Analysis of regen on an ebike

[justin_le]

Has anyone else been doing any data logging and studies of their regen statistics?

While not formally accumulating regen data as you have been, I keep coming back to a single conclusion: I really love regen as a way of slowing down an electric vehicle, vs. using a mechanical "brake". Lately I've been splitting my miles between ebikes and a Tesla Model S, and in both cases I'm convinced that I would keep the regen in the engineering mix *even if* there was no current going back into the battery pack at all.

That's quite an interesting conclusion. I always try to emphasize the "saving wear on your brake pads" side of regen, but there's also a big psychological thrill of seeing power go back into the pack. With the Phaserunner field oriented controller I've got now, it will continue to do regen even at low speeds where it needs to draw power FROM the battery pack in order to sustain a given braking force. If you short all the windings you get nothing back into the battery, but the brake force diminishes as you approach a stop. But you can allow high levels of braking right up to a complete motor stall if you're willing to drain the battery a bit in this situation. In practice I only get this when I'm below about 5-6 kph, but I usually cease the regen if I see my watts above zero instead of below.

So far I've only encountered one annoying wrinkle on the ebike regen function -- it doesn't work when the battery pack is fully charged. So, if starting out with a topped-off pack, I try to run the pack down a bit before I think I'll need to be using the brakes... dumb, but there you have it.

This is another really good case for setting your charger to like an 80 - 90% charge level instead of always topping it to 100%. Then right from the getgo you have plenty of room to absorb regen energy into the battery, and as a plus you get several times the cycle and calendar life from the cells. As partial charging becomes more the norm then this one minor wrinkle with regen should cease to come up.

 

[rowbiker]

With the Phaserunner field oriented controller I've got now, it will continue to do regen even at low speeds where it needs to draw power FROM the battery pack in order to sustain a given braking force. If you short all the windings you get nothing back into the battery, but the brake force diminishes as you approach a stop. But you can allow high levels of braking right up to a complete motor stall if you're willing to drain the battery a bit in this situation.

Justin -- I don't know how I missed the Phaserunner controller thread(s) until your reference above, but I did. I'm now reading all posts regarding it I can find, but in the meantime, I want to make sure I'm in the queue for obtaining one. I've been pursuing true proportional regen from the very beginning of my interest in electric vehicle propulsion, with "full stop electric braking" being a slightly bigger motivator (focal point) than energy recuperation. I wasn't aware that wheel had been invented and built, and possibly even available.

I also agree with you about the 80 percent recharge limit to leave headroom in the pack for ebrake regen capacity. Just need to save up enough for a smart charger like the Satiator, which I assume has that capability.

 

[John in CR]

I have some controllers that have electric braking, but not regen. It's some kind of plug braking with no current ever to or from the battery. It's actually smoother than fixed current regen in that no initial sudden "hit" and even with my high Kv hubbies, which have softer regen for the same current, it provides a smooth braking force that feels continuous and works all the way down to a stop. On a high power rig you do have to be careful to avoid continuous activation from high speed, because it's too much energy to dissipate. The only controller I've popped in the last 5 years was one of those before I knew it wasn't regen, and the first time I rode the ebrake all the way down from 60mph+, one of the pair died. After that I just always alternated between the two controllers when slowing from high speed or long mountain descents.

I'm with rowbiker and would use electric braking if only for the benefit of reduced brake maintenance. With my faster than typical ebikes whose brakes also have greater work due to my higher than typical load, brakes are a maintenance hog without regen. Now I only have to change the front pads maybe once a year, vs nearly once a month before I had regen. Living in the mountains electric braking has a safety advantage too, since my mechanical brakes are always cool and ready for emergency stops...no brake fade.

My only issue with regen has been that the cheap Chinese controller makers need to include variable force regen braking. I've had hall brake handles waiting for it since 2008. At least I'm better off than most, since I have multi-level regen because my motors use 2 controllers. I can use one or the other or both at the same time for 3 different levels of force.

 

[LuboN]

Hello, I really love the regen brake, it feels so smooth and my golden motor magic controller brakes stronger at higher speeds - the opposite to mechanical brakes. On longer mountain rides I can get up to 30% of regen (peddling a lot and a little also downhill with regen engaged - this way I am able to get to 50% at only the uphill/downhill part of the trip). It extends my range with my moderate 14s 10Ah LiPo battery (which I don´t charge to 100% or discharge too low). Also I can ride uphill until battery is discharged, then I ride downhill with regen and little peddling (to keep legs in motion) and I can get enough charge for the remaining way home.
Recently I am thinking of separate regen braking module to get the regen I allways wanted with any motor controller. It would consist of three mosfets capable of shorting phase wires to ground driven by pwm according to signal generated in analog (throttle type) hall sensor mounted on my brake lever with some magnets. Generated current will flow through motor controller mosfets body diodes to battery. Does anybody know if there is any catch to do this, and what PWM frequency should be used? My motor is Golden Motor MP2.

