# Regenerative Braking



## mattW (Sep 14, 2007)

*What is regenerative braking?
*Regenerative braking is a way of slowing a vehicle down where some or all of the vehicle's kinetic energy is saved rather than being wasted as heat. Electrically this is achieved through the use of a generator (often the traction motor with the terminals reversed) and energy storage devices such as batteries or capacitors. This generator is usually an AC or permanent magnet DC motor.

*What sort of range can be gained from this feature?
*The typical stated range gain for regenerative braking is about 10%. AC Propulsion states as high as 30%, US Electricar measured as high as 20+%, Toyota RAV4 owners report as high as 25%. This would obviously be more effective in city driving rather than highway where little braking occurs.
*
Is regenerative braking possible on a series wound DC-motor?
*Yes, but it is difficult and can be dangerous to implement. Some controllers, such as the ZAPI H2 have regen abilities built in but some have questioned the controller's reliability. One successful DIY attempt  by Otmar Ebenhoech of Cafe Electric is documented here. Early 90's Soleq brand EV's were DC and had regen built in.

*Why is it so hard to use regenerative braking on a series wound motor?
*Series wound EV motors have field coils instead of magnets, which are not normally energised when the car isn't being powered forward. No current in the fields results in no current generated when the motor is turned by the inertia of the car coasting/slowing down. Specialised controllers can energize the fields, but even that isn't a simple solution because series motors run above 96V are usually "timed", that is the brush, armature, and field magnet positions are calculated and set in an advanced position to optimize the motor producing torque when it is powered by batteries. Attempting to cause that motor to become a generator will mean that the timing optimization is exactly opposite what it should be for producing power instead of consuming it, and the effective result is that excessive arcing and/or plasma is generated on the armature's commutator, usually ruining it. The motor can be timed in a "neutral" manner, but then it makes a less efficient drive motor, and the regen it might put out will be consumed just making up for the efficiency it lost by being mis-timed.

*Is regen possible on a Permanent Magnet DC motor?
*Regenerative braking is easier with a permanent magnet motor because the magnets do not need to be energised. Regenerative braking is achieved by having the controller reverse the terminals to the motor so that current flows in the opposite direction. Since these motors are also brushed they suffer from the same advanced timing problems when used at voltages greater than 96V. Typically though PM motors are smaller anyway and therefore run with neutral timing and lower voltages. Thus many motorcycles and small EVs run regen using PM motors at less than 96V.

*Can an alternator or generator be attached to the drive train to implement Regen?
*Theoretically a generator (or alternator) that is connected to the drive train when the brake pedal is pressed would be a simple way to implement regenerative braking. In practice it could be quite difficult to mount a generator of sufficient size (approximately the size of the motor for similar braking power to your acceleration) or enough 12V alternators to charge the individual batteries in the pack in an already crowded engine compartment. Also the additional complexity of a clutch system to remove unnecessary drag when not in use and controlling the current spike into the batteries further complicates things. All of this for a 10% gain in range is often not deemed to be worth the money, time and additional weight it would take to implement it. That said it would be an interesting experiment and challenge, and serve as a good example if someone was willing to try such a set-up.

Hybrid gas/electric automobiles now use a completely different method of braking at slower speeds. While hybrid cars still use conventional brake pads at highway speeds, electric motors help the car brake during stop-and-go driving. As the driver applies the brakes through a conventional pedal, the electric motors reverse direction. The torque created by this reversal counteracts the forward momentum and eventually stops the car.

But regenerative braking does more than simply stop the car. Electric motors and electric generators (such as a car's alternator) are essentially two sides of the same technology. Both use magnetic fields and coiled wires, but in different configurations. Regenerative braking systems take advantage of this duality. Whenever the electric motor of a hybrid car begins to reverse direction, it becomes an electric generator or dynamo. This generated electricity is fed into a chemical storage battery and used later to power the car at city speeds.

