In our first initial drives with the Mini, we were getting apparent ranges that matched my calculations for range initially without regen. We had assumed we would get some extension in range from having our first car with regenerative braking.
After just a few drives, it became apparent to me that with 100AH pack, and just under 1 AH per mile, that our range would be limited to about 100 miles – really quite similar to the BMW MIni-E. We had a larger pack, but also a larger car as we used the Clubman. But I was assuming we would get something from regenerative braking.
The question was of course what? Most comparisons indicate AH consumed and AH recovered and the Toyota RAV-4 even has a little indicator showing you your efficiency gains from regen based on this. I rather suspected that a real world comparison didn’t yield the 21-25% gains that this measurement routinely provided. But I assumed 6-8% would be “real.”
In the case of the Mini, it didn’t look like we were getting much.
But we were busy completing the Speedster build and we have already heard from a number of people who want to purchase it from Special Editions Inc as a finished car when they can produce it. They can’t produce it until we send them the prototype and bill of materials, so priorities being what they are, we’ve been playing Speedster.
Our first Speedster featured a series DC motor from Netgain, the venerable Warp 9, and a Kelly DC series controller with two banks of 90AH Thundersky cells. This makes a powerful package, but there just isn’t an easy way to have regenerative braking with a series DC motor. The longest distance we drove this Speedster was 107 miles on a single charge, but to 80% DOD it’s probably an 85 mile car safely.
In doing the new Speedster, we used just a bit smaller motor, the AC-50 from High Performance Electric Vehicles, along with a newish Curtis 1238-7501 3 phase controller. This does give us a regenerative braking capability that is quite flexible, and also allowed us a badly needed 1/4 inch to wedge in CALB 180AH cells in a single string. The AC induction motor is purportedly more efficient, though in very small numbers.
I did a max range test a couple of weeks ago and what number do you think came up on the odometer at the end? It was just over 107 miles.
What happened to the range gain from regen? Surely I must get something. In fact, the car is a bit lighter with the CALB cells but still got about the same range as the original. Now that’s a rough approximation. Bringing cells to the brink of destruction WITHOUT destroying them is something I’m pretty good at, (though bettter at actually destroying them) but it’s not precise. But we were pretty much run out. Even at 6 or 8% gain from the regenerative braking, I shoudl have hit 115 miles on the drive. What happened to the regen?
Regenerative braking, of course, is the concept of using the motor as a generator to slow down and stop a vehicle, instead of using the “friction” brakes that basically convert forward motion into heat in order to stop the vehicle.
So like Jerry McGuire – SHOW ME the REGEN. Where did it go.
Such questions are actually very good for me. They distract me from my full time vocation drinking whiskey and chasing younger women – though I’m easily distracted from that important work by technical questions.
To find out, we devised a “test drive” featuring lots of “regen opportunities” and of some length. Actually it was 48 miles or 76.6 km and takes an hour and a half to drive it.
We did the drive three times in three different modes, fully charging the car between each drive.
In the first drive, we used regenerative braking on the brakes only. We had fitted a 0 500 psi 5v pressure transducer to our Speedster and used it as an input to the brake pot input of the Curtis 1238. Curtis allows you to map this quite flexibly across the 5 volts and we used 100% regen braking current mapped across the first 2.5 volts of the pressure transducer output. This gave a great feel and we could do regenerative braking pretty much to max before the friction brakes came smoothly into play. We observed braking currents of up to 100 amps using this technique and typically still made current down almost to a full stop.
The drive departs EV Motor Verks up Spanish Street to William Street/Route K and proceeds west through a very busy area to U.S. Highway 61, known locally as the KIng’s Highway. It was originally El Camino Real a Spanish Trail throught he territory.
It is now an unlimited access four lane with hundreds of shops and restuarants and commercial enterprises stretching about 8 miles out to Jackson Missouri.
We continued the drive through Jackson and out Country Road 335 to the site of Southeast Taxidermy, where on the first drive we had fried fish and Stag beer. We then backtracked to Jackson, went through the downtown area, and back onto Highway 61 out to Route 177. All of this is quite urban and quite busy.
177 is a two lane new “highway to the Proctor and Gambol plant. Speed limit 55 with really nothing along its length and so no stop and go here, though gentle elevation changes and curves. We turned off on county road V which is a narrow two lane blacktop road that is quite curvy and quite hilly. This reconnects to 177 back into Cape Girardeau, which we take through downtown main street back to EV Motor Verks.
The results of drive 1 indicated we had travelled 76.6 kilometers amd consumed 78.2AH of energy from the pack for a net 1.02088 AH per kilometer. Brian went along as camera man and so he was rather required on the two subsequent drives so that the car would be loaded the same.