 

[justin_le]

On longer mountain rides I can get up to 30% of regen (peddling a lot and a little also downhill with regen engaged - this way I am able to get to 50% at only the uphill/downhill part of the trip). It extends my range with my moderate 14s 10Ah LiPo battery (which I don´t charge to 100% or discharge too low). Also I can ride uphill until battery is discharged, then I ride downhill with regen and little peddling (to keep legs in motion) and I can get enough charge for the remaining way home.

These are some impressive numbers for sure, and I can believe it since I see similar results if I zone in just on the uphill/downhill segments of a trip rather than the full overall span.

Recently I am thinking of separate regen braking module to get the regen I allways wanted with any motor controller. It would consist of three mosfets capable of shorting phase wires to ground driven by pwm according to signal generated in analog (throttle type) hall sensor mounted on my brake lever with some magnets. Generated current will flow through motor controller mosfets body diodes to battery. Does anybody know if there is any catch to do this, and what PWM frequency should be used? My motor is Golden Motor MP2.

If you can be 100% sure that the main motor controller fets are completely off and disengaged during this period, then what you propose is possible in principle. You would need to actively set the phase with the lowest voltage relative to the other two as your ground, and then do PWM on the other phase which is at the highest potential. The tricky bit is that you'll need to commutate which phase you have grounded and which of the other two is doing PWM as the wheel rotates, most straightforward by using the hall sensors and hall mapping. If at any point the main motor controller tried to turn on its mosfets while your add-on regen circuit was running, then there would be smoke guaranteed. Anywhere from 10kHz - 20 kHz PWM frequency will give you the least trouble.

Of course from a system perspective it makes zero sense whatsoever to have an add-on circuit to do regen, when the motor controller itself can already do this with no additional parts. But if you've got a large supply of controllers that don't have regenerative braking enabled and you want to keep using them, then of all the harebrained proposals I've seen this one at least would work and give you variable braking control.

 

[LuboN]

Thank You Justin for Your opinion and advice, when I get to do some tests I will share with my experience.

 

[lumen0]

Hello, I really love the regen brake, it feels so smooth and my golden motor magic controller brakes stronger at higher speeds - the opposite to mechanical brakes. On longer mountain rides I can get up to 30% of regen (peddling a lot and a little also downhill with regen engaged - this way I am able to get to 50% at only the uphill/downhill part of the trip). It extends my range with my moderate 14s 10Ah LiPo battery (which I don´t charge to 100% or discharge too low). Also I can ride uphill until battery is discharged, then I ride downhill with regen and little peddling (to keep legs in motion) and I can get enough charge for the remaining way home.
Recently I am thinking of separate regen braking module to get the regen I allways wanted with any motor controller. It would consist of three mosfets capable of shorting phase wires to ground driven by pwm according to signal generated in analog (throttle type) hall sensor mounted on my brake lever with some magnets. Generated current will flow through motor controller mosfets body diodes to battery. Does anybody know if there is any catch to do this, and what PWM frequency should be used? My motor is Golden Motor MP2.

I had this same idea for a simple add on regen unit and think there are some very good reasons for doing so.
First is the low RPM issue where the motor is rotating too slowly to regenerate battery volatages.
Second is that regen should be transparent and engaged only when braking or using the brakes to slow down.
Regen braking should also appear to be linear in stopping or similar to standard brakes.

With all that said, it should be clear that regenerative braking could only be a benifit other than some additional cost.
I put some thought into a circuit that may achieve that goal and should not be complicated but could recover more lost energy by allowing lower volatges to be recovered.

To be simple it operates something like this (in theory):
A three phase bridge rectifier converts hub energy to DC and is smoothed a bit by a small capacitor. This voltage is then up converted by PWM and a small transformer like from a computer supply but operating in reverse.
Something like 5v in is converted up to about 90-100v by a single FET by PWM which is increased proportionally with the brake handle.
The higher voltage has much less current because every time the voltage doubbles the current is halfed. The output is converted to DC and fed back into the battery.
The only other thing required is a limiter to clamp the high voltage to something slightly higher than the batteries BMS cutoff, so if the battery is full and stops taking a charge, you still have brakes!

The brake handle needs a linear hall sensor for feed back to increase the PWM frequency and the pulse with is constant at a resonate frequency with the transformer.
It's actually a simple DC to DC up converter with a voltage limiter pushing all braking power into the batteries (other than some losses) and the more the brakes are pulled the more energy transfered until you reach the end where the pads contact.

Just a thought for now.

 

[Alan B]

I had the same idea awhile back, but have not actually tested it.

The up converter is called a "DC-DC boost converter", there are plenty of designs to look at.

The 3 phase rectifier makes it simple to implement.

Some of today's modern controllers already do this and more, since they can also route battery power to provide strong braking right down to zero speed, and without additional parts, just using the FETs that are already present. The amount of power consumed by this braking is actually very low since it is only used when the motor RPM is very low, thus the back EMF is low and the current mulitplication is very high. So a small battery current generates a large reverse motor current for braking.

