Regen vs Neutral and speed for max range

[jray3]

Thank you for the information.

Wonder if that would mean pulling the car while regenerating would actually pump up the battery really quickly

Yes, as this is essentially what happened when I took road trips with the pusher trailer. Considering that I could add bars while cruising at highway speed, it made for far faster progress than a CHAdeMO road trip, equaling the gasser trip times, but as a 20 mpg *******ized hybrid.

I still think the I-MiEV could make a great RV Tadpole, providing not just local transportation at one's destination, but boosted hill-climbing and acceleration in addition to sweet regen.

 

[Aerowhatt]

Aerowhatt, I can tell this is going to be a protracted discussion, and I appreciate your presenting the math which I need to stare at a bit. One of the variables compounding the problem is the non-linear relationship of aerodynamic drag with speed.

In trying to find an example that might accompany this discussion, I went back to a trip I made five years ago soon after I bought the car (and before CaniOn) that might be of some use - it briefly shows the fuel gauge going up by one bar on a fairly short stretch of highway in the Santa Cruz mountains. There was nothing special about this trip, as it had no serious hypermiling associated with it nor was I trying to maximize regen as regen was simply used to slow the car down to keep from exceeding the 50mph speed limit in the hills. The datataking was simply me reading the pertinent i-MiEV gauges and Garmin GPS readings into a voice recorder. FWIW, here it is (I just added the GPS Visualizer graph and you need to click on the link to see the primary pdf graph):
http://myimiev.com/forum/viewtopic.php?f=28&t=269&p=993#p993

I'll be taking your comments into consideration and also try to take some CaniOn readings to see if I can develop a scenario that will quantitatively further develop this topic. For now, we are at an impasse.

Although the ball is in my court, everyone is welcome to join in.

To be continued...

[All this Marching for Science and Climate Change over the last two weekends has me motivated to play with the numbers!]

Initially I had trouble with my data too because of the relationship of aero drag with speed. That's one reason I kept it to myself and used canion on every permutation of the mountain drives. It goes against our intuition that going faster in (N) could be as or more efficient. It took a lot of corroborating data to sway my intuitive preconception.

The post just above where I bumped this thread was about a 500 meter elevation climb and descent with up to 8% grades. Four bars to climb and one bar returned on a regen required for a speed controlled descent. Your drive where you had one bar return was a ~1,800ft decent where "regen was simply used to slow the car down to keep from exceeding the 50mph speed limit in the hills" some of the grades were steep enough for effective (efficient?) regen. There seems to be a rough consensus about how much power can be returned by regen based on elevation traversed since these numbers roughly agree with mine.

Since we know that there is a baseline power usage to have the inverter maintain an active zero torque field in the spinning motor. It follows that regen is more efficient the harder it is braking the car. Hence it yields good results on steeper grades. For example with made up numbers - Lets say that 10 battery amps are need to maintain that rotating field and run the accessory loads. The stronger the regen the more efficient the energy capture process is. Don said early in this thread that being in D with the power needle in the zero position down a grade is the same as coasting. Reasonable enough I thought at the time. It turns out though that what it really is 0% efficiency regen! All the regen is feeding the overhead of regenerating to begin with. Slip that car into (N) and it will pick up speed! In the example it takes ten amps worth of braking force on the car to break even. On my steep mountain grade I see spikes of -68 amps on canion. Efficiency (electric only) would look like 68/68 +10 or 87% -- pretty good right! At lower regen amp rates though. Say 10 amps indicated going into the battery looks more like 10/10 +10 = 50%. With 5 amps indicated going into the battery becomes 5/5 +10 = 33%. So for gentler grades it takes 15 amps worth of regen braking force to net 5 amps of current back to the battery. 15 amps of braking force feels reasonably significant by the seat of my pants. Which qualitatively helps to understand how it might be roughly equivalent to the significant difference in wind drag force between 35 and 65 mph??

Aerowhatt

 

[jray3]

I like the turn this thread is taking. If there is a lower amperage limit on regen efficiency, below which you do nothing but create heat in the motor and inverter, perhaps there is a good reason that Mitsu makes regen go away below about 12 mph. If you're just making heat, far better to do that in the brakes than in the electronics.

One thing that has been appealing to me about other EVs like the BMW i3 and the BOLT is their ability to smoothly regen down to a nearly complete stop. Maybe that's mostly for show...

