12 clicks - 5 = 7 clicks.
Regen efficiency = (5 clicks/7 clicks) = ~73% +- ~7%.

I was hoping for higher efficiency, but this course was a little too short for an accurate calculation.

<<End Edit>>>
Because I had to apply light brake pressure most of the time on this steep grade, I probably lost some energy to friction braking. If I can find a steady grade not quite so steep, with a higher safe speed, where I don't need to apply any brake pressure, I can hopefully get a higher regen efficiency reading.

Whenever I have my foot on the brake, even lightly, I worry that some friction braking may be engaged. I am not sure how to “feel” whether there is friction braking, at least at low brake pressures. At higher pressures I am sure friction is engaged.

Assuming one has plenty of spare battery capacity, so that all double circles show, I have found two tricks for maximizing my regen. One is to keep the speed up, at least 20 mph. The other is to apply a brief strong braking pulse. This will often help kick the regen into 4 or 5 circles. Of course this also briefly engages friction braking, but often most of the higher regen level will remain after I have removed most or all brake pressure. Have others observed this ?
 
Nekota said:
The KW loss going from the battery to the wheels is described by Nissan to have an efficiency range of 87% to 95%



Which leads to the question of what the 80KW and 30KW values represent? Are they the energy at the input of the motor or inverter or battery?
I have also wondered about this.

This is a very nice graph. Do you have a link to the report from which it was extracted ?
 
tbleakne said:
If I can find a steady grade not quite so steep, with a higher safe speed, where I don't need to apply any brake pressure, I can hopefully get a higher regen efficiency reading.
The road that we use to reach our mountain town (CA-330) seems pretty good for that purpose. When there is not much traffic and the SOC is sufficiently low, it is possible to use ECO mode regen to keep one's speed down with little to no use of the brake pedal. You'll have to come visit! :D
 
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