Posts
Thursday, July 22, 2010
Cliff Notes on shows.
Eric Kriss of Krissmotors has been following our progress for some time. He has successfully built an excellent Cobra kit car in the past and has some experience with the Saab Sonnet fiberglass sports car.
He is working on a Speedster Part Duh and has been following the shows and offering some suggestions with peculiarly keen insight. He recently sent me an index of our shows through July 2 in PDF format that is just particularly well done.
We're starting to get a lot of questions of the nature "Which show did you cover the EVInterstellear Drive Flavis Waven in. This Show index of Mr. Kriss's may help you a bit on that front.
We do a show each week. I can't quite recall myself. Many thanks Eric. I found this particularly well done.
Regards;
Jack Rickard
Friday, July 16, 2010
4C LiFePo4 Power Tests
I was able to post this week's video ON Friday. We shot a good bit of it last week. So this week we do a 4C load test of four CALB 100AH cells.
The test actually consists of 40 pulses of 400AMPS lasting 2.5AH or about 30 seconds. The CALB cells list 4C for 30 seconds on their spec sheet. We do a full 100 AH with every single tron coming out at a 400A rate (well, until the end when it more or less collapses).
Here is the Microsoft Xcel spreadsheet chart of voltages and temperatures experienced across the test.
The good news is that the cells will indeed do 400 amps quite handily. I think the bad news is that at that rate, the Peukert effect does come into play and you will get a little less than 100 AH from the cell.
Overdischarging the cell of course is damaging. We had venting after 95AH and indeed lost a cell. But this was not entirely accidental. One of the things we wanted to demonstrate was failure under a substantial load from overdischarge. No fire. No explosion. No thermal runaway. In fact, when we did discontinue after 99.9AH, all four cells cooled fairly rapidly.
Interesting test. You'll want to watch this one.
Jack Rickard
The test actually consists of 40 pulses of 400AMPS lasting 2.5AH or about 30 seconds. The CALB cells list 4C for 30 seconds on their spec sheet. We do a full 100 AH with every single tron coming out at a 400A rate (well, until the end when it more or less collapses).
Here is the Microsoft Xcel spreadsheet chart of voltages and temperatures experienced across the test.
The good news is that the cells will indeed do 400 amps quite handily. I think the bad news is that at that rate, the Peukert effect does come into play and you will get a little less than 100 AH from the cell.
Overdischarging the cell of course is damaging. We had venting after 95AH and indeed lost a cell. But this was not entirely accidental. One of the things we wanted to demonstrate was failure under a substantial load from overdischarge. No fire. No explosion. No thermal runaway. In fact, when we did discontinue after 99.9AH, all four cells cooled fairly rapidly.
Interesting test. You'll want to watch this one.
Jack Rickard
Sunday, July 11, 2010
More Regenerative Braking Results. It gets worse...
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.
I'
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.
Jack Rickard
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.
I'
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.
Jack Rickard
Monday, July 5, 2010
Regenerative Braking - Entirely a Myth?
Rather interesting week. It started with a longish drive in the new Speedster design. We reached about 107 miles when it seemed politic to bring it into the barn and check some batteries.
One notable thing about this was that 107 miles was the max drive we ever actually did with the original Speedster. Over the winter, we took out all the toe-in on the front end, went to a Redline MTF fluid in the transaxle, and made a couple of other minor tweaks that might perhaps get us another mile or so. But I never did get around to actually doing the drive.
But the new Speedster design features an AC induction motor and 3 phase Curtis controller that has more than adequate provisions for regenerative braking. As we are using a very similar battery pack between the two cars, we should have gotten SOME increase in range from the regenerative braking.
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.
It is fairly obvious to anyone that this must offer some efficiencies for an electric car. And indeed, the idea is not only not new, but has been actively discussed and deployed for some thirty years.
I have for some time suspected all is not as it seems in this. The thing that bothered me was the law of conservation of energy. Broadly applied this indicates that a mass in motion will tend to stay in motion, and the energy of interest is usually referred to as kinetic energy or "stored" energy held in the forward inertia of the mass. But its best and highest use is of course to continue in motion. If we convert it to electricity and try to put it back in a battery, we naturally fall prey to all the inefficiencies inherent in that.