I can see this in operation on the Borg's Sabvoton controller. There is power generated during the ebraking cycle until very low speeds, then there is a small consumption of power, but the braking stays very strong. At a low speed like 1 mph it drops out. This value is adjustable in the controller, but I have not experimented with setting it lower. In actual use it is low enough to stop the ebike with a foot, or a tiny application of the friction front brake.

 

[Will N too]

Thought I'd add my two cents.
10% isn't much. I live in Los Angeles, my house is at 500 feet on a hill about 100 feet above the surrounding terrain. There are a lot of hills & mountains where I like to ride.
I wonder how much more than 10% will be regenerated by hills and mountains.
It might come down to regenerative braking being an option for people who ride a lot of hills. When I got my Prius V I drove it (baby-driving) about 80 miles up into the mountains (to about 5000 feet) then back down again. For this trip I averaged over 90 mpg, normally I get about 42. Regular driving up to the mountains and back, not nearly as good.) If you could get this kind of boost with an E-bike, regen brakes and a mountain, people might seek out mountains instead of trying to go around them.

I'm also wondering if another advantage of regenerative braking might be to supplement, back-up and extend the life of regular friction brakes.

Will

 

[lumen0]

Well here's something, I was building another ebike from a cheap walmart bike with front and mid mono shock after my first solid frame ride which was incredibly rough, when I wanted to extend the wires on the controller so the connections could be inside the frame and protected from the weather.

I opened the controller and was soldering longer wires to the board and noticed the connection points had some short letters indicating the connections operation.
That's when I noticed one labled "EBS". This was the cheapest ebike conversion on Ebay so I figured it must mean Electronic Braking System..... right.
In fact it is...EBS! But it's better than that, It actually regens also. But it's better than that! By simply connecting the point to ground the system then simply operates as normal until you pull a brake and feel the ramping drag as it pulls to a stop, and at any time the brake is released it stops instantly and will again ramp into action when pulled again.

It's very easily controlled and it regens the power back to the battery. I set the LCD to indicate battery voltage and could peddle with the brake slightly pulled and see an increase from the brake current surge when pulled.
I almost killed myself trying to peddle with the bike strapped to the bench with the brake on. (I'm to old for that)

 

[justin_le]

Thought I'd add my two cents.
10% isn't much. I live in Los Angeles, my house is at 500 feet on a hill about 100 feet above the surrounding terrain. There are a lot of hills & mountains where I like to ride.
I wonder how much more than 10% will be regenerated by hills and mountains.

The highest stat that I have seen is over 60% range increase from regen by a a guy in a hilly part of Israel, who locked the clutch in a geared eZee motor which was able to do substantially better low speed regen efficieny than a direct drive hub. Have a look here:
https://endless-sphere.com/forums/viewtopic.php?p=1269754#p1269754

It might come down to regenerative braking being an option for people who ride a lot of hills.

This is a silly thing to say. Regenerative braking should be a de-facto standard for anyone who has a direct drive hub motor. There is zero reason not to have regen in that case since it's all win (small extra boost range from the battery and huge extension on the life of your mechanical brakes). The only time you could argue against regen is if you have an overrunning clutch (for instance a geared hub motor or a mid-drive motor) where there is no ability to transmit braking torque from the motor to the wheels. In those cases, you can argue that you prefer the freewheeling nature of no drag when you are pedaling the bike. But if you read this thread from the begining, you'll see that purely from an energy perspective this doesn't hold water. You easily gain more energy back from regen than you put into overcoming the motor drag for those times when you aren't using the motor.

When I got my Prius V I drove it (baby-driving) about 80 miles up into the mountains (to about 5000 feet) then back down again. For this trip I averaged over 90 mpg, normally I get about 42. Regular driving up to the mountains and back, not nearly as good.) If you could get this kind of boost with an E-bike, regen brakes and a mountain, people might seek out mountains instead of trying to go around them.

With a regen setup I find that my wh/km stats increase only slightly in hilly terrain vs flat terrain if I keep the speeds similar, while with a non-regen system you'll see wh/km figures that are 20-30% higher in hilly terrain than flat terrain.

I'm also wondering if another advantage of regenerative braking might be to supplement, back-up and extend the life of regular friction brakes.

That's not something to wonder about, that's hands down one of the greatest things about regen I had an eZee geared motor on my cargo bike for a couple years, so no regen possible, and I'd have to adjust the disk brake pads every 5 weeks or so and replace them every 6-8 months. With the DD motor and strong variable regen enabled with a field oriented controller, I haven't had to adjust the brakes once. The mechanical brakes have gone from the largest recurring maintenance item on the bike to something that doesn't need touching.

 

[rowbiker]

I "second that emotion" ...

In fact, the only remaining 'issue' that I believe remains on the table when it comes to regen used as a non-friction electric brake is the first mile or two of riding when you have just topped off your battery. Since regen dumps juice back into the battery, there must be some room for it to go there. I've gotten in the habit of 'wasting' some electricity on the beginning portion of my rides now just so I can begin using regen braking sooner. I live two blocks from the Mississippi River and it's fantastic parallel bike trails, but it's all downhill to get there.