 

[JoeS]

The following is Off Topic but related to this topic, so I thought I'd share…

----------------------------------------------------------------------------------------------------------
Science Saved My i-MiEV (and a Deer)

I've been lucubrating on Aerowhatt's insightful writeups and thought it's about time to start getting a feel for the data we have available on CaniOn.

Where I live, from the point my side road enters the winding country road there is about a 1-1/2 mile (2.4km) gentle downhill on that country road with a 30mph(48km/h) speed limit before hitting the stop sign at the bottom. My habit for 40 years when getting onto this road is to accelerate to about 35mph (56km/h) and then drop the car into Neutral and simply coast all the way down to the stop sign, using regen in B to slow down just before the stop sign. The i-MiEV holds that speed constant very nicely, whereas my previous Gen1 Honda Insight and now Tesla accelerate to the point where I needed to start slowing them down. I attribute this to wind resistance and their lower Cd than the i-MiEV.

This morning on my way to Jazzercise, after accelerating and kicking the i-MiEV into Neutral I wondered what would happen if I simply dropped into D and let the car slow down of its own volition. Since there was no traffic, I did just that and was dismayed as the car slowed down more and more (after all, it is a gentle grade) … as it got down to 15mph I was just about to accelerate back up when a deer jumped right in front of the car! My reactions are still pretty good, and both the deer and car escaped unscathed (LOVE the i-MiEV brake rate application response which seriously applies heavy braking in an emergency). Had I been traveling at my usual 35mph, both the deer and the car would have crunched! Had to share…

----------------------------------------------------------------------------------------------------------

Aerowhatt, I appreciate what you wrote this morning, I'm still digesting all your information. I'm considering formulating a scenario that would enable us to perform some controlled tests. What's funny is that almost all of my (now aggressive) i-MiEV driving is invariably interspersed with stints in Neutral, and I've never faced a situation where I needed to eke out every last mile(km) of range and actually put this downhill regen vs. N into practice. That said, I will admit to occasionally restraining the Tesla on downhills and holding down its speed with slight regen in the belief that it would improve that trip's Wh/mi (Wh/km) readout.

 

[Aerowhatt]

The Tesla may work very differently in this (N) vs regen respect. I would be happy to do my best to quantify the Teslsa in this regard. Just loan it to me for 6 months of mountain trips

Aerowhatt

 

[Aerowhatt]

The following is Off Topic but related to this topic, so I thought I'd share…

----------------------------------------------------------------------------------------------------------
Science Saved My i-MiEV (and a Deer)

I've been lucubrating on Aerowhatt's insightful writeups and thought it's about time to start getting a feel for the data we have available on CaniOn.

Where I live, from the point my side road enters the winding country road there is about a 1-1/2 mile (2.4km) gentle downhill on that country road with a 30mph(48km/h) speed limit before hitting the stop sign at the bottom. My habit for 40 years when getting onto this road is to accelerate to about 35mph (56km/h) and then drop the car into Neutral and simply coast all the way down to the stop sign, using regen in B to slow down just before the stop sign. The i-MiEV holds that speed constant very nicely, whereas my previous Gen1 Honda Insight and now Tesla accelerate to the point where I needed to start slowing them down. I attribute this to wind resistance and their lower Cd than the i-MiEV

Really glad you missed the deer, and it you, Really weird how something seemingly unrelated can affect your behavior, or timing, saving you a hassle, or catastrophe. Then again if you had just gone your normal 35 mph you might have been well past there when the deer crossed. Just strange how often this comes up in my universe.

So this trip down your road got me thinking. Since Canion only reads what the car gives it and mostly that is what is happening at the battery interface with the rest of the system. It can't show what the regen efficiency is at different speeds. I know what that rotating 0 torque field uses up at freeway speed but I would doubt that the requirement is a constant. Makes some sense that at lower motor RPM the field might require less power to support. Your road offers a possible opportunity to get a reading for 35 mph. So, you just match the steady state (N) coasting speed for the grade and take a few Canion battery amp draw numbers while in (D) and feathering the accelerator as steadily as possible to maintain the exact same speed. Average the readings and we have a 0 torque rotating field amp draw for a slower speed. I don't have any grades like you describe, that I know of. Everything around here is too steep, too flat, and/or too short. Just please have someone else read Canion so you can see the deer coming

I like the turn this thread is taking. If there is a lower amperage limit on regen efficiency, below which you do nothing but create heat in the motor and inverter, perhaps there is a good reason that Mitsu makes regen go away below about 12 mph. If you're just making heat, far better to do that in the brakes than in the electronics.