*What are AC motors like at providing Regen?
*Unlike brushed motors, AC motors (ie induction or perm mag) can provide regen very efficiently. AC motors can usually regen at almost the same efficiency as when motoring. Regen for AC motors also comes _at no extra cost_ to the existing controller, although bare in mind that AC motors and controllers cost more than brushed controllers. Before purchasing an AC drive system it's worth investigating its regen capabilities.


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## Thalass (Dec 28, 2007)

*What kind of range boost can I expect using Regen?


If accelerating at a certain rate costs me 200A, how much charging current can I expect from a similar rate of de**celleration? 
*

Just some things I was wondering.


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## mattW (Sep 14, 2007)

I typically hear that regen gives about a 10% increase in range, obviously that will vary with city/highway driving but 10% is the number that is always thrown around. And I think that the prius regen returned about 30%, so you would get 60A back. These aren't solid numbers from studies or anything, just what most people say when talking regen... hope that helps.


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## deanbo (Jun 7, 2010)

Can regen create more electricity than the batteries can handle? Or should I say does some of the electricity created by regen get wasted because batteries can only accept so much current all at once?


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## major (Apr 4, 2008)

deanbo said:


> Can regen create more electricity than the batteries can handle? Or should I say does some of the electricity created by regen get wasted because batteries can only accept so much current all at once?


I guess so. Much depends on the system. Like the particular battery. And then the application. Like most often, the scenario used is that the EV is fully charged by the guy who lives at the top of a hill. So he goes down that hill and tries to regen into a fully charged battery. What happens? The system is smart enough not to regen. So there is no "electricity" wasted. Electricity is not generated and therefore no braking torque. What's the guy to do? Duh, use the original braking system. That is what it was designed to do. And if the guy was smart enough to figure out his driving mission ahead of time, maybe he would not fully charge his battery so he could take advantage of the potential energy available at his location.

And yes, some of the electricity used to charge batteries is wasted, whether the charge current comes from a battery charger or from regen. Batteries, like most things, are not perfect. So there are losses associated with charging and with discharging batteries.

Regards,

major


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## DJBecker (Nov 3, 2010)

mattW said:


> I typically hear that regen gives about a 10% increase in range, obviously that will vary with city/highway driving but 10% is the number that is always thrown around. And I think that the prius regen returned about 30%, so you would get 60A back. These aren't solid numbers from studies or anything, just what most people say when talking regen... hope that helps.


I suspect the Prius 30% figure is solely for optimistic regenerative cycle efficiency: how much of the power slowing down can be returned in later traction effort if everything is perfect.

A quick back-of-the-envelope calculation shows this to be likely. The motor and controller are each about 85% efficient. A very good battery is about 70% efficient at return charge energy. Multiplying these together, remembering that the power is making two trips through the motor and controller, gives you 0.85*0.85*0.70*0.85*0.85 = 36%. Factor in the drivetrain losses, at 90% efficient (although a dyno operator will give you a 130% factor for wheel-to-shaft horsepower) and you get very slightly under 30%.

Considering that each of these numbers is optimistic (e.g. a cold, loaded differential can absorb 10HP at highway speeds, the regenerated voltage will be far from optimal for charging), I suspect the 30% number is for the traction/power unit only, not as-installed: you won't be able to climb a 300ft hill from the power going down a 1000ft hill.

Consider regenerated power a tiny extra bump in efficiency, with a bigger bump in braking effectiveness. It's not a major win in range, despite what the press stories would have you believe.


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## algea07 (Oct 1, 2010)

what efficiency does a pm dc motor get in regen? you mentioned that AC motors are quite good but what about pm dc?

might as well ask about SepEx while i'm here.

thanks for your time.


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## frodus (Apr 12, 2008)

it always depends on the particular motor, and only the manufacturer will know that.


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## mikep_95133 (May 20, 2009)

I hope it's ok if I updated this part of the wiki on regen. It's very valuable. Soleq's use sep ex to generate regen. I've personally measured 20-40% regen return on my vehicle.

