Super Capacitors - Sähköautot - Nyt!
The lithium-ion batteries have a higher power density than lead or nickel based batteries. The nominal power output available from the pack should surpass the maximum power output of the motor by a factor of two or more, unless the pack size is very small. Likewise, the charging can be done as fast as discharging, which means that any adequately sized battery pack will be able to handle all the power you can push into it with good efficiency. After all, you're not going to slam your brakes through the floor every time you want to stop.
A 1 ton car travelling at 10 m/s will have 5 kJ of kinetic energy and bringing it to a halt over 2 seconds would mean a constant regenerative power of only 2.5 kilowatts, which is a fraction of what the battery pack can take.
The main power loss in regenerative braking is in the power conversion, because the voltage and current waveforms generated by the motor as it spools down have to be conditioned before they can be efficiently fed into a capacitor or battery. To fill the capacitor, you need to provide higher and higher voltages from a generator that is turning slower and slower, which means that you have to pump the voltage up somehow, and that process wastes a lot of the energy.
It is easy to see, though, how little energy there is available through regenerative braking. Due to the low running speed of the car in city traffic, the regeneration efficiency and recoverable energy is inherently low, and the amount of accelerations and decelerations drops sharply when moving on to high-speed commutes where the main energy loss is through rolling and wind resistance. Spending money on the regenerative braking system thus has somewhat diminishing returns. It just looks good in the advertisements to have it.
The beauty of super capacitors is in regenerative braking. Where do you put the energy when you are braking - short, high power surges - supercaps! The motor(s) can with stand much higher energy conversion for a short time (when braking) than their rated load - we should use this capability!
The use of supercaps for braking utilizing a converter is not the way to go for the reasons you have outlined. What is useful for regenerative braking is FIFO (first in first out) switched rotation of capacitors - as each cap reaches it's voltage, a new drained cap is inserted in the circuit. Charged caps are constantly being discharged into the batteries using a similar technique where the caps are sequentially series connected to maintain a charging voltage to the batteries. Complicated arrangement, but nothing difficult for a CPU or embedded logic device. Difficulty is in the switching devices and the losses in them - this would look like a bridge for each capacitor.
If I may, I'd like to point out few things in your calculations.
The kinetic energy of a moving object is Ek=0.5*m*v² (1) where m equals the mass and v the velocity of the object. When m=1000kg and v=10m/s the amount of the kinetic energy Ek=0.5*1000kg*(10m/s)²=50,000J=50kJ.
Now if bring that car to a halt in two seconds, dt=2s, from equation Pavg=Ek/dt (2) we get the average power of Pavg=50,000J/2s=25,000W=25kW.
Now we have another problem: Power can also expressed as follows P=F*v (3) where v is velocity and the force F=m*a (4) is product of mass m and acceleration a. From the equations (3)&(4) we get P=m*a*v (5). Now from (5) we can see that when decelerating at constant power, and m being constant, the deceleration (acceleration a) must go up when velocity v goes down.
Example 1: What is the rate of acceleration at the begining? From (5) we can solve a=P/(m*v) and from that we get ast=Pavg/(m*v)=25,000W/(1000kg*10m/s)=2.5m/s²
If the deceleration must go up when slowing down, how high it can go. Realistic maximum acceleration (or deceleration) rate is ~1g=9.81m/s² so let amax=10m/s².
Example 2: At what velocity we reach max acceleration of amax? Again from (5) we get v=P/(m*a) => v1=Pavg/(m*amax)=25,000W/(1000kg*10m/s²)=2.5m/s ~ 9km/h
From examples 1&2 we find that if decelerating at constant power of Pavg=25kW we end up with deceleration rates which slam the passangeres to seatbelts despite of the wery gentle start with acceleration rate of 2.5m/s². And that is unless we run out of torque from the motor. Which is more than likely to happen well before 1g deceleration.
- Jari W
"What is useful for regenerative braking is FIFO (first in first out) switched rotation of capacitors"
It is also called a charge pump, and the capacitors provide an impendance that limits the rate of power that the system can absorb power at. Impendance is a big problem, because the charge pump has to operate at a very high frequency to switch the capacitors in and out fast enough.
Supercapacitors, while excellent at absorbing greater amounts of energy than conventional capacitors, also have a nasty habit of having a high ESR (equivalent series resistance) which basically means that their impendance is relatively high, so they are not well suited for operation in a charge pump. Their ability to handle large current surges is better than a battery's, but worse than a conventional capacitor's.