I now typically burn off some battery before I head downhill to catch the trails, allowing me to use regen braking closer to the beginning of the ride. I've thought about just not charging to 100%, but I've got the eZee battery (from Grin) and just the simple bulk charger that came with it, and I'm concerned about cells becoming unbalanced. Maybe xmas comes early and I'll get the Satiator, which might let me safely charge the battery and leave some room for start-up regen. Or, Justin (in his unlimited spare time) might come up with an easy way to jettison electrons when the battery can't take them. I'm thinking a port on the Phaserunner that let's me connect an external ballast of some sort, like a bank of supercaps (or a Jacob's Ladder). The controller decides where to send the regen electrons, based on all the usual suspects, like battery voltage, etc.

At any rate, worrying about this 'issue' is a 'high class problem'.

 

[justin_le]

Or, Justin (in his unlimited spare time) might come up with an easy way to jettison electrons when the battery can't take them. I'm thinking a port on the Phaserunner that let's me connect an external ballast of some sort, like a bank of supercaps (or a Jacob's Ladder). The controller decides where to send the regen electrons, based on all the usual suspects, like battery voltage, etc.

I can say with certainty that this is NOT going to be on the table ! From a circuit perspective though, you could potentially have the controller jump right into plug braking where it alternately does very high intensity regen (effectively shorting the phase wires together) and then cuts off and coasts. By varying the shorting time vs. the coasting time you could control the average braking force pretty well, but all the braking energy becomes heat in the copper windings rather than going back to the battery. It'd be rough and would cause a lot of vibration, but it would get around this issue without external energy dump.

In your case, if you actually are concerned about the first mile issue going the 2 blocks downhill to the river (personally I wouldn't be, that's not nearly enough overcharge to do any damage), then either open the charger and find the small potentiometer that will let you tweak the output voltage to like 4.1 V/cell, or just hook up a diode inline with the charger output cord so that it's 0.7V less.

At any rate, worrying about this 'issue' is a 'high class problem'.

It is nice at least that the state of ebike technology is such that we can start fussing about really first world issue details like this

 

[rowbiker]

The diode ... now why didn't *I* think of that? And, I suppose if one diode isn't enough, then... Thanks Justin ... again!

 

[billvon]

I can say with certainty that this is NOT going to be on the table ! From a circuit perspective though, you could potentially have the controller jump right into plug braking where it alternately does very high intensity regen (effectively shorting the phase wires together) and then cuts off and coasts. By varying the shorting time vs. the coasting time you could control the average braking force pretty well, but all the braking energy becomes heat in the copper windings rather than going back to the battery. It'd be rough and would cause a lot of vibration, but it would get around this issue without external energy dump.

I think you'd still have some trouble with that energy dumping. When you short the windings you build up current very fast in the windings, and when you unshort them that current has to go somewhere, and since the winding is basically a big inductor that comes out in the form of high voltage, which the body diodes in the FETs rectify and send back to the battery.

At least normally. I guess if you shorted them long enough that the windings saturated, most of that energy would likely go to heat instead of power. Is that what you were thinking? Can you do that with a duty cycle long enough to saturate the windings, but short enough to feel like smooth braking?

 

[TimJ]

I came across this post and was really interested in the analysis. One thing I don't understand is that the maximum energy regenerated is only 50% of the energy available from the initial kinetic energy. I'm not an electrical engineer, so I didn't follow the argument for why it would be limited to this level. For hybrid cars, though, I remember reading in a couple places that the regen efficiency (kinetic energy to stored electrical energy) is around 80% typically, and the conversion back from the battery to kinetic energy is about the same, which yields a "round trip efficiency" of about 65%. It sounds like Justin was saying that the (one-way) regen efficiency is limited to about 50%. Is there a fundamental difference in what is normally used in cars vs. the configuration for regen in an ebike that accounts for this difference?

Thanks for the great information!
Tim

One thing that is slightly counter-intuitive with regen is that applying more regen braking force does not necessarily increase the regen current going back into the battery pack. This is easy to understand when you realize that the maximum regen braking torque occurs when all the motor phases are simply shorted together. In this situation, the regen current simply circulates through the windings of the hub motor generating heat, and none of that energy ever flows back into the battery pack. At the other end, when the duty cycle (D) of the motor controller is such that the effective voltage going to the hub motor (Vbatt * D) exactly matches the back emf voltage of the motor, then there is also no current flowing into or out of the battery as well, though in this case there is no braking torque either.

View attachment 3

In between these two points the current flowing into the battery follows a parabolic shape, with the maximum regen battery current occurring exactly in the middle. This is the point of maximum power transfer of energy back into the battery pack, and the hub is generating exactly half the braking torque that it would produce if all the windings were shorted. That's where you would want to be to get maximum amperage into the battery pack, but the efficiency of turning kinetic energy into useable battery energy is only 50% at this point.