On the second drive, we added a 50% of max regen current level to the “neutral braking” of the accelerator. What this means is that a certain map of the first volt of throttle signal does not accelerate the car, in fact it progressively decelerates it – though in reverse. At the top of the volt, you have minimum regen and as you remove pressure from the throttle it builds to 50% of the max current available from regen.
Driving this way, you can actually come to a full stop as if you had applied brakes simply by taking your foot off the accelerator. Indeed we hardly touched the brake pedal and accomplished nearly the entire drive just by modulating the throttle in this way.
We DID miss the turn to 335. So we turned around and went back into Jackson anyway, cutting about a kilometer from the drive.
This, what I consider more “aggressive” regen is actually pretty interesting when you get used to it. You are pretty much always either accelerating or decelerating. The “sweet spot” of zero regen and zero acceleration is not very broad. The results of drive two were 75.2 km traveled, 77.7 AH and 1.03324 AH per kilometer. With MORE regen, it appeared it was actually LESS efficient????
In drive three, we disabled all the regen. I DID make the turn and so we recorded the exact 76.6 kilometers for 78 AH or 1.01827 AH per kilometer.
We’re not answering questions here. We’re just bringing up new ones. Not only was there NO gain from regenerative braking, but our most efficient energy use was WITH NO REGEN AT ALL. And the three drives were in a direct line from MAX REGEN being least efficient, to SOME REGEN in the middle to NO REGEN as the best. How can this be?
This week we repeated the test and we used the same drive. But we used a different car. The 2009 Mini Cooper Clubman.
This unit features regenerative braking as well. But it has a different motor – the MES-DEA 200-250 and a different controller – the TIMS 600. All the parameters are of course different. But it is interesting in that it is FRONT WHEEL DRIVE instead of rear wheel drive. And instead of 2000 lbs, it weighs 3500 lbs. It is a different pack size at 375 volts and 100 AH, as opposed to the Speedster’s 120v and 180AH. In fact, almost everything about it is different except for the measurement device, a TBS Expert Pro with a 50 mv 500A shunt.
The regen is used on both brakes and throttle. On the throttle, it is very mild, simulating the back pressure from the compression of an ICE engine very nicely. On the brakes, we did not employ a pressure transducer but trigger it on the brake light, which comes on at the lightest touch of the brake pedal. And it is progressive in that it builds over a 2 second rate. Often, on long stops, this leads to a pulsing feeling. As the braking builds, you let off the pressure but it resets and builds again very quickly so that the effect is of a gently pulsating regenerative braking.
On both cars, we can easily observe regenerated currents of up to 100 amperes. And indeed on a long downhill you can observe the AH counter tick backwards.
On drive 1 in the Mini Cooper, our odometer recorded 48 miles and our elapsed time, which we tracked for this version of the test, was one hour 28 minutes for an average speed of 32.72 mph. The drive end to end required a total net energy from the pack of 47.1 AH.
For drive 2, we simply switched the regen off totally using a switch we had mounted on the dash for this purpose early on. The car feels best taking off in 3rd gear, and in fact, we drove both these drives in 3rd gear the entire time. In first gear or reverse, the additional gear leverage makes the regen very uncomfortable at low speeds. So we had wired this switch to turn it off while backing up, parking or in inclement weather.
One of the downsides of regenerative braking is that it is reported to be a bit hard to control the car on ice and snow. So we had installed this disable switch.
We recharged and drove the same drive the next morning with regen totally disabled, again recording 48 miles on the odometer which does not provide tenths, and required one hour 25 minutes to complete for an average driving speed of 33.88 mph. It required 43.8 AH – some 3.3 AH less or 7% less than our original drive with regen.
Again, I find this result astounding, but very much in keeping with what we found on the Speedster. The reason we were not enjoying any gain in range in either the Speedster or Mini Cooper is that regen wasn’t providing any. Indeed, it seems to carry a penalty.
The comments since have been both frustrating and entertaining. The heart of the angst seems to be that they cannot envision what happened to the recovered energy during braking. They seem strangely willing to forfeit the throttle gain, but unwilling to do so in the case of the brake peddle.
The results are the results as far as I’m concerned at this point. We started with the observation that we were not seeing any gains in efficiency from regen, and after five 1.5 hour drives, we pretty much know why. Regenerative braking, either throttle actuated or brake actuated, does not provide any.
But it is not unusual for the answer to a question to trailer along a series of further questions. The regenerative braking traces back to very early electric cars. The 1906 Krieger Electric Landaulet featured regenerative braking and sported a range of 50 miles and a speed of 18-20 mph.