You also still face the problem where the motor will inherenty produce lower and lower voltages when it spins slower through the braking process. There is a reason why regenerative braking generally doesn't work at low speeds and cannot bring a car completely to a halt:
How regenerative braking actually works with AC induction motors is, the controller sends magnetization pulses to the coils which excites the field in a way that turns the motor into a generator. The resulting back-EMF current pulses are captured into capacitors and fed back into the batteries. Simply put, the controller deliberately tries to spin the motor the wrong way and captures the energy when the magnetic fields slam back against its face.
As the motor turns slower, the voltage of the back-EMF pulses decrease and the controller compensate this by increasing the current of the excitation pulses. Each excitation pulse consumes energy from the system through losses in the coils, which means that the efficiency of the system drops the slower the motor turns. There is a speed below which using regenerative braking actually consumes more energy than it returns into the battery because of this. Within these limitations, about 70% of the kinetic energy can usually be recovered in typical sub-urban driving at 50 - 80 kph.
If we assume that half of the 50 kJ calculated above can be recovered, we get 25 kJ which is roughly 7 Wh. A positively tiny amount of energy in a car that is expected to use about 150 Wh for each kilometer it travels. From this we can extrapolate that once your need to stop falls to less than once per 1/4 a mile or so, the returns are no longer large enough to be measurable through everyday driving with varying conditions and the human factor behind the wheel.
Inter-city driving returns close to nothing anyways because the speeds stay well below 40 kph where the motor no longer works efficiently as a generator.
Thanks Anonymous (88.193.143.38) that was very clear and informative.
The way I look at it (low average speed regenerative braking) is that the fuel used in city start/stop traffic - even though speeds are less than 40km/h - is a significant part of most motorists fuel costs. These are the conditions that can potentially deliver maximum returns for motorists. This is where we can get buy-in from many, many more people who live in cities and experience low speed, stop/start driving regularly. (Most motorists I suspect).
With an electric vehicle, the long stops in start/stop driving don't require any energy, but ancillary uses like lighting, heating and air conditioning do and are related not to speed or distance but to time spent in the vehicle. So adding what you are saying in relation to regenerative braking to this scenario, there will be even less return on investment for this kind of driving from the use of regenerative braking! In fact it accentuates the energy storage issue (battery weight and cost) as this kind of driving will require more reserves of electric energy.
The logical conclusion then is to focus on other, perhaps non-electric methods of capturing and storing braking energy. It remains the single most concentrated source of energy available to a motorist.
Perhaps we need to be considering hydraulic or compressed air technology to recover braking energy and store it for use, perhaps for things like heating and air conditioning, or even for propulsion?
Is it time we started to consider hybrid technologies? Hybrid Electric/Hydraulic Motors?
Unfortunately, both hydraulic and compressed air technologies would require the use of torque converter or pumps which introduce more friction into the system and lower the efficiency of the car as a whole, unless you want to invent a system of automatic clutches that engage the pumps as needed. That in turn gives you another point of failure and more operating costs: clutches wear out. Pressurized air also works at pretty low efficiency, because compressing air warms it up and energy escapes as heat. There are serious problems with each solution; otherwise they'd already be using them.
The regenerative braking system in an AC induction motor comes essentially for free, since the basic circuitry needed for it is already present in the motor controller: the controller has to have a way to deal with any feedback current from the motor as a part of normal operation, because there is inevitably some back-EMF anyways. Some controllers simply dump the energy out as heat, but it costs relatively little extra to pump it back into the controller's primary capacitor bank from which the controller draws its power. Then, if the capacitors get charged to a high enough voltage, current flow from the battery reverses and you know the rest of the story.
What would cost a lot extra is if we tried to draw every joule of energy back out of the system, because it would make the motor controller significantly more complex. The inherent problem is in making the motor work as a generator - not in storing the energy thus generated. The solution may even require to change the motor itself in ways that are yet to be discovered and are thus out of the scope of this particular project. Pretty much same thing applies to flywheels and pressure vessels etc. because they increase the complexity of the system as a whole and require expensive custom parts that need maintenance over the long run. Most of that stuff isn't tried and proven, and the R&D required is still years of work, for very small gains.
The electric car doesn't use much power in stop&go traffic anyways. Idling an engine is what consumes the most energy. The power an ordinary car engine uses at idle would easily be enough to drive an electric car up to 60 kph and that's not even with the AC on. It all goes out of the tailpipe just like that.
