Is Regenerative Braking Real? Does it work as we thought?
Is Jack Rickard correct about there being zero gain -- or even a loss, when regenerative braking is used in an EV?
If you have an EV, can you try to duplicate his results? He is even seeing a 7% *loss* when regenerative braking is used.Like him, I am very surprised at these results -- they make no sense. EVTV.ME: More Regenerative Braking Results. It gets worse... He is already working with Illah Nourbakhsh (at Carnegie Mellon), so hopefully we can sort this out? Regenerative braking in the Aptera - is it merely dogma? - ApteraForum.com - Unofficial Aptera Car Forum |
FYI, when looking at using a freewheel on my bicycle vs a fixed gear, the figure I saw was that the regen would return %18 of what you put into it, and the constant friction reduced the bikes ability to coast well beyond what you would get back from %18 of your braking energy. So most bike sized EVs opt for free-wheel.
Not an EV only problem though, hybrids have the same concern. |
The three modes that Jack Rickard tested (on the first car) included one where it could coast freely, and only had regen on the brake pedal. This still returned no better results than when *no* regen was used...
Something does not compute. And Jack is puzzled, too. |
Regen works on my (PH)EV. Evidently, when he replaced his DC motor and controller with AC, he selected less efficient components.
Also, he was talking about being constantly in regen or assist. If he doesn't have a neutral position in his transmission, then he needs a "zero amps glide" setting on his throttle. DWB and long coasts work better than regen. |
Didn't check the links but I'll say that a properly driven vehicle doesn't usually need much braking at all on the non-urban flatlands so regen maybe wouldn't be worth any cost/equipment penalty, but I've spent enough time in hilly areas to really, really wish I had regen there!
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What is he using a the regen element, the motor rotor? or and independent charge item like edy brakes?
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Yeah I'm not sure what systems hes using either, but if the regen setup is working correctly its going to recapture energy, period. Sounds like his system isn't properly designed, or his testing is flawed.
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hello!
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I know regen works on my Prius. But it's set to about 10% of max with the the throttle off (to simulate engine braking on a conventional car). I usually apply just enough throttle to eliminate the regen when I want to coast - the Prius has a display that tells the driver when the regen is on and off so it's easy to modulate. Then when I approach a light or stop sign, I let off the throttle and let the regen resume and apply just enough brake (trying to stay within the regen zone so as not to engage the mechanical brakes) to get a smooth stop. I know the regen is working because I can see the battery charge level rise.
Possible reasons why the tester didn't see a benefit from regen: A) Different conditions between tests. B) Didn't do the math correctly. C) Regen level set too high so that he was "hunting" between regen and applying power to maintain speed, wasting energy (due to system losses) along the way. |
I vote for C. Neutral coasting would be better in most cases.
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Coast to a stop best, regen next best and brakes bad. Any system that converts your motion into energy will not be 100% but will be better than converting motion to heat and loosing it.
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Practical example with an Insight (since I don't have an EV): start by climbing up to Carson Pass (westbound) on Calif. 88, which is about 8500 ft. This will definitely drain the battery pack. Keep going west until you reach the valley floor, about 60 miles. You will have used very little gas - my trip mpg shows its 150 mpg max - and your batteries will be fully charged.
It's not a perfect test, since there are several upgrades where I need to use engine + assist, but I will repeatedly drain the pack on those upgrades, and recharge on the next downhill. So how does this happen, if regen doesn't work? |
I've really enjoyed all this debate, because even if it ends up being a hoax, it stirs up thought. While most people just jump on the bandwagon and say "well the ONLY explanation is your data is wrong and/or you tested wrong" (because conventional wisdom is that regenerative braking works), a few people have given ideas as to why it might NOT work. If you just sit back and think "how would it be possible for regen NOT to work" it's kind of a fun brain exercise. The way I see it is that maybe we can come up with a way of using regenerative braking more efficiently.
I know I'm in the minority here, and my reasoning will be quickly dismissed by some more technically minded than me, but when I was working on my adapter plates, I used my cordless drill A LOT. Well, obviously I would drain the battery A LOT. But what I noticed was that my best use of it was always at the beginning of the day: if I tried to put it back on the charger between uses, it just didn't work as long as that first use. It appeared pretty significant. Granted, apples and oranges, and it's a NIMH battery vs LiFePO4, but I wonder if perhaps it might just be possible that charging/discharging/charging/discharging might very well lead to reduced total amp hours a battery can give out than if you just discharged all the way? Jack has already dismissed that hypothesis, as well as at least two others, but I am still not convinced..... |
Sounds like the whole P&G vs. P&DFCO thing, except in this case it's P&R(egen).
So basically, my thought to him is this: EVs - You're doing it wrong. |
I'm thinking that it very possible that Jack Rickard did not change his driving style when changing the ways that regen was set up; and when you do not take advantage of coasting (or if the cars he drove do not coast very well?), then that would not improve things.