Mike


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## Duncan (Dec 8, 2008)

Hi Mikep
Those numbers sound incredibly high - they imply that 30% of your energy is wasted in the brakes
I have only seen numbers of that level in inner city buses with frequent stops


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## mikep_95133 (May 20, 2009)

Duncan said:


> Hi Mikep
> Those numbers sound incredibly high - they imply that 30% of your energy is wasted in the brakes
> I have only seen numbers of that level in inner city buses with frequent stops


I get that a lot. I've just observed it in other vehicles from ACP, RAV4, and my own. Of course the number can drop very low if the drive is all freeway. But my work commute was 20-40%.

Mike


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## Duncan (Dec 8, 2008)

Hi Mike,

Those numbers seemed high so I did a little calculating
I looked at the EPA cycles - do you guys drive like that?
City has 23 stops in 11 miles!
Even the highway is only over 10 miles!

Anyway I took all of the decelerations in each cycle and made a best case that ALL of the energy for these decelerations would go into re-gen

I took two extreme cars as examples

2003 Hummer -------- weight 3000Kg -- CdA 2.44 m2
1999 Honda Insight --- weight 840Kg -- CdA 0.474 m2

City Cycle 
- deceleration energy
Hummer -- 2.1Kwhrs
Insight --- 0.59 Kwhrs

- Aerodynamic energy (just using average speeds)
Hummer --- 0.68Kwhrs
Insight --- 0.13Kwhrs

I assumed that 90% of the re-gen was captured

To my surprise I got 
Hummer -- 2.78Kwhrs total - 1,89Kwhrs regen = 68%
Insight ---0.72Kwhrs total -- 0.53Kwhrs regen = 73%


Highway Cycle 
- deceleration energy
Hummer -- 0.58Kwhrs
Insight --- 0.16 Kwhrs

- Aerodynamic energy (just using average speeds)
Hummer --- 3.32Kwhrs
Insight --- 0.64Kwhrs

I assumed that 90% of the re-gen was captured

I got 
Hummer -- 3.90Kwhrs total - 0.52Kwhrs regen = 13%
Insight ---0.80Kwhrs total -- 0.14Kwhrs regen = 18%

These are theoretical maximums assuming no deceleration by just letting the car slow down and no bearing/tire drag but still they are *much* higher than I expected,

Looks like re-gen is more useful than I thought
Not so much for around here (Southland) but in busier areas .....


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## mikep_95133 (May 20, 2009)

Hi Duncan,

Nice piece of math. You'd be surprised at the return regen will give you. It's also a great way to charge your pack should you run out. Just get a tow!

Max regen values are equal to max output values. Mine puts out 300 amps, and it regen's 300 amps. Well, at least it does if it's not raining. But the gating factor is always how the motor controller is programmed. ACP adjusts theirs with a slide pot on the dash in the older cars. I don't remember if they still use that on the new stuff. Mine is setup in what ever 'gear' I select on the 'shifter'. The manual transmission is locked in second gear so the shifter is for the computer. Drive=0%, 2=50%, 1=100%.

Mike


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## tomofreno (Mar 3, 2009)

Duncan,

Can you elaborate on your calculations? Are you estimating energy used per mile at some constant speed by estimating energy lost to rolling resistance and air drag, and adding this to the energy used to accelerate the vehicle to that speed to determine total energy used over some interval in the EPA cycle before stopping? Then assuming 90% of the vehicle kinetic energy is converted to potential energy in the battery pack as a upper bound for regen, and repeating this for each interval in the EPA cycle to determine the ratio of regen to total energy used?

I expected to get somewhat higher regen on my car, especially going down some of the large hills around here, but like I said I've not seen over 20% down a hill, and usually more like 10-12% in flatland driving. But then I don't drive like the EPA cycles, more on secondary roads with more like 8 -12 stops per 15 miles, less when I drive on the highway. The EPA cycle is about 3x more stops per mile than this, so I might expect something like 70/3 = 23% based on your calculation? I would expect a hybrid like the Prius to do better since it has a separate generator, optimized for generation, whereas the drive motor is not.