That 50 kJ to reach ~35 kph is very small amount of energy. You could make more than two - three thousand accelerations on a single charge on any adequately sized battery pack. I doubt you're going to run across much more than twenty traffic lights on your way to anywhere in a city, which means that stop&go traffic eats less than one percent of your battery even without regenerative braking. It really is that little. You could easily triple the weight of the car and still see no apparent difference in driving range, as long as the speeds are kept low.
It would be interesting to re-program the computer of a Prius to keep tally of how much energy it has regenerated from the wheels. If my estimates are right, one day of normal commuting through the city will get you enough to make a pot of tea.
(1 litre of water at 4.19 kJ/kg/K times 80 degrees = 335 kJ or something like 15 stops at the traffic lights)
Wouldn't look good on the advertisements, though. They keep pressing how regenerative braking is the next best thing since sliced bread to sell those cars that still rely on gasoline to move. It's just fluff, really.
Yes, I see that regeneration is of little overall value to this project.
Here is a specific example calculation for a vehicle (my car) on a long drive of 5 hours.
I need 10 litres or petrol per hour to sit on 100km/hr average over a road that starts and finishes at the same height above sea level. I therefore use 10 litres x 8.76kw-hrs (Petrol has roughly 8.76kWh per litre), of which say 15% is used for propulsion = 13.14kw to sit at 100km/hr. (I'm not sure how accurate this is but it tallies with other info I have read)
Air-conditioning, lights etc. say we need: Lights, radio etc. say 250 watts or .25kw-hr Air con/heating (say I use a 750 watt reverse cycle air conditioner unit - i suspect this would easily keep the car cool or warm.
So all up I need around 1 kw-hr for creature comfort and safety.
So in my car, I need 14kw in round numbers to overcome losses due to resistance (wind, rolling etc.) and making the vehcile livable during the time in it. That is I need 70kw-hrs of energy for the 5 hour trip just to overcome losses.
Add to that conversion losses and the figure is possibly around 100kw-hrs for the 5 hour trip without hills or acceleration (which are the recoverable part of the trip, since each acceleration will have a corresponding de-acceleration and an uphill will have a corresponding downhill).
I need energy to accelerate to 100km/hr, go up hills and pass other cars etc. This is the only energy that can be recovered as all losses (approximately) have been accounted for above.
Lets take accelerating to 100km/hr - my car wieghs 2000kgs. (assume batteries + motors = engine/tranmission/fluids etc. in weight). F = m x a, so to accelerate at say 1m/s/s (3.6km/hr/s I need a force of 2000 newtons. At 3.6km/hr/second I need 27.8 seconds. Work is f x d, so in 27.8 seconds (,0077 hrs) at an average speed of
50km/hr I will cover 386 meters. So Work is 2000 x 386 = 772 kJoules. This equates to .214 kw-hrs (from http://www.unit-conversion.info/energy.html) per 0 - 100km/hr acceleration event.
Let's say I would like to stop every hour in this 5 hour journey = about 1 kw-hr for acceleration from 0 to 100km/hr.
Now let's say I need to climb a total of 1 km - perhaps a hill here and a hill there adding up to 1 km in vertical distance. (This is a guess - I've never stopped to estimate this)
F = 2000kg x 10m/s/s = 20,000 newtons. So work done for 1 km = 20,000,000 joules (20,000 x 1000m) or 20,000 kJ which is 5.6kw-hrs.
So a 5 hour trip will require: Constant 100km/hr 13.4 kw-hrs x 5 = 65.2kw-hrs - non recoverable Aircon & Lights @1 kw-hr/hr = 5 kw-hrs - non recoverable Accelerating 5 times to 100km/hr = 1 kw-hr -recoverable
Climbing hills = 5 kw-hrs - recoverable
So the maximum energy we could possibly recover in this scenario is 6 kw-hrs of the a total of over 100kw-hrs (including conversion losses which are also not recoverable).
The thing is that this recoverable energy would not be recovered by some process of regeneration wehave created but rather it would naturally be recovered in part by not using 13.6kw when I am deceleration or going down hill.
This just measn that the total energy available for recovery through regeneration is less that the 6kw-hrs calculated above. I don't know how much less, but whatever it is, it makes adding regeneration capability a fairly low value exercise in this scenario too.
There is one way of overcoming part of the conversion losses in the system, but it requires the use of a different type of motor and a different controller suitable for the task. Again, not very useful for this particular project, but future projects will benefit.