In a real drive of a real car, how much of this regenerative braking would actually be useful? While the concept of driving a car down the street doesn't sound like rocket science, there are really a LOT of different things going on more or less simultaneously and in various sequential time frames that is in reality quite complex. Yes, we do accelerate, and yes we do brake, but we do a lot of other things as well, and so does the car.
So I had always said it is probably a little overhyped, and my sense was regenerative braking could provide 6-8% efficiencty gain, not the 15-25% I've heard bandied about.
More recently I fell under the spell of a fascinating speaker, Dr. Illah R. Nourbakhsh. Dr. Nourbakhsh is Associate Professor of Robotics, The Robotics Institute, Carnegie Mellon University. He was the former Robotics Group Lead,
Ames Research Center, National Aeronautics and Space Administration.
Dr. Nourbakhsh drives a RAV-4 EV, and has noted some startling efficiencies in real world driving. Unfortunately, on reviewing his data, it becomes apparent that he actually measures AH consumed going forward, AH produced during regen, and assigns efficiency values based on those measurements. Bu the hasn't actually done anything with actually measuring how much energy is required to move the car. This tends to ignore driving dynamics.
He has started a web site and is aware of the concept that regen will do various things on various driving patterns, and indeed his web site at http://www.chargecar.org purports to collect actual drive data from various people across the country. But he appears to be focused on variations in the two measured quantities - AH out and AH in.
They are actually working on a design for an ultracapacitor implementation that will make capture of this regenerative braking more efficient.
Unable to resolve all this, we set up three tests.
In the first test, we drove the car 76.6 kilometers with full maximum current regenerative braking tied to pressure applied on the brake pedal. We drove the route almost entirely in 2nd gear. And we ignored how much was coming out and going back in, and looked for the TOTAL AH consumed on the 76.6 mile drive. We drove through very urban area mostly with dozens of traffic stop lights and signs, lots of vehicular traffic, and plenty of hills and curves that should have made regenerative braking quite attractive.
In test two, we drove the same route at the same temperature and SOC and again used 2nd gear all the way. But this time we added neutral braking. Neutral braking is braking applied by using the motor as generator but driving that with the accelerator pedal. The car accelerates of course when you press the brake pedal. But when you start to release the throttle pressure, regenerative braking kicks in. I missed a turn on this lap and so we totaled 75.2 km.
In our third test, we drove the same lap under the same conditions but with ALL regenerative braking disabled.
Obviously, we expected maximum efficiency from the more aggressive regen derived from brake and throttle both.
We would expect the 2nd greatest efficiency from the car with regen deployed only on the brake signal.
And NO regen should bring up the rear.
Surprisingly, here's what we found.
Drive 1 76.6km 78.2 AH or 1.02088 AH per kilometer
Drive 2 75.2km 77.7 AH or 1.03324 AH per kilometer
Drive 3 76.6 km 78.0 AH or 1.01827 AH per kilometer
This shows the LEAST amount of energy used to accomplish the 76.6 km drive to be when using NO regenerative braking at all. The differences were pretty close - probably in the noise level. But what IS apparent immediately is that regenerative braking in ANY form would appear to have no efficiency additions AT ALL.
Yes there were plenty of AH going out and coming back in during regenerative braking, but over the course of the drive, it appears to have nulled out to zero as a benefit.
I found this a bit astounding. Chevrolet's Volt, the Tesla Roadster, almost ANY serious attempt at an electric drive train pays homage to regerative braking. Has NO one ever done an end to end test to see if it actually worked?
I'd repeat the test, but I haven't a clue what I would do different.
Jack Rickard
One notable thing about this was that 107 miles was the max drive we ever actually did with the original Speedster. Over the winter, we took out all the toe-in on the front end, went to a Redline MTF fluid in the transaxle, and made a couple of other minor tweaks that might perhaps get us another mile or so. But I never did get around to actually doing the drive.
But the new Speedster design features an AC induction motor and 3 phase Curtis controller that has more than adequate provisions for regenerative braking. As we are using a very similar battery pack between the two cars, we should have gotten SOME increase in range from the regenerative braking.
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.