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## Duncan (Dec 8, 2008)

Hi Tomofreno

I thought that the numbers Mikep was quoting seemed high so I did a "best case" calculation

I looked at the EPA city and highway cycles, 
I took all of the decelerations and noted the initial speed and the final speed,
for each deceleration I calculated a V squared term and summed them for the cycle

With this I could calculate the total possible re-gen energy for the Hummer and Insight,
I took an arbitrary 90% for max possible re-gen energy

To get the total energy I added a term for the aerodynamic losses - I ignored the rolling resistance

The aerodynamic term will be too low as I just used average speed and energy used goes up with the cube of speed,

These were just to give an idea of what could be done - I was expecting much lower numbers

I could add rolling resistance to the numbers - but I am not sure where to get numbers for rolling resistance?

CdA and mass were from Wikipedia

_I would expect a hybrid like the Prius to do better since it has a separate generator, optimized for generation, whereas the drive motor is not._

I used 90% - which is almost certainly far too high even with a dedicated generator as it needs to include storage losses as the batteries are charged as well as generator efficiency

These were just rough numbers to see how useful re-gen could be

*They represent absolute Maximum possible - plus a bit*

A lot of the numbers quoted on the forum have been very low -5% - not sure if that is 
"my DC motor won't do re-gen but its not worth it anyway!"

It looks as if 30% in city and 10% general would be achievable re-gen


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## tomofreno (Mar 3, 2009)

Duncan,

Bob Brant gives F = C*W*cos(phi) for rolling resistance, with C = 0.012(1+v/100), v is vehicle speed. He adds 0.003 brake resistance and steering/suspension drag to this (Build Your Own Electric Vehicle, Bob Brant). Using this, and the equation he gives for still air drag force, Cd*A*v*v/391, I estimate that the combined rolling resistance plus brakes/suspension is 50 lb, and drag force is 53 lb at 60 mph for my car. Rolling resistance force is larger at lower speeds, for example 43 lb and 18 lb at 35 mph. I think 90% efficiency into the pack is optimistic for a generator and would not expect that for a motor. Edit: Bob references the Bosch Handbook for his equations.


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## tomofreno (Mar 3, 2009)

I went through a similar exercise using the EPA city cycle here:
http://www.fueleconomy.gov/feg/fe_test_schedules.shtml

I already had power at constant speed and acceleration energy calculated for my car (2250 lb, Cd = 0.32, A = 18 square ft) in a spreadsheet. I estimated time at each rough average speed in the EPA cycle and took the product of that and power to estimate energy used at constant speed. Acceleration energy was estimated from the equation for force given by Bob Brant, Ci*m*a, where Ci depends on the moment of inertia of rotating parts such as wheels and flywheel, and overall gear ratio (final drive and transmission), m is vehicle mass, and a is vehicle acceleration. I used an acceleration of 4 mph/sec (about 13 sec from 0 to 50 mph). I made the simple assumption that regen was 90% of acceleration energy. With these I got total energy used at constant speed of 2393 Wh, total acceleration energy of 772 Wh, total regen energy of 695 Wh, and ratio of regen to total energy used of 0.22 for my car, which falls right about in the middle of your estimate of 10 to 30%. I would expect it to be lower for a vehicle with higher drag force, as well as for less stop/starts per mile. For example, at half the duty cycle (half the number of stop/starts same overall run time), the energy used at constant speed would about double, and the acceleration and regen energies would halve, giving about 0.07 ratio of regen energy to total energy used.


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## mikep_95133 (May 20, 2009)

I finally found my notebook. I had spend something like 6 weeks focusing on regen vs no regen on work commute for my last job in 2007. Going to work with and without regen netted an 19% gain in efficiency using regen. Going home, with and without regen netted a 18% efficiency gain using regen. This was a no highway commute. City streets only with 40mph max. Not too congested. Total commute distance one way was 5.7miles. Some elevation change. Fairly flat. Some long stretches with no stops. Truck weighs 4000 lbs. Summer time daylight temps. These numbers were taken with the hardware set to 250 amps max on output and regen. It's currently set to 300a and 300a. This commute resembles an EPA highway driving cycle, except the speed never exceeds 40mph. 