Just guessing here, but I think Mitsu purposely made the car act like a normal automatic ICE as best they could. Being first to market and an electric car being such a foreign concept to most of the population. Likely a good Idea to make it "feel" as normal as possible??

Aerowhatt

 

[JoeS]

...Your road offers a possible opportunity to get a reading for 35 mph. So, you just match the steady state (N) coasting speed for the grade and take a few Canion battery amp draw numbers while in (D) and feathering the accelerator as steadily as possible to maintain the exact same speed. Average the readings and we have a 0 torque rotating field amp draw for a slower speed. ...

Good idea, and I'll start doing test runs first to see where and how exactly the car behaves in Neutral (I like 35mph, for various reasons) and then simply start recording the data each time I go to town at that exact speed. I recall how difficult it was when, in the pre-CaniOn days, I had a Hall-effect ammeter clamped onto the battery cable and tried to hold the car at a steady speed while measuring amps. Amazingly close, when later validated by CaniOn.

Over all these years of driving the i-MiEV, I've always "felt" that keeping the power needle at the zero point when in one of the drive settings was simply not the same as kicking it into Neutral. Now perhaps I'll have some data to either validate or disprove this supposition. Edit: I should have noted that this is yet a different test that I would like to run in addition to the one described above

I keep my CaniOn set to metric, as the primary figure of merit I look for on each trip (when I have CaniOn on) is Wh/km, with my goal being less than 100 (including freeway driving) if I'm trying to drive efficiently. I've never bothered with the instantaneous reading, but perhaps now should start looking at it just to get a feel for it. For the first 8000 miles of ownership I was obsessed with recording my i-MiEV's wall-to-wheels consumption, so when you say miles/kWh I think in terms of end-to-end and not just battery-to-wheels.

Will post here when I have some meaningful data.

 

[JoeS]

So, as I'm doing my Jazzercise (I'm the token male in the class), led by a beautiful blonde cheerleader, I try to keep my mind occupied and thus this morning formulated the following summary statement attempting to define what it is we are attempting to prove or disprove. I'm putting this on the table for discussion and refinement:

The Hypothesis:

On any given downhill, placing the i-MiEV into Neutral and coasting is more efficient than leaving the car in "gear" and regenerating.

I would like to define Efficiency as based on the difference in State of Charge (SoC) at the bottom of the hill vs. SoC at the top of the hill. This thread's title uses the term 'max range', and I submit that the simple concept of having a larger SoC at the bottom of the hill using one or the other technique will demonstrate this.

I recognize that this statement encompasses uncontrolled variables such as time, speed, distance, altitude, and road grade, to name a few. May I suggest that, in order to bound our investigation, the following variables be limited:

Speed - lower end of 35mph, which also happens to be the truck speed limit on many Interstate steep (≥6%) downgrades; I'm suggesting an upper limit of 70mph so we can both stay legal and not over-rev the i-MiEV motor.
Note: one of the parameters I hope we can establish is a grade % vs. coasting speed for the i-MiEV at any given altitude.

Time and distance - I am putting no restrictions on these two as they will be bounded by the distance travelled and the car's speeed.

Road Grade - this will probably be naturally limited by the availability of grades on which to perform our tests

Altitude - this will be significant, as the vehicle coasting in Neutral will achieve much different losses for any given grade depending on altitude.

Other variables such as air temperature, windspeed (and its relationship to the car), etc. should simply be noted.

Ok, I'll shut up now and open this up for discussion - do we have agreement on the Hypothesis (or whatever we wish to call that statement)?

 

[JoeS]

Well, this was an interesting, but frustrating, exercise. Don't bother reading any further, as the only one who might be interested in the following is Aerowhatt.

Here's the elevation chart of the road I take:

The objective was to first get the car up to 35mph, kick it into Neutral, and then coast all the way down. The idea was to repeat this with the car in D (or Eco or B) and hold the accelerator steady to hold the 35 mph speed and then record the power consumption (ammeter reading). Simple, huh?