It is fairly obvious to anyone that this must offer some efficiencies for an electric car. And indeed, the idea is not only not new, but has been actively discussed and deployed for some thirty years.
I have for some time suspected all is not as it seems in this. The thing that bothered me was the law of conservation of energy. Broadly applied this indicates that a mass in motion will tend to stay in motion, and the energy of interest is usually referred to as kinetic energy or "stored" energy held in the forward inertia of the mass. But its best and highest use is of course to continue in motion. If we convert it to electricity and try to put it back in a battery, we naturally fall prey to all the inefficiencies inherent in that.
In a real drive of a real car, how much of this regenerative braking would actually be useful? While the concept of driving a car down the street doesn't sound like rocket science, there are really a LOT of different things going on more or less simultaneously and in various sequential time frames that is in reality quite complex. Yes, we do accelerate, and yes we do brake, but we do a lot of other things as well, and so does the car.
So I had always said it is probably a little overhyped, and my sense was regenerative braking could provide 6-8% efficiencty gain, not the 15-25% I've heard bandied about.
More recently I fell under the spell of a fascinating speaker, Dr. Illah R. Nourbakhsh. Dr. Nourbakhsh is Associate Professor of Robotics, The Robotics Institute, Carnegie Mellon University. He was the former Robotics Group Lead,
Ames Research Center, National Aeronautics and Space Administration.
Dr. Nourbakhsh drives a RAV-4 EV, and has noted some startling efficiencies in real world driving. Unfortunately, on reviewing his data, it becomes apparent that he actually measures AH consumed going forward, AH produced during regen, and assigns efficiency values based on those measurements. Bu the hasn't actually done anything with actually measuring how much energy is required to move the car. This tends to ignore driving dynamics.
He has started a web site and is aware of the concept that regen will do various things on various driving patterns, and indeed his web site at http://www.chargecar.org purports to collect actual drive data from various people across the country. But he appears to be focused on variations in the two measured quantities - AH out and AH in.
They are actually working on a design for an ultracapacitor implementation that will make capture of this regenerative braking more efficient.
Unable to resolve all this, we set up three tests.
In the first test, we drove the car 76.6 kilometers with full maximum current regenerative braking tied to pressure applied on the brake pedal. We drove the route almost entirely in 2nd gear. And we ignored how much was coming out and going back in, and looked for the TOTAL AH consumed on the 76.6 mile drive. We drove through very urban area mostly with dozens of traffic stop lights and signs, lots of vehicular traffic, and plenty of hills and curves that should have made regenerative braking quite attractive.
In test two, we drove the same route at the same temperature and SOC and again used 2nd gear all the way. But this time we added neutral braking. Neutral braking is braking applied by using the motor as generator but driving that with the accelerator pedal. The car accelerates of course when you press the brake pedal. But when you start to release the throttle pressure, regenerative braking kicks in. I missed a turn on this lap and so we totaled 75.2 km.
In our third test, we drove the same lap under the same conditions but with ALL regenerative braking disabled.
Obviously, we expected maximum efficiency from the more aggressive regen derived from brake and throttle both.
We would expect the 2nd greatest efficiency from the car with regen deployed only on the brake signal.
And NO regen should bring up the rear.
Surprisingly, here's what we found.
Drive 1 76.6km 78.2 AH or 1.02088 AH per kilometer
Drive 2 75.2km 77.7 AH or 1.03324 AH per kilometer
Drive 3 76.6 km 78.0 AH or 1.01827 AH per kilometer
This shows the LEAST amount of energy used to accomplish the 76.6 km drive to be when using NO regenerative braking at all. The differences were pretty close - probably in the noise level. But what IS apparent immediately is that regenerative braking in ANY form would appear to have no efficiency additions AT ALL.
Yes there were plenty of AH going out and coming back in during regenerative braking, but over the course of the drive, it appears to have nulled out to zero as a benefit.
I found this a bit astounding. Chevrolet's Volt, the Tesla Roadster, almost ANY serious attempt at an electric drive train pays homage to regerative braking. Has NO one ever done an end to end test to see if it actually worked?
I'd repeat the test, but I haven't a clue what I would do different.
Jack Rickard
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