In general, the less the regen braking current the higher the efficiency, since you have less I^2 R copper losses in the windings. However, if you are trying to slow to a stop, then applying a meager amount of regen torque won't increase the total amount of regen energy captured because even though the hub motor/controller may be converting a small amount of kinetic energy into battery energy with very high efficiency, you'll meanwhile be loosing much more energy to air drag and rolling friction while you come to a prolonged stop. Likewise, if you come to a very abrupt regen stop, you won't loose much of your kinetic energy to air and rolling friction, but the efficiency of converting that kinetic energy into battery energy via regen will be quite poor. Somewhere in between these two points is some optimum regen braking torque which would get a maximum amount of energy back into the pack.

Justin

 

[999zip999]

I live in a hilly area and have max regen ( yes trying to reprogram ) I see a 126amp spike on my muxus 4t 80v 90 amp controller. It must just be a momentary Spike reading on my C.A . As I average 8 - 10% over a ride.

 

[TimJ]

Can anyone provide more input on this?

Thanks,
Tim

I came across this post and was really interested in the analysis. One thing I don't understand is that the maximum energy regenerated is only 50% of the energy available from the initial kinetic energy. I'm not an electrical engineer, so I didn't follow the argument for why it would be limited to this level. For hybrid cars, though, I remember reading in a couple places that the regen efficiency (kinetic energy to stored electrical energy) is around 80% typically, and the conversion back from the battery to kinetic energy is about the same, which yields a "round trip efficiency" of about 65%. It sounds like Justin was saying that the (one-way) regen efficiency is limited to about 50%. Is there a fundamental difference in what is normally used in cars vs. the configuration for regen in an ebike that accounts for this difference?

Thanks for the great information!
Tim

 

[justin_le]

At least normally. I guess if you shorted them long enough that the windings saturated, most of that energy would likely go to heat instead of power. Is that what you were thinking?

Yes exactly. You need not saturate the windings. What I'm saying is leave the windings shorted until the current is leveled off at Vemf / Rwinding, so it's dissipating a ton of heat without putting charge back into the battery. It's only when you open circuit the windings again that you'd have a bit of inductive freewheeling current into the pack. You then control the ratio of time that the windings are fully shorted vs. fully open to adjust the effective braking force.

Can you do that with a duty cycle long enough to saturate the windings, but short enough to feel like smooth braking?

That is of course the question. Smooth braking I think is out of the question, but if you say periodically shorted and then open circuited the windings at a frequency of like 200 Hz or so, it would likely vibrate like heck and be a little unpleasant but I think your velocity would slow down steadily with most of the energy going into copper losses and only a very small amount back into the battery pack. Achieving effectively the goal of varying regen even in a situation when the battery pack is fully charged and can't take more current.

 

[justin_le]

Can anyone provide more input on this?

Thanks,
Tim

I came across this post and was really interested in the analysis. One thing I don't understand is that the maximum energy regenerated is only 50% of the energy available from the initial kinetic energy. I'm not an electrical engineer, so I didn't follow the argument for why it would be limited to this level.

You don't need to be an electrical engineer, just familiar with physics. The electrical efficiency of regen with typical direct drive ebike hub motors is about 80%, similar to the motor driving efficiency. But you don't get 80% of your kinetic energy back to the battery pack because while you are slowing down under regen; your kinetic energy is also getting lost by rolling resistance and air drag, so those take a chunk of the available energy pie too.

The different with a hybrid car is that they have much greater mass, so the ratio of air drag to kinetic energy is lower and hence the losses to air resistance while slowing down account for a much smaller piece of the pie. If I did the same tests on my ebike and added like 400 lb of dead weight on the rack, then I'd similarly get more like 65-70% recapture rate of kinetic energy back to the pack. Same thing if I did the tests on the moon where there is no air drag.

Most likely though in the "couple places you read" about regen efficiency, they weren't even factoring any of the air and rolling drag into the equation.

 

[MexicanElmo]

TimJ wrote:
I came across this post and was really interested in the analysis. One thing I don't understand is that the maximum energy regenerated is only 50% of the energy available from the initial kinetic energy. I'm not an electrical engineer, so I didn't follow the argument for why it would be limited to this level.

You don't need to be an electrical engineer, just familiar with physics. The electrical efficiency of regen with typical direct drive ebike hub motors is about 80%, similar to the motor driving efficiency. But you don't get 80% of your kinetic energy back to the battery pack because while you are slowing down under regen; your kinetic energy is also getting lost by rolling resistance and air drag, so those take a chunk of the available energy pie too.

The different with a hybrid car is that they have much greater mass, so the ratio of air drag to kinetic energy is lower and hence the losses to air resistance while slowing down account for a much smaller piece of the pie. If I did the same tests on my ebike and added like 400 lb of dead weight on the rack, then I'd similarly get more like 65-70% recapture rate of kinetic energy back to the pack. Same thing if I did the tests on the moon where there is no air drag.

I think that when we are expecting high numbers on Regen we are being kind of unfair agains this hubmotors when we expect efficiencies around the 70-80%.

We are told by manufacturers of Ebikes and hub motors that efficiencies are in the 80% region... however if you look closely into the motor's performance curves you will see that the motor performs at best efficiency(70-85%) between 1/2 top speed to little below top speed (By top speed I mean it's maximum RPM rating while operated under no load). Lower speed operation always will lead to lower efficiencies.