Here's what happens normally -

Looking at the above graph, it is fairly level for the first 0.1-mile with a fairly sharp curve so I use that distance to accelerate up to 35mph, kicking it into Neutral at the 0.1-mile mark.

The car maintains a speed of 33mph-35mph for the distance from 0.1-mile to 0.8-mile, despite the road being somewhat curvy in places. In this mode, CaniOn reads 1.3A and instantaneous 8Wh/km. At the 0.8-mile mark the car starts accelerating and by the time it gets down to the 1.2-mile mark it is almost up to 40mph.

I realized why I never see that 40 mph in my everyday driving: at around the 0.9-mile mark (last curve followed by straight stretch) I reflexively drop it into D then Eco then B and consciously slow the car down using regen so that it is crawling by the time it hits the stop sign at the bottom (1.3mi). Also, that last stretch is occasionally a speed trap by the local sheriff (the speed limit is 30 mph), so by playing with regen I automatically avoid that issue. That's kinda my force-of-habit method of driving that road daily in the i-MiEV for the past five years.

Incidentally, with CaniOn, my figure of merit is to drop down my cumulative trip consumption to zero Wh/km by the time I hit that stop sign - it takes that long to counteract the sharp initial acceleration power draw, and the final regen really brings that number down fast.

So, what happened today when I tried it in D -

What I decided to do was make a few runs in only that nice steady first portion of that drive: between 0.1 miles and 0.8 miles, and record the data using a voice recorder. In a nutshell, in my first few attempts today, trying to keep a steady foot on the accelerator in order to keep the speed in the 33mph-35mph range and then read the Amps on CaniOn proved to be impossible. The Amps were all over the map, from a negative maybe 2A (slight regen) to a positive maybe 9A, as the car is operating in a very lightly-loaded region. The amps number was different every time I looked at it! I need to keep my foot from twitching or else I need a mechanical throttle with a vernier on it!

FWIW, it was our first very warm summer day today, afternoon temperature 87degF (30degC), battery was at 65%SoC, battery average temperature was 26degC (79degF) and RR=44 - just some meaningless data points.

On the drives back up the hill I was better able to steady out the fluctuations, with perhaps an average of 25A at 35mph on that stretch between the 0.8mi and 0.1mi markers. Have to think about it, as that info might prove to be significant.

What does all this tell us? Nothing! Over the next few days I will now see if I can develop a really gentle non-twitchy featherfoot to keep that accelerator pedal constant with the car at 35mph and then glance over at CaniOn for the amp reading, without worrying about deer jumping out in front of the car. To be continued...

Edit: I just had a duh! moment, as I forgot that I can upload the CaniOn data and can then dissect it. Also, I can take a screenshot of the CaniOn graph...

 

[Aerowhatt]

I like to analyze things qualitatively and then quantify it if needed. Why? simply because it is easier. So just as a suggestion, I would tackle evaluating your descent more simply first.

Record SOC at the top and the bottom and also record Wh/km at the bottom for each case. Then compare them and see which delivered better power stingy results.

So you would have SOC difference between the top and bottom for (N) and SOC difference between top and bottom for (D). Plus Wh/km for both to compare.

It's might also be better to use Eco for the "in gear" descent since it is easier to be steady on the throttle with that modes less sensitive throttle. Then your only focus it to match the speed in both runs. This would give clean simple data that anyone can wrap their head around. If the two runs are super close in power consumption then it could be quantified more to discern the winner.

Aerowhatt

 

[JoeS]

Aerowhatt, thank you for the excellent and very logical suggestions, especially the one driving in Eco which reduces the pedal sensitivity. I'm pretty tied up for the next couple of days but will attempt to do this every time I go to town.

 

[JoeS]

Brief update: using SoC over this short stretch doesn't work, as both CaniOn and EVBatMon only show a 0.5% resolution, which doesn't change over the course of this downhill, using either technique.

Will keep trying to see if Wh/km is doable, as controlling the amount of energy going into the car during acceleration is difficult/unmeasurable?, as is keeping a steady foot on the accelerator.

Science Saved Me A Ticket. Good thing I was going slow doing this testing, as yesterday there was a barely-visible Sheriff standing in the shadows with a handheld laser gun point up that final downstretch...

 

[Aerowhatt]

Brief update: using SoC over this short stretch doesn't work, as both CaniOn and EVBatMon only show a 0.5% resolution, which doesn't change over the course of this downhill, using either technique.