Normal AC motors create the magnetic field in the rotor by external excitation into a short-circuited cage of coils that reacts to the changing field by creating an equal and opposite magnetic field. That field then turns with the AC field in the stator. This design has a good efficiency, but it does have ohmic losses in the conductors of the cage because of the current generated in the windings, and the slower it turns and the more torque is required, the higher the currents which means that the efficiency is low at very low running speeds.
There are more technicalities about how many turns of coil and how thick they have to be, thermal design considerations, and tradeoffs between weight and low-end torque etc. but it can all be summed up pretty much by saying that the rotor invariably turns out to be somewhat heavy. Winding the coils so the rotor is balanced well is difficult and that in turn places great loads on the bearings if you want it to run at high RPM. This means that you have to compromise in efficiency to obtain reliability. This is why the motor is "only" 85 - 95% efficient slowed down, even though it can be more than 95% efficient at nominal speed.
Permanent magnet motors deal with the magnetization problem by using high-grade neodymium magnets in the rotor to replace the induction cage. This means that the motor is inherently more efficient across the whole range, and the rotor can be made lighter. The whole motor weighs less and can turn at very high speeds, 50,000 - 100,000 rpm, which enables it to produce whopping amounts of power from a small package. I've seen 100 kW figures quoted on 12 kilogram disc rotor type designs used in trams.
The tradeoff is that since the magnetization is permanent, the torque characteristics differ from the flat line of the AC induction motor. On a limited maximum voltage, it starts high at 0 RPM and drops linearily through the speed range. It behaves more like a regular DC motor, because it is pretty much a brushless DC motor in spirit, or rather: a brushless DC motor is more like a permanent magnet AC motor with the DC/AC switching controller integrated in the motor.
How all this relates to regenerative braking is, without the need of external magnetization, the back-EMF is automatically generated by the motor and all you need to do is grab it and pump it out. Again, you need more complicated electronics to do so, because you can't control the voltage at which it comes, but you do get more energy out of it regardless since you're not spending any of your own to get it. (technically though, the charge pump does require some power to switching)
Overall, the system is more expensive now, but in the future the mass production of such motors and controllers will bring the price down. The lighter motor in itself helps with the problem of battery weight and the better regeneration efficiency significantly improves city mileage because it actually works at those low average speeds.
Each kilogram you shave off, you can add one in the battery, and with batteries fast approaching a capacity of 1 kilogram per kilometer, shaving 40 kilograms off of your typical AC induction motor will get you that 40 kilometers extra range.
The Brusa motor proposed for this project is a permanent magnet brushless DC motor. It therefore has the superior regen suggested. Capacitors as an assistant to the battery has almost zero payback and the capacitors will be very expensive. The scenario where the combination capacitor and battery is viable is when very low power batteries like nickel, sodium, chloride (aka Zebra), and does not apply to the Lithium Ion suggested for this project.
Piilota Re: Super Capacitors luonutThat superior regen still depends on the controller and the speed range of the motor.
Generally speaking, if the motor is designed to operate at relatively low voltages and speeds, it will still perform poorly in regenerative braking, because the output back-EMF voltage will consequently be low and more is required of the controller and cabling to effectively pump the energy out.
There is a fixed resistance involved in the charge pump/voltage booster, or rather, the workings of the pump can be reduced and represented as a resistance because the pump has to rely on the current to "flow in" on its own. It's not like a water pump that actively pulls stuff in. It charges up a capacitor or an inductor with the voltage and current generated by the source, and then pumps that energy to a higher voltage. You can think of it as a guy holding a bucket under a faucet and then lifting it up to another fellow upstairs at precise intervals.
As the generated voltage gets lower, the current through the resistance diminishes, therefore the output power drops and the rate of energy extraction per switching event of the pump drops until practically no current flows (the car is inching along very slowly). Meanwhile, each switching event takes a fixed amount of energy, therefore you end up with more energy used for lesser gain. The bucket gets less and less water each time as the pressure from the faucet drops, but the guy still runs upstairs with his bucket even if it was empty.
There is still a point where the regenerated voltage is so low that running the regen voltage pump requires more energy than what you can pull out. If we imagine that this point comes at, say, 6 volts, and we further assume that a motor will generate its nominal voltage at its nominal speed, we can figure out the speed at which the regen stops working. In reality, a motor will not generate its full nominal voltage at its nominal speed, because the attempt to draw current out of it will lower the voltage due to the internal resistance. This justifies the rather high value of 6 volts chosen here for demonstration.
A motor designed for 120 volts and geared for 120 kph will reach zero gain at 6 kph, 50% regen efficiency at 8.5 kph and 75% at 12 kph.