With a lot of regen, then you have to use a lot of accelerator, and since regen is less than 100% efficient, you are bound to use more. Coasting is always better, since you can use less throttle and if you underestimate, just add a little more throttle. It is too easy to overestimate, especially if the regen is on the throttle and set too strong, so you over accelerate and then over brake, then you can easily waste energy. |
well the edison 2 folks have a pretty picture, looks like ~22% return in an EPA drive cycle here in a VHC (Very Heavy Car) with next to nothing in a light car
http://sphotos.ak.fbcdn.net/hphotos-..._2607636_n.jpg |
...and yet I wonder if regen on even the lightest bicycle EV would have merit in a place like steep hilly California...
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Yah, that would be the best scenario for regen, something where you would have to ride the brakes a lot to prevent overspeed anyway.
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The energy from regen is proportional to weight (just like braking and acceleration) so the weight doesn't affect the proportion of energy you can get from regeneration.
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Neil, Edison2 doesn't seem to think so:
"Other conclusions that can be drawn are that there is less energy available to regenerate in a light car because less has been spent accelerating it, and that as weight shrinks the benefit of regenerative braking becomes vanishingly small." Edison2 | Facebook |
Edison2 is owned by Oliver Kuttner, and I know him -- he invited me to be on the Edison2 X-Prize team, so I could be there at the X-Prize. Their cars are not electric (though they may be putting an electric drive in one in the future). He is correct in that it takes less energy to accelerate a low weight car, and the regen would be proportional to that. So, it takes less energy to accelerate it, and you get less back, as well.
But that doesn't mean that regen is worthless; because even if you only get back a little -- it is worth just as much to you, because you don't need as much. If they put an electric drive and battery in their VLC, it would be heavier than it is with the ICE they use now. If you need brakes to slow down (and what car doesn't?), then regen offers you braking where you can recover some of the energy. |
but I think the case can be made that it does not work as well as people think, generally.
And if you are building a lightweight fuel car, useable regen would be extra weight. |
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Edison2 has obviously determined that the weight penalty for making their VLC a hybrid erases their fuel savings. DIY EV'ers may find that for the $ cost of going AC, they could instead buy more batteries to achieve greater range. Folks who buy one-ton or heavier vehicles will probably say regen is well worth it, because it's a real boon on hills, and it's the best way to keep a hybrid's 12V battery charged. |
one thought I have is what is the net energy profit? If it takes x amount of extra equipment to use regen braking, how much is the cost to move that equipment down the road? If x=0, then 1% recovery of the energy otherwise lost is infinitely larger than 0%
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If you have to use the brakes in heavy urban traffic situations like my 40 mile 46 traffic light round trip daily drive then regen is helpful. When my battery was fully charged I could pulse with the battery and coast in neutral to increase my mileage in my CVT Insight.
I could manage low to mid 70s MPG, while averaging about 38 MPH for the two legs of my drive. If I got unlucky and hit more than a couple of lights my just replaced by Honda battery would be just below 20 bars on the gauge and I would pulse and coast to use some of the capacity. Out in the country where IO might not have to stop for many miles, I would still use the same technique on grades when the battery was full. I probably can't quantify the percentage benefit but it was most likely in the range of 10% under those circumstances. I never tried the daily trip without battery assist but I would guess it was 20% under those circumstances. regards Mech |
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But I have no clue how they made the graph, it fits nicely with their philosophy though. and at 750 lbs the return looks really tiny if at all. "Our analysis of efficiency showed that the energy gained from regenerative braking simply was not worth the cost in added battery weight. One 6 lb gallon of gasoline contains the energy of 500 lbs of batteries; at least 1000 lbs of batteries would be needed if our 750 lb VLC were electric. That is a lot of weight to push around for 200 miles." |
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well it does get a bit complicated down the aero path, the aero has much less impact at lower speeds, where I have to do most of my braking.
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Another thing to consider is the amount of braking power given by regen brakes. Seems like I read about Ben Nelson's friend with the AC powered electric car who hardly used his service brakes due to the regen braking strength of his motor.
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Something I've noticed, my "antique" 05 48v miles zx40 mini van with a series wound motor is that it uses about 65-75 amps at top cruising speed which is typically (NOW) in the 26-29mph area, using field reduction I can go 32-38mph and draw 100amps. (usually ends up at about 35/36 depending on SOC) The funny part is the same car AC powered 72v Miles ZX40 uses the SAME number of amps to go the same speeds I go and its 72v NOT 48v. Seems to me the motor must not be as efficient (or controller) or its just that my tire inflation and lubrication practices make that big of a difference. So perhaps in these situations motor and controller efficiency are more important than some other factors like regen? |
Here's a post I made to the EVTV.me blog:
Jack, thanks for the testing and publishing your results. How long had the car sat before the test? If the car sat for a few days, I could see the oil being stiffer and the batteries less responsive for the regen test. Then the batteries would be a bit stronger for the non-regen test the next day since they have been recently exercised. Ideally you'd do "ABABAB..." type testing -- regen one day, nonregen the next, multiple times. This would accomplish 2 things: One is to look for system drifts, like batteries working better due to recent use. The other is to give you some statistics -- if the numbers are jumping around you might not have a statistically significant difference (or sameness). I have a car with regen, I'll have to try your test myself. One thing I have noticed: If I do strong regens (like occasional 200+ Amps), my lead acid batteries stay very well balanced with no BMS (all within about 0.03 Volts). With weak regens (50 Amps), the batteries quickly drift to be 0.5+ Volts apart. Jack have you seen anything like that with your cars and lithiums? |
I noticed a voltage driftt aswell, are you measuring the volts identically ?