Will keep trying to see if Wh/km is doable, as controlling the amount of energy going into the car during acceleration is difficult/unmeasurable?, as is keeping a steady foot on the accelerator.

Science Saved Me A Ticket. Good thing I was going slow doing this testing, as yesterday there was a barely-visible Sheriff standing in the shadows with a handheld laser gun point up that final downstretch...

I was afraid of that. It's a pretty short road at only 1.3 miles. Wh/km should be able to show up a difference even on a short hop. That's what's nice about my "mostly" descent, it's 27 to 34 miles long depending on the chosen starting point. Sections that I look hard at because of their consistency of conditions are 5 miles or more each. Problem is I can't test it at 35 mph. The speed limits for the most part are 45 to 65 mph. Drivers around here tend to add at least 5 mph to that. Just not a safe option to go slower!

I did a little refresher on aerodynamic drag while waiting in stopped traffic today. Comparing 7,500 ft elevation to sea level. Air density
is a nice round 80% of sea level at 7,500ft (the averaged altitude of my journey). Since air density is a single multiplier in the drag equation it gets simple. Basically, given the same velocity one would be fighting 80% of the resistance that they would be at sea level.

Aerowhatt

 

[JoeS]

Aerowhatt, thanks for doing the drag comparison. Hey, the only reason for this whole discussion is the aerodynamic drag effect!

Well, I've been totally unsuccessful in trying to get any meaningful data by trying to keep the accelerator in a constant position and a constant speed. The combination of curvy and undulating road, short distance, and slow speed makes it not doable, IMO. On this stretch, it is soooo much easier to just kick the car into Neutral and not worry about it. For maximizing range, this low speed test doesn't address the comparison we're trying for, anyway. Neutral gives me a steady-state current draw of 1.3A (correction, I had written 1.8A) at around 50%SoC.

During my recent Tesla trip I played with Neutral as well as holding the speed down on steep slopes and regenning like crazy. The problem with Neutral and the Tesla is that its Cd is so low that the thing just keeps going faster and faster...

Without a controlled test on the same stretch of road on which one has the ability to go both at 70 mph and 35 mph to do comparative measurements, we're still at an impasse. An early Sunday morning on the Interstate near me might work at 35mph for a few minutes...

 

[JoeS]

Forum readers will recall that Aerowhatt and I have been embroiled in a discussion as to whether regeneration and slower downhill descents will increase an i-MiEV's range as compared to putting the i-MiEV into Neutral and simply coasting down the hill. Aerowhatt is a proponent of coasting and, from a practical standpoint, I also do that all the time; however, the theoretical challenge on the table is still there.

The discussion boils down to whether there is a higher or lower amount of stored energy which results from a lowered aerodynamic drag (from going slower) and regen power plus the power consumed by the engaged motor/inverter combination when compared to simply coasting whereby the car is still consuming a steady (1.3A?) irrespective of speed. [That's an awkward sentence that needs to be rewritten]

The metric to prove/disprove this was the change in State of Charge between the top of the hill and the bottom of the hill. Unfortunately, the resolution of this reading (via CaniOn) is only 0.5%, which is inadequate for short-stretch controlled testing. In my case, I don't have a long-enough downhill where the car will coast at a fixed speed in Neutral.

Never one to give up I've gone back to looking at a short stretch (0.4 mile?) of freeway with a nice steady downhill where the i-MiEV holds a constant 65mph in Neutral. I'm willing to go back on a Sunday morning with negligible traffic and kick the i-MiEV into regen and thus go down this hill slowly in an attempt to prove (or disprove) the hypothesis.

Let's ignore SoC, and try CaniOn's energy consumption over distance readout.

I haven't tried it, but if we reset CaniOn at the top of the hill and then take the Wh/km (or non-metric equivalent) reading at the bottom of the hill, using both driving techniques, will that prove anything? Let's discuss it before I try it.

Aerowhatt, you had published some compelling numbers in another thread which showed that engaging the motor/inverter was terrible compared to coasting in Neutral, but I couldn't readily find them. Perhaps summarize your findings on this thread?

Incidentally, the location of my "test" hill is (using Google Maps) 37.3860966,-122.2056714, but I have to re-learn to use GPS Visualizer and come up with an exact Start and Stop point on this road for the test.