A motor designed for 300 volts and geared for 120 kph will reach zero gain at 2.4 kph, 50% regen efficiency at 3.4 kph and 75% at 4.8 kph.
In the first case we see that the regen stops working properly when the car is still moving at a considerable speed. In the second case, that speed is lowered to a walking pace, meaning that the car can almost stop on its regen brakes. Gearing the car to go faster will raise the cutoff speed and vice versa. Of course, if we put more gears in the car, we could get more energy out, but then the car would suffer from extra weight and less efficient transmission.
Anonymous(..254) Your description and analogy is very good, and I now see that your intent is to use the capacitors for low voltage only and then use a DC-DC converter to transfer the charge to the working voltage of the batteries. I had previously assumed that the capacitor would work at the battery voltage, so I must now say yes, the low voltage capacitors are a good idea. This arrangement does require a little more complicated regen circuit in the controller so the motor receives its power at one set of high voltage terminals and directs its regen to a second set of low voltage terminals. I think the small extra cost of this would be well worth it!
Piilota Re: Super Capacitors luonutYou misunderstand the physics involved.
It's a physical property of the motor. When the controller applies a voltage to the terminals of the Permanent Magnet DC motor, current starts to flow in. As the motor spools up speed, it automatically starts to generate an opposite voltage to what the controller is pushing out. The back-EMF voltage. These two voltages "battle" each other so that when the motor gets enough speed, the voltage it generates equals and opposes the voltage of the controller and thus negates it. No more current flows in, no more power is generated and the speed cannot continue to rise.
This is the situation when the car has reached its travelling speed after acceleration. Now of course, the air resistance and all that drags the car and the motor back, so the speed of the motor will never rise high enough to completely nullify the voltage from the controller, which is why the motor continues to draw current and consume energy to move the car. The system goes into a balance where, if more load is put to the motor, it will slow down and generate less backwards voltage, which results in more current flowing in and more torque being generated to match the load. It means that the back-EMF is never higher than the battery voltage. For that to happen, you would have to travel faster than the highest speed you can accelerate to with the battery voltage.
Now, if the controller should suddenly stop putting voltage to the motor, the motor's own backwards voltage would still exist. The controller has to shunt that voltage somewhere to draw out the current. If we weren't interested in taking that energy back, we would simply direct the voltage around and short circuit the motor, which means that the energy would be lost as heat in the resistance of the coils. The motor would start to brake rather hard in that situation. Or, we could simply ignore the voltage, which means that we open all the switches and let no current flow anywhere. The motor continues to wheel freely and a voltage exists across its terminals as if it were a battery itself.
If we want to take that energy back, we have to direct the voltage to somewhere we can make use of it. We can't put it straight into the battery in any case, because the battery voltage is higher and that would make the motor turn even faster when we're trying to slow it down. We have to direct it somewhere where the voltage is lower. In theory, we could put it into a 6 volt battery which would then charge up happily until the motor turns so slow that it generates precisely 6 volts where the charging stops. This approach has its problems, because the huge voltage difference at start would probably cause the battery to explode in a sudden rush of high current, and the passengers would be slammed against their seatbelts due to the rapid deacceleration. The same problem would occur should you place a capacitor in there. The voltage of an empty capacitor is zero, so while the capacitor would survive the huge inrush, the passengers would have their noses against the windshield.
So in practice, there is an inductor. A simple coil of copper wire, perhaps with some iron in the middle. An inductor resists the change in a current going through it so when you connect it to the motor, it takes some time for the current to build up. Conversely, when you disconnect the inductor, it wants to maintain that current it already has and generates a huge voltage spike up to thousands of volts in an effort to find some route for the current to flow. This is because the inductor stores energy into a magnetic field when the current builds up (but not when the current is steady) and when the current stops, the magnetic field collapses back, releasing all the energy in the form of an electric jolt through the coil. This is how the basic boost DC-DC converter works.
Now, all you need is a capacitor that can take in such a huge inrush. And where you find such a capacitor is in the motor controller's input filtering capacitor bank that buffers the power from the batteries for the motor controller so that the batteries don't have to suffer from the jittery and choppy power draw of the controller, which is why they're excellent for filtering the choppy and spiky power surges from the regen brakes, smoothing down the thousand volt surges into something more manageable. As those surges come in one after another, eventually the capacitor bank voltage rises above the battery voltage and current simply flows back into the batteries in a steady stream until the capacitor voltage goes back down again. Now that you have the inductor and the capacitor, you can keep flipping the switch, connecting the inductor to the motor, then the capacitor, then the motor and so on, and it will pump the energy out of the motor and back into the electrical system at a controlled rate. The practical limitation to the pumping is that as the voltage from the motor decreases, the rate of energy you can pump out decreases while your switching losses do not, which is what I explained previously.