Has anyone of you performed an acceleration test to measure efficiency from 0 km/h to lets say 30km/h?
My guess is that it would be as inefficient as the regen on the same bike. (just a tiny bit better due the electronic voltage boost required to perform regen). Actually during the past week I tried to perform such a test but due to lack of equipment I couldn't.

Also we are expecting a bit to much from this experiments due to the fact that the mentioned motors may be a little bit underpowered, I remember the motor Justin Le used for it's initial tests being a 500 W DD motor, when let's say that a 100 Kg man+bike is traveling at 8 m/s (28.8 Km/h) has a kinetic energy of 3200 J, and lets say that in order to avoid as much as possible aerodynamic drag + rolling resistance on a lenghty slow down we would like to brake from 8 m/s to 0 m/s in 4 seconds, we would need 3200 J / 4s = 800 watts of power capability within that motor, which of course we have as manufacturers guarantee in their spec sheets, but on that 800 W input regime it's efficiency is quite lower. So the short slowdown is not the best method, however if we take a lenghty slowdown we may face the other side of the problem, lots of aerodynamic+rolling losses.

Of course we would be able to get better efficiencies than 50% on bikes... but not by much, how? my best bet will be to use higher power motors wound for high torque and lower RPM, we would end up with a low top speed motor, but would be great for quick take offs.

The difference with electric Hybrid cars I believe is mostly about their aerodynamic construction, their motors are basically designed to push a 1.5-2 ton machine from 0 to 100Km/h in 7-3 seconds so they are really powerfull and can take back a great amount of energy back if their system allows them and last they are operated at high voltage, Rimac concept one I think is used with DC at 600 V, Teslas batteries are around 320 V but their motors are AC so the actual AC voltage should be higher, Nissan Leaf is 480 and so on, you know, just as transmission lines, high voltage means low currents, less heat, less copper can be used. I wouldn't consider mass an advante on cars to have a better regen number as it's kinetic energy is directly proportional to it's mass to both, the bike and the car.

I just made a build of an electric bike with no batteries but supercacitors, certainly it wasn't a success but I won't give up on supercaps now, I will post my results on another thread. But basically it was all about this same topic: Regenerative braking.

Regards.

 

[MexicanElmo]

please take a look at my post on my supercap build
https://endless-sphere.com/forums/viewtopic.php?f=3&t=90197

I need to get my hands on a geared motor and start playing with it just like that guy who welded his to avoid the ratchet mechanism.

 

[justin_le]

We are told by manufacturers of Ebikes and hub motors that efficiencies are in the 80% region... however if you look closely into the motor's performance curves you will see that the motor performs at best efficiency(70-85%) between 1/2 top speed to little below top speed (By top speed I mean it's maximum RPM rating while operated under no load).

That's only true when you are looking at the full throttle power and efficiency curve. In practice, most people when moving slow are riding part throttle, and the motor efficiency can still be very good (>80%) even at low speeds when you are running at the correspondingly lower power that is required.

But when doing regen, most people are after as much braking force when they are going slow as when they are going fast, and so you do see a very significant dropoff in the electrical efficiency of regen as the bike comes to a stop.

Lower speed operation always will lead to lower efficiencies.

That's broadly true but not strictly true. At high speeds and small torque levels the motor can become less efficient than generating the same torque at lower speeds.

My guess is that it would be as inefficient as the regen on the same bike. (just a tiny bit better due the electronic voltage boost required to perform regen). Actually during the past week I tried to perform such a test but due to lack of equipment I couldn't.

You actually don't need to have a bunch of test equipment to see this kind of data, since the motor regen behavior is also pretty neatly modeled on the trip simulator.
http://www.ebikes.ca/tools/trip-simulator.html

For example, if I pick an H3540 motor, 120kg mountain bike, and create an elevation profile that's a 10% downhill grade over a 400m elevation drop. The input looks like this

It's important for this to check the "regen enabled" box on the controller parameters. Then we can set the speed limit to clamp the vehicle to a certain speed with regen and select a section to view the total regen watt-hours recaptured, and also set the cursor on the graph to look at the motor efficiency.



You can then change you speed limit input to see how much speeding up or slowing down affects the motor efficiency during regen. And if you really want to have fun with it, we've provided a "Run Simulation Set" button in the bottom left. This will let you have the trip simulator automatically step one of the simulator inputs between two values. In this case, I'll run the simulation from 10 kph to 40 kph, and choose to have both the cursor data (for motor efficiency) and the selected section data (for total regen Wh) saved.


When you run this and then plot the output data, you see that limiting the speed to 10kph via regen has very low motor efficiency (30%) and corrspondingly low energy recapture rate. As you allow the bike to go down faster, then the motor efficiency increases quite a bit (getting to 80% at 27 kph) and continues to improve at higher speeds. However, the watt hours recovered reaches a max of 74 Wh at 25 kph. As you go downhill faster than this, the electrical efficiency of the motor keeps improving to 83-84%, but the amount of energy recovered and put into the battery pack starts to decrease. While the efficiency is better, more of the total potential energy is now going to wind drag.