I have a 5 mile test loop mapped out here that I call the San Jose loop. I'll test drive it to get more regen data. Now that it's winter here, it will be interesting to see if the percentage changes. The energy required is definitely higher in cold months. I could also, given the time, record the drive with the data logger to see how it compares to the EPA cycles.

Mike


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## GizmoEV (Nov 28, 2009)

FWIW, Here is a quote from my blog where I just posted results of driving my Gizmo for 1 year and 5500 miles. 



> Also, the difference in the CA total of 12,544Ah and my total of 12,365Ah is due to the fact that the CA records total Ah that the batteries deliver whereas my figure is from adding up all the individual trip Ah which include regenerative braking. From this you can calculate that regenerative braking gained me 1.4% range over this time period. *[edit: I just realized that 7597Ah came from back calculating for the time I did not have the CA and are based on the kWh used to charge from the wall. A more accurate representative result would be a 3.7% gain from regen.]* Not a whole lot, however I have gone over three times farther on this set of brake pads than I did before I had variable regenerative braking so it is definitely a benefit. Not to mention the ability to come down steep long hills and not have brake fade and the much quicker panic stops than without.


Some may argue that my numbers are off because I used Ah and not Wh but since all the discharges were on the level part of the discharge curve there is very little, if any, measurable difference with my setup.


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## loertini87 (Mar 16, 2011)

i guys,
i'm new in this field , so i'm going to ask some simple question just to understand better the problem. I have a brushless motor has a three phase winding on the stator and permanent magnets on the rotorals; when i'm going to brake in output from my motor i will have a three phase AC,so if i want to recharge a battery i need to convert it in DC, isn't it?
an other question is, which is the dependence between the regenerative voltage and corrent, and the rotation speed of the rotor? for example,i don't know, the voltage is constant and only the corrent changes.
Sorry if i made stupid question....


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## major (Apr 4, 2008)

loertini87 said:


> i guys,
> i'm new in this field , so i'm going to ask some simple question just to understand better the problem. I have a brushless motor has a three phase winding on the stator and permanent magnets on the rotorals; when i'm going to brake in output from my motor i will have a three phase AC,so if i want to recharge a battery i need to convert it in DC, isn't it?
> an other question is, which is the dependence between the regenerative voltage and corrent, and the rotation speed of the rotor? for example,i don't know, the voltage is constant and only the corrent changes.
> Sorry if i made stupid question....


Hi loertini,

Not a stupid question, however it is confusing 

The details of V and I and RPM and all that can be complex concerning the regeneration mode with a 3 phase motor. But lucky for you, you don't need to know any of that stuff. The motor needs a controller. This controller, among other duties, converts DC from the battery to AC for the motor when motoring. It also converts the AC from the motor to DC for the battery when regenerating. It will set all the proper voltage, currents and frequency automatically. All that is required from you is some initial parameter settings and then a command signal, like from the throttle position. The controller does it all for you 

Regards,

major


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## loertini87 (Mar 16, 2011)

So, using the controller, i can choose the voltage with which i'm going to recharge the battery during the braking? for example, i can take a constant voltage, and make change the corrent with the changing of the braking power? 
Thank you very much!


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## loertini87 (Mar 16, 2011)

major said:


> Hi loertini,
> 
> Not a stupid question, however it is confusing
> 
> ...


So, using the controller, i can choose the voltage with which i'm going to recharge the battery during the braking? for example, i can take a constant voltage, and make change the corrent with the changing of the braking power? 
Thank you very much!


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## major (Apr 4, 2008)

loertini87 said:


> So, using the controller, i can choose the voltage with which i'm going to recharge the battery during the braking? for example, i can take a constant voltage, and make change the corrent with the changing of the braking power?