So you see, it is not my intention. It is how the system already works. In practice, the controller uses both inductors and capacitors to control the rate of current moving in the system so the whole thing doesn't just jam in a split second and burn everything up. There's very little to be done by adding supercapacitors in the mix, because they have properties that aren't suitable for the task in hand. For example, the equivalent series resistance (ESR) of a supercapacitor is somewhat higher than of a conventional capacitor, which makes it less suitable for power filtering tasks. That's because they resemble more a battery than a capacitor. And modern day batteries are more than adequate to handle the braking power in a car. If they can supply half a megawatt in an instant, they can take half a megawatt in an instant.
Of course, you could in theory, add some capacitors that are switched in when the output of the motor drops very low and use them as a charge pump. If the output is 2 V, then you can charge 100 capacitors in parallel to get 1 V in each. That's because the motor output voltage also drops to 1 V as it slows down more.
And then you could connect them in series to get 100 volts. The next time you get 0.5 Volts and make 50 Volts out of them. Then 0.25 and 25 V and so on…
In practice, each switch or diode that connects each capacitor in a charge pump has a voltage loss of around 0.3 - 0.7 volts, and you need multiple switches in series to build a charge pump, which means that you will get nothing out of it. It simply doesn't work because semiconductors need a certain threshold voltage to operate, and the higher the currents you want to handle, the more voltage loss you will see. It has everything to do with how transistors and diodes are made.
Operating DC-DC converters from low voltages invariably results in significant efficiency losses. That is why it is wise to avoid low voltages. The higher the voltage you use in the electrical system, the lesser your losses.
Avoiding low voltage in a regenerative braking simply means that you use a motor that is designed for higher voltage, because that means it also generates a higher voltage.
Thank you for the explanation of regen anonymous. I drive an ev with a 3 phase AC induction motor (HPEVS AC50) and controller with 130V max voltage (Curtis 1238-7501). From some spot checks of Ah into/out of the LiFePO4 battery pack it appears I typically get about 20-25% of charge back into the pack going down a hill compared to what was used to go up. Of course the charge goes in at higher voltage than it went out, so energy in is a higher percentage of energy out than this. With regard to regen at slow speeds, this of course depends on the gear used since lower gears give higher motor rpm, so more regen is obtained by driving at the lowest possible gear for the vehicle speed. There is another benefit to regen, in that I rarely use my mechanical brakes. I control the vehicle speed using only electric braking, and many times come to a complete stop at stop lights using only electric braking. This saves wear on mechanical brakes, and limits the heavy metals that wear deposits onto roads, some of which eventually ends up in waterways.
Piilota Re: Super Capacitors luonut tom (vieras), 03 Jun 2010 15:30Another practical comment on regen: I currently have regen programmed at 55% of full scale on my controller. Setting it higher would not give much greater energy recapture in normal driving because it would mainly effect regen at low vehicle speeds on hills or in sudden stops at higher speeds. During normal driving you typically do not decelerate all that quickly, so you do not require more aggressive regen to maximize energy recapture. I typically slowly ease back on the throttle pedal to gradually increase electric braking to slow down, rarely using the max amount to slow at the pace of traffic - even when coming to a complete stop. If I completely release the throttle at high motor rpm, say 5800 rpm, the max regen current will be about 195A, or just a bit over 1C for my cells, which they easily handle. This typically only occurs at a sudden stop light change when I am very close to it, and is rare. I could get more regen in this case if I set it higher because I have to use my mechanical brakes to aid stopping, but since it is a rare event, this would result in very little extra energy capture. It would also increase energy capture on hills, but only if slowing to a complete stop, as I currently can slow to about 10 mph or less on a 5% grade using only electric braking. Of course, if I needed to stop more quickly on such a grade, a higher regen setting would permit recapture of more energy. I have not set it higher because that would result in larger current spikes in a sudden stop scenario as described above, and I prefer to keep the regen max at around 1C to be easy on the cells. Because the scenarios where I would gain more energy recapture with a higher regen setting are fairly rare, I don't see significant advantage to setting it higher. And in this application, you can see supercapacitors are not at all required.
Piilota Re: Super Capacitors luonut tom (vieras), 03 Jun 2010 16:22