The initial potential energy available is 120 kg * 400m * 9.8 / 3600 = 130 watt-hours. So you can again recover a little over 50% of your watt hours, but this does NOT mean that regen is only 50% efficient. It was close to 80% efficient.

Of course we would be able to get better efficiencies than 50% on bikes... but not by much, how? my best bet will be to use higher power motors wound for high torque and lower RPM,

This is a fallacy. You don't wind a motor for high torque and lower RPM. The motor winding has very little effect at all on how much regen efficiency and regen braking force a motor can produce. In practice you will see a difference if you have a simple controller with just a fixed regen level, but in any controller where you can set the regen phase current higher for the faster motor so that you are commanding the same braking torque, then the behavior and motor efficiencies will be the same with fast or slow winding hub.

I wouldn't consider mass an advante on cars to have a better regen number as it's kinetic energy is directly proportional to it's mass to both, the bike and the car.

The added mass doesn't change the efficiency of regen from an electrical perspective, but it most definitely increases the % of kinetic energy that you will typically recapture as a result of regenerative braking. As a thought experiment, imagine your car weighs almost 0kg but still has all the rolling resistance and air drag of a normal car. What kind of regen numbers would you expect to get?

I just made a build of an electric bike with no batteries but supercacitors, certainly it wasn't a success but I won't give up on supercaps now, I will post my results on another thread. But basically it was all about this same topic: Regenerative braking.

Hey congrats on building that. There was a time maybe 10-15 years ago when Supercaps looked like they might have a place in EV's to to buffer the higher current flow of regen when dealing with batteries that could only take limited charging current. But I think you would find now that the same weigh of lithium battery will have no problem handling the regen current levels, and then carries like 100x more total energy as well, which can be kindof useful.

 

[MexicanElmo]

Hello! thanks, I wasn't aware of your simulator, it's really neat indeed. Actually I was doing some calculations on a spreadsheet but this saved me lots of time.

Now let me tell you, that I wasn't trying to mimic a normal ebike, I was trying to boost my leg power, hence I used ebike parts as it was the easiest way but If I come up with a better Kinetic Energy Recovery System I will try it. Why the supercap module? because I only want enough energy storage for a couple of normal stops from 30 km/h to 0 for the average rider, lets say 9000J or 2.5 Wh (that would be less than 1 kg, less than 1 Lt in volume and less than 200 USD), and that energy use it to reach a confortable speed (that's the part where the freewheel feature comes in handy) to pedal until the next red light, hence refill that supercap module and then all over again.

As most cycling is done in urban areas with slight hilly terrain I want to be able to capture that stoping energy. As Justin pointed out at high speeds most kinetic energy is lost outside the motor/controller, while on the other side we have low speeds that mostly lead to high losses within the motor. However this is FOR CURRENT DESIGNS! that target for an efficient sweet spot/region that is over 200 RPM, because that's the region where the motor will operate mostly, right?

I simulated the same Justin did, but with a different motor, I know it's geared and regen can not be done unless the motor is modified or placed backwards but it illustrates my point, the gear ratio, the winding and overall design WILL have an impact on where the efficiency sweet spot/region is. This 12T MAC was designed for 200 RPM operation.


However a motor can be designed for an appropiate output and appropiate speed.

Here's a graph of the same motor efficiency along the overall bike energy recapture efficiency during regen.



Those numbers are not that bad and most of them are over 50%

This is a fallacy. You don't wind a motor for high torque and lower RPM. The motor winding has very little effect at all on how much regen efficiency and regen braking force a motor can produce. In practice you will see a difference if you have a simple controller with just a fixed regen level, but in any controller where you can set the regen phase current higher for the faster motor so that you are commanding the same braking torque, then the behavior and motor efficiencies will be the same with fast or slow winding hub.

My bad, I wasn't just talking about winding, I was talking all around motor design. But winding has indeed an effect on where the motor sweet spot will be at.

The added mass doesn't change the efficiency of regen from an electrical perspective, but it most definitely increases the % of kinetic energy that you will typically recapture as a result of regenerative braking. As a thought experiment, imagine your car weighs almost 0kg but still has all the rolling resistance and air drag of a normal car. What kind of regen numbers would you expect to get?

The different with a hybrid car is that they have much greater mass, so the ratio of air drag to kinetic energy is lower and hence the losses to air resistance while slowing down account for a much smaller piece of the pie. If I did the same tests on my ebike and added like 400 lb of dead weight on the rack, then I'd similarly get more like 65-70% recapture rate of kinetic energy back to the pack. Same thing if I did the tests on the moon where there is no air drag.

Are you sure that you would be getting 65 -70 % on the same bike? I don't think your bike could handle 400 extra lbs, however if it did the rolling resistance will increase, also your motor won't be able to capture all that kinetic energy in the same distance, hence the rolling resistance will increase even more. My point is, the bike + rider is mostly a non aerodynamic entity, it was not designed to be so, however cars were. But yes both mass and aerodynamic parameters are within equations so both have an effect on the energy recaptured I was just under the impression that under the "the different with a hybrid car is that they have much greater mass..." could lead to people assuming that a higher efficiency EV will be most likely heavy.