So the theory is not precisely recent.
So the question remains: Why no efficiency gains, either in experienced range or measured energy usage, from two different cars with entirely different regenerative braking schemes?
We don’t know. But it has always seemed to me that the basic concept was a little flawed. The highest and best use of the kinetic energy stored in the forward motion of a vehicle has to be to remain in motion. Traffic laws being what they are, you eventually have to brake to slow down. And regenerative braking is pictured as a way to recover some of that energy. Whether it is a large amount that is recovered or a small amount is not really the problem. You don’t have to be faster than the cheetah. You only have to be faster than the smallest gnu in your herd.
So how do we have a NEGATIVE amount???
There really aren’t a lot of options here. We must have a phantom drive at play. And I think this is where the answer lies. The drive not driven. The path not taken.
Cars and drivers interact. Adding regenerative braking absolutely alters the driving characteristics of the car. It is my observation that with regenerative braking on the throttle, you really don’t coast along very much. You are always either accelerating or decelerating. There is a bit of a sweet spot at around 0 amps but it’s a little tricky to find and stay in. And it is just easier to accelerate and decelerate as traffic demands. You quickly adjust to the car and you do indeed drive it differently because of that.
What if the drive with NO regenerative braking is significantly different than the drive with regenerative braking?
By way of a thought experiment in simplest form, let’s picture a drive of 5 miles on perfectly straight, perfectly flat road. In the first case, we will accelerate to 45 mph and on achieving that speed we will maintain it precisely at 45 mph until 200 feet before the end of the drive, where we will manfully haul in the binders and bring the car to a stop.
There is a certain level of energy required to accelerate the car to 45 mph. This energy must be sufficient to overcome the rolling resistance of the car, the wind resistance, but it also must be sufficient to accelerate the full mass of the entire vehicle to 45 mph ON TOP OF THAT. And that is beginning at an inertia of zero.
Once there, the energy required to maintain 45 mph is relatively trivial. We need only replace the energy from the parasitics of rolling resistance and wind resistance.
Let us say that the total energy on this drive requires 5 AH.
On a second drive, we accelerate to 50 mph. At the moment we hit 50 mph we immediately decelerate to 40 mph. Once at 40 mph we immediately accelerate again to 50 mph. And we continue this oscillation until the end of the 5 miles, averaging 45 mph for the entire distance.
It took more energy to accelerate to 50 than it did to 45. We recapture some of the kinetic energy stored in the forward motion of the car when we decelerate using regenerative braking. But our efficiency in doing this is less than unity and in fact let’s use the number 80% for the sake of argument. On reaching 40 mph we again have to apply energy to accelerate back to 50 – in fact 100% of the energy required to do so. But once there, we again decel at an 80% efficiency. We do 200 such oscillations in the course of the 5 miles.
It would appear obvious that it would require notably and measurably more energy to drive the car with the oscillations, even with the resulting recoveries from regenerative braking, than to maintain a steady state of 45 mph.
But our real world drive was much closer than that. Ok. Let’s revise the thought experiment. Let’s say we accelerated to 47 mph. And we decelerated to 43 mph. Are we getting closer?
Over the 48 miles of stop and go driving, a lot is going on. We know with some certainty that our measured results showed not only little gain, but negative gain. And that is the clue.
The regen induces oscillations in the driving characteristics of the car, and we as drivers adapt to them. If we compare the measured gains from the regenerative braking to the THEORETICAL drive of the SAME energy use that we WOULD have had without regen, the gains are real and apparent.
But if we compare to a REAL drive that we ACTUALLY drove without regen, we get a very different result. And this is because we lack the changes induced by the regen itself.
We do know that driving style can dramatically alter the energy consumption in both gasoline and electric cars. Electric cars SEEM more sensitive to this. We can readily observe that just such oscillations are induced when we have regen on the throttle.
With regen on brakes only, most people’s ability to picture this breaks down. However, the drives indicate the following:
Throttle plus brake – least efficient
Brake regen only – in the middle
No regen – most efficient.
It would fit that while the effect may be LESS pronounced with brake only regen, it remains a factor. And this would explain a penalty for regenerative braking of any kind, compared to a REAL drive with no regenerative braking at all.
So that’s our working theory at the moment. I’m actually quite confident of the test results. They match the initial observation that we weren’t actually gaining any advantage from regenerative braking. And they explain that rathe nicely – there aren’t any such gains.
On the why, I’m a little less confident. I like the scenario in that it accounts for all the observations. But it is curiously difficult to communicate succinctly. Usually, if I struggle to present, it is because I don’t REALLY understand it yet viscerally.