 

[Don]

All in all, I think neutral is hard to beat if you have a straight downhill stretch where the car will coast at about the speed you want without the need for either slowing or accelerating on portions of it - Anytime you introduce electronics into the equation, there are losses, caused by getting DC from the power source, converting it to AC to power the motor, or regenning AC and converting it back to DC to recharge the batteries. If you were on an Eco Challenge mileage run, you would be using the coast mode as much as possible and you would be going further on less juice, no question

It's a shame Mitsu didn't give us an easy way to make use of the coast mode - A paddle on the steering wheel where you don't need to fiddle with the darned shifter every few seconds would have been really nice. But as we all know, it was a new thing and their concentration was to make it drive as close to a conventional ICE as they could. Thinking like that left lots of good, low cost ideas left on the curb - A MAX regen paddle on the left side of the wheel and a coast paddle on the right side could have probably been done for less than $100 per car. But then . . . . some elderly driver would get confused which was which and drive through a store front and everyone would be saying "Them darned EV's are just UNSAFE!"

For everyday driving in traffic where other drivers affect how and when you can coast or slow, shifting the shifter all the time is just way too much trouble, especially when all you're really doing is making and braking switch contacts - What a ridiculous way to accomplish that . . . . and then we read about people who have shifter problems after a time, where it needs to be readjusted to get it back in sync

I've never had any range problems and the wife even less so - We got used early on to just pulling the shifter all the way back into 'B' and just leaving it there. One pedal driving. You get used to where to ease up to make the curves and corners without the need to touch the brakes. I like it even better in the Volt, as I can ease out of the throttle later and then when I need even more regen to come to a complete stop, pull the paddle on the steering wheel. No question we could be adding a few percentage points to the energy savings by constantly fiddling with things and if there was a need, maybe we could be bothered - But after 6+ years driving in B mode, it's just so simple and you don't even have to think about it . . . . it's just the simple, normal way to operate, at least for us

Don

 

[Gorfllub]

Don,

I just made a "Coast/Neutral" button! Works great!

Going to start a new thread about it in modifications section.

 

[JoeS]

Found this very good article discussing regeneration and its limits, further strengthening Aerowhatt's position in our regen vs. neutral debate.

https://chargedevs.com/features/reg...oser-look-at-the-methods-and-limits-of-regen/

Dang, gotta do some controlled tests to see if I can bolster my argument. For those of you wondering what this is all about, see this post which defines the hypothesis -

http://myimiev.com/forum/viewtopic.php?f=28&t=1685&start=20#p32826

 

[JoeS]

In a previous posting on this thread, I defined the hypothesis that Aerowhatt and I are debating. Summarizing:

On any given downhill, placing the i-MiEV into Neutral and coasting is more efficient than leaving the car in "gear" and regenerating, defining efficiency as based on the difference in State of Charge (SoC) at the bottom of the hill vs. SoC at the top of the hill.

Unfortunately, I then bounded it by limiting the upper speed limit to 70mph which means that for steep downgrades one must kick the car out of Neutral and do some regenning as opposed to simply coasting in Neutral and let the car find its own maximum speed for any particular grade. I also limited the lower speed limit to 35mph. Sadly, these two limits complicate matters to such an extent that, to date, I have found it impossible to perform a controlled test.

Nevertheless, not being one to give up easily, last week, after climbing up to Lake Tahoe and then from Truckee up to Donner Summit (7,000ft elevation) on Interstate 80 in my i-MiEV, I was really looking forward to the >50-mile mostly downhill run into Sacramento with grades of 4%-6% in order to take some data and see if I could make any sense out of it to either bolster or refute some of the arguments Aerowhatt and I were having.

My best-laid plans blew up! Here's why -

I had anticipated having the ability to drive at a leisurely pace and play with both coasting and regen and was even considering going back up a particularly suitable grade to repeat this experiment (if I found one) and doing it a few times ... BUT ... what I had not counted on was the truck traffic. Interstate 80 is a major East-West transcontinental thoroughfare and what I experienced was a never-ending closely-spaced convoy of trucks significantly exceeding the state's 55mph truck speed limit on the downhills. No way was I going to drive even at 55mph hypermiling along using regen - I'd have been eaten alive!