Hey congrats on building that. There was a time maybe 10-15 years ago when Supercaps looked like they might have a place in EV's to to buffer the higher current flow of regen when dealing with batteries that could only take limited charging current. But I think you would find now that the same weigh of lithium battery will have no problem handling the regen current levels, and then carries like 100x more total energy as well, which can be kindof useful.

Supercaps are way more than a nanotechnology/chemistry promise, they have an important role in some vehicles and most electronics. If you want to stick to the old known batteries that's fine but please, take a look at the following application notes from mouser electronics.

http://www.mouser.com/applications/new-supercapacitor-applications/

Also I want to say that the whole idea of the supercap bike was due to these buses

https://en.wikipedia.org/wiki/Capa_vehicle

They are cheaper than battery based buses and 30-40% more efficient than traditional trolley buses, they do not require transmission lines all along the route. They are just a beautiful type of transportation. Please take a look.

 

[Mahe]

Excellent thread, especially start and end, now understanding quite a bit more on regen efficiency, but given the tradeoffs of various systems in term of hill-climbing efficiency, I would appreciate your insights on the overall performance in a mountainous area. The irony is that while it is arguably the most sensible use environment for regen use (both in term of regen and lower brake wearing), it is also where mid-drives or geared motors are typically recommended since they work at maximum efficiency. Hoping to get Justin involved in this, I'll start quoting ebikes.ca (https://www.ebikes.ca/product-info/grin-kits.html#geared-vs-direct-drive):

- Pros of Geared Motor: Better hill climbing efficiency
- Pros of Direct Drive: Regenerative braking

(I understand that mid-drive would stand there as: Even Better hill climbing efficiency -- whatever that means)
You will agree that for whoever wants to purchase his/her first ebike, that is confusing : )

Can you give any number on that? Even theoretical.
Here a more quantitative analysis that I made the other day. For a planned 260km (two days in ebike) trip in a mountainous area in South Italy (gpx track from Komoot here) I obtain the following:

The assumptions are:

• 100kg weight

• 100W human power

• 250W motor power most of the time, with 80% efficiency (extremely simplified, none of the refined assumption from ebikes.ca)

• speed calculated at constant power (human + motor x efficiency) but imposed to be between 10 and 32 km/h

• when balance speed would be over the max, the extra energy is added as potential for regen braking, diagnostic only (100%), not actually applied

• CdA = 0.5 for air drag, and Crr = 0.005 for rolling friction -- all as simplified friction law

(https://github.com/perrette/ebikesimulator for more info)

What you can see from this figure is that at 80% efficiency the motor would draw 1700Wh from the battery, while 600Wh are lost braking downhill. If 80% from that could be recovered (I finally understood toward the end of this thread that is what we are talking about, and that the confusing 5-20% relate to overall saving in battery usage during a trip, accounting for air drag etc -- that is quite different), that would be significant gain, close to 28% overall.

On the other hand, if instead of using an efficient mid-drive one has to use a less efficient direct drive, that could be entirely offset ! Even without considering the increase in weight (3.5kg for TSDZ2 mid-drive, vs 6-10 kg for DD+controller, right?).
For instance, considering 50% efficiency in the motor leads to 1000Wh additional consumption !! If that were 70% efficiency that would be an additional 250Wh, less than recoverable regen -- good.

So my main question here is: is there any rule of thumb to calculate motor efficiency, so that the question of mid-drive vs DD+regen-braking in the mountains can be adequately answered? I realize that is exactly the kind of question the ebikes.ca simulator could answer but the many motor types and other options make it a little confusing to me. Can anyone help? Here some quotations from the above thread:

the motor performs at best efficiency(70-85%) between 1/2 top speed to little below top speed (By top speed I mean it's maximum RPM rating while operated under no load).

That's only true when you are looking at the full throttle power and efficiency curve. In practice, most people when moving slow are riding part throttle, and the motor efficiency can still be very good (>80%) even at low speeds when you are running at the correspondingly lower power that is required.

Climbing probably means low speed and full power...

TimJ wrote:
my best bet will be to use higher power motors wound for high torque and lower RPM, we would end up with a low top speed motor, but would be great for quick take offs.

I understand this statetement as a hint towards higher motor efficiency.

I was considering the TSDZ2 mid-drive for myself and my g/f to move around this region, but if the regen argument really makes a DD or regen-enabled (no clutch) geared motor look good then I would reconsider. Especially if that allows to have a less intrusive front hub motor instead of giving up on the front gears. Which motor?

EDIT: I just found out that the GMAC 10T has quite a good efficiency over a broad range of speeds, with ebikes.ca simulator giving about 70% at 10 km/h with a 10% slope. For that kind of numbers (hypothesized in the post above, by chance), it seems that the regen would bring more back than is lost from sub-optimal use uphill. That's very interesting. Seems to only exist as rear motor, and it is currently out-of-stock anyways.

Again, any help / advice greatly welcome !!

Thanks