could be that the batts have different internal resistances. |
I've watched the videos and my first comment is they desperately need
to find an editor. The videos could easily be cut in half and increase their content ratio by 100%. Maybe Count DaMonay can help. :rolleyes: I'm not competent to comment on the purely electrical elements and calculations, especially when carried out to 4 decimal places. That said, I think there was some experimental design errors or misunderstandings in the original 356e tests that has lead to this brouhaha. As I understand it, three conditions were tested with the 356e: C1 - Brake pedal regen only Brake pedal - deceleration by regen and friction brakes No pedals - freewheeling ( I'm not saying "glide" as it has Prius-specific meaning and includes a small power draw on the HV battery.) Accelerator - acceleration only. C2 - Max regen Brake pedal - deceleration by "aggressive" regen and friction brakes No Pedals - constant "aggressive" deceleration by regen Accelererator - acceleration only C3 - No regen Brake pedal - deceleration by friction braking only No pedals - freewheeling Accelerator - acceleration only The perceived problem came up when the figures suggested that the car used less power over the 47.6 mi. course for the second condition over the first, and the third case - no regen whatsoever - over the second. Overall conclusion: regen in an EV offers no energy savings/efficiencies. In C1 there is freewheeling and some regen via the brakes. In C2 there is no freewheeling and a lot of regen. In C3 there is a lot of freewheeling and no regen. My observation is that C2 is not about regen at all. It is about essentially constant deceleration at all times except when the accelerator is depressed. It would be like driving with a dragging brake shoe. You would have to accelerate -- that is use power -- to maintain speed when you would otherwise be freewheeling. To me, this is not a surprising experimental result. It would also be less efficient to drive around dragging a 200 lb. anvil behind. I do not yet understand why C3 -- no regen -- looks more efficient than C1 -- some regen. I suspect that, as is noted later in the series, it has to do the nature of the road course driven, having to do with stops/starts, topography, and ratio of city vs. country, regen "friendly" vs. "unfriendly" driving. There may be two additional things that would confound accurate energy use and efficiency analysis between C1 and C2. I am at/beyond the limits of my EV/battery understanding here. Both the test runs for C1 and C2 were made on the same day. 1. Someone said long ago that to properly warm up a Prius for a FE/MPG, trial, the car needs to be driven for 20 miles or so before the event. In this way you get, for lack of a better term, a "whole car" warmup, including all rotating equipment; transmission, rear axle, wheel bearings, pump bearings, etc. There is no mention of the 356e being warmed up prior to C1, this could mean that the the 356e would have been less efficient on the C1 run compared to the C2 run. 2. The internal temps of the drive batteries during the runs could have fuzzed-up the data. I believe that the C1 and C2 test runs were done on a warmish, 90 degF day. It is well known for the Prius anyway that when the HV battery gets up over 100 degF, it suffers from both reduced capacity and reduced ability to receive a charge and deliver power. I don't know how effective the 356e's drive battery cooling system was/is. Without taking temps before/during/after each run there is the possibility that temps were not equal for both runs. Which run this would favor would depend which run was at the hottest part of the day. |
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Common sense says wise use of regen braking is better than friction braking. Or am I alone in this foresight? |
Good points.
As to the second; only one run of each condition with unaddressed variables... inconclusive results. |
From Jack Rickard's blog post of 7/16:
". . . We're kind of going over it now, but we've changed things a bit and gotten quite a different result. In my mind, the URBAN part of the drive was about half. That's because it takes about half the time. As it turns out, it comprised 11 of the 48 miles. The rest were more freeway like (about another 10 or 11) and rolling country blacktop county road (26 miles}. PLenty of hills and curves on the county road, but really one of the reasons I like to drive that is the Porsche handles it so well. That' s probably not good for regen, and probably IS good for free coasting. We did a very different drive out US 61 through Cape to Jackson, the urban part, and then the urban part back. Though 21 miles, it takes almost the same amount of time as the 48 mile drive we were doing. And we're getting very different results on both the kWh meter and the AH meter - and showing really quite impressive gains in fact. So I'm probably going to have to fall on my sword on this one. I was measuring the percentage of road style based on time instead of distance, and probably mischaracterized the drive pretty badly. The latest results are based on precisely TWO drives, so it's still pretty preliminary. I'm learnign that this stuff is quite a bit more variable than I had thought. So before going into further detail, we're going to do a series of drives in the MIni Cooper. . ." |
hello!
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My 94 VX has just passed 59k miles and the original front brake pads have 50% left.
I guess that confirms that coasting is better, but coasting and regeneration would be even better in the inevitable circumstance of forced stops. I read somewhere that a single 60-0 stop wastes the energy that could maintain the original speed for .7 mile. The key is to have the ability to regenerate without a massive weight penalty, as well as to store and reapply the energy at much higher efficiencies than are possible with electric configurations. regards Mech |
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