So what I ended up doing was roaring along with the other car traffic, passing the trucks (it's only two lanes in one direction) which caught me completely unprepared as I consciously blew past a key charging stop only to find that I didn't have enough charge for a major planned side trip. No problem, as I ended up continuing on down the hill to the next CHAdeMO and having to take a different route than originally planned - I was so rattled that I forgot to take data or even CaniOn screenshots.

But, with all those downhills on the Sierra side roads, I had time to think (I know, that's dangerous...) as I watched the SoC creep up on CaniOn and the fuel gauge gain bars -

So, let's revisit this without limiting constraints -

For any serious downhill, we have two competing conditions, and are asking which is the more efficient:

Coast in Neutral vs. limit car speed using Regen

1. One argument is that the car's electronics draws significant power when the motor is engaged, negating the power gained using regen. A quick check of quiescent power draw: when the car is stopped, foot on the brake, shifting from Neutral into Drive results in no change in the amount of current drawn by the entire i-MiEV system (about 1.2A as seen by CaniOn), with the car not moving.

2. If the car is moving and regenerating, it is indeed stuffing current into the battery and increasing the State of Charge of the battery, despite any losses it may have with the drivetrain engaged. OTOH, in Neutral, the car continues drawing its 1.2A.

3. At the bottom of a steep 20-mile downhill, if the car had been in Neutral the whole time (speed unchecked), the SoC would be unchanged (or maybe even lose a bit) compared to what it was at the top of the hill.

4. OTOH, going down that steep 20-mile grade and slowing down the car using Regen will result in a significant increase in SoC by the time the car gets to the bottom of the hill. Like I tell people, it's fun watching the fuel gauge go UP when going downhill!

Thus, I still contend, on downhills it is more efficient to regen than to coast.

Flame suit on...

(Or am I so confused by now that I should throw in the towel and lock the thread on this discussion and fugetaboutit? - I'm presently on the road in the Tesla killing time in Oregon)

 

[Don]

How fast does the car end up going down a long 5% grade? You are limited by aero drag and the mechanicals in the car, since there is in actuality no real 'neutral' which allows you to disconnect the drivetrain and just coast . . . . and the car's Cd is just terrible at the speeds involved in such a long run down such a steep grade. No question you would get much different results in the Tesla . . . . you might be going 140 mph at the bottom of the hill!

It's easy to see where using (our) neutral down a long grade limited by aero and friction to whatever speed that gets you is less efficient than regenning and going down the long grade even just 3 or 4 mph slower will have you with a fuller battery at the bottom, no question . . . . but, what have you proved??

That an iMiEV has a terrible Cd - We knew that already!

If this same test is done on lesser hills at 35 mph, you won't have those huge aero losses and that limiting of speed done while regenning might mean you have scrubbed off momentum recharging that you're going to have to pay (in electricity) to get back to either maintain whatever speed you want to maintain after the coast, or you pay more to make it up the next hill - I suspect the latter is true for sure. Then, you have the energy conversion losses to add into the equation too which are probably more than the cars quiescent 1.2 amps that you were losing while coasting. So, you can't make a blanket statement about coasting down hills without factoring in the aero losses based on the speeds involved - It might be more efficient to coast at 35 mph than it is to coast at 70 mph or more and regenning just enough to reduce the aero loss just enough is more efficient when aero is playing a big factor like it does at higher speeds, yet the calculations for half that speed would give you a completely different answer . . . . right? I'm pretty sure they would

I think if you're going to design an EV to win the Eco Challenge, you would want a real neutral, plus the ability to shut down all current draw in the car while coasting . . . . which just isn't safe to do in a real car driving on real roads among real traffic

Truckers love to tack on an extra 10 or 20 mph at the bottom of a hill to 'use' to make it up the next hill and they know that's more efficient than riding the brakes down the hill (where they get no energy return) and then burning extra fuel to make it up the next hill. Now you're trying to prove whether or not an EV which can return 80% or so of one of the the speed limiting forces is better than just coasting while using the cars constant quiescent current . . . . or not . . . . and that's probably very speed dependent

IMCUO (In My Completely Uneducated Opinion) I think you guys set out to try to prove something one way or the other by carefully examining the 6 or 7 factors you thought governed the situation, only to discover in the end that there are actually 9 or 10 factors to consider *and* those unexamined forces threw all your calculations out the window :lol:

But, I might be completely wrong!

Don