Yes, the AC controller designed to be used on an EV will have software which does this automatically.


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## zeroexcelcior (Aug 2, 2011)

What about simulating engine braking, more as a way to slow the vehicle down rather than recharging the pack? I realize that regenerative braking is better suited for AC motors but is this something than can be done with a DC motor like those from Netgain?


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## UCFhybrid (Sep 26, 2011)

Hey guys, I have a question about choosing between an AC or DC motor.

So with AC, you have to convert the power from the DC batteries first, and that will lower efficiency right? But at the same time, isnt an AC motor more efficient than a DC motor, even though DC motors won't require the coverter?

So is the AC motor still a better choice even though you will lose some efficiency from the power converter?


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## GizmoEV (Nov 28, 2009)

My understanding is that with a quality system of each type, properly built, the differences come out nearly a wash. The regen will net you anything from 0% to 10% from what I have heard. With my SepEx system I am just under 3% after 6000 miles.


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## DavidDymaxion (Dec 1, 2008)

AC is slightly more efficient, but you'll be much further ahead (literally and figuratively) to apply the difference in cost to buy better and/or more batteries.


UCFhybrid said:


> Hey guys, I have a question about choosing between an AC or DC motor.
> 
> So with AC, you have to convert the power from the DC batteries first, and that will lower efficiency right? But at the same time, isnt an AC motor more efficient than a DC motor, even though DC motors won't require the coverter?
> 
> So is the AC motor still a better choice even though you will lose some efficiency from the power converter?


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## Siwastaja (Aug 1, 2012)

DJBecker said:


> A quick back-of-the-envelope calculation shows this to be likely. The motor and controller are each about 85% efficient. A very good battery is about 70% efficient at return charge energy. Multiplying these together, remembering that the power is making two trips through the motor and controller, gives you 0.85*0.85*0.70*0.85*0.85 = 36%.


This is an old post but it needs to be corrected. When the numbers add up like here, it's important to get the input data right, otherwise it is garbage in garbage out. These numbers are clearly incorrect.

Typical efficiency for a motor is about 90%, and for a controller, it's about 95%. (These are typical, conservative numbers, not optimistic; there are motors that can perform at 95% eff and controllers that get 98%. But it's pretty extreme.)

Li-ion battery is between 95-100% depending on charge/discharge rate, battery pack size and the "quality" of battery (specific internal resistance). (You said "very good" so it's near to 100%.) This gives us, for example:

0.90 (motor) * 0.95 (controller) * 0.98 (bat charge) * 0.98 (bat discharge) * 0.95 (controller) * 0.90 (motor) = 70%. But the drivetrain losses are very prominent, including losses in tires. Typically, 50% return is quoted including all losses. But it is true that 50% return does not translate to 50% saving, because most of the losses come not from acceleration and braking, but driving forward (rolling resistance, aerodynamic drag). Hence, I believe that 0 - 30 % is the range for usual savings, 0% saving when driving highways only, 30% saving when driving in a mountainous city with red traffic lights in every corner. 10% might be typical in a real city.

As a reference, metro train systems over the world have achieved savings between 10% and 50% when switched to regenerative braking. 50% is pretty extreme and shows that that the whole system is very well designed for regeneration (and the distance between the stops is very short.)


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## major (Apr 4, 2008)

Siwastaja said:


> As a reference, metro train systems over the world have achieved savings between 10% and 50% when switched to regenerative braking. 50% is pretty extreme and shows that that the whole system is very well designed for regeneration (and the distance between the stops is very short.)


The train system has a couple big differences. The energy is returned to a grid and not stored in battery. It uses steel wheels. The mass to power ratio is considerably higher. I would think these factors contribute to ability for higher regeneration efficiency compared to battery electric cars.

And the best part.... It is free, practically. It takes little or no additional equipment to regenerate.


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## tomofreno (Mar 3, 2009)

Test results on the Tesla S:

"As you can see in the video below the Tesla’s total energy consumption was *1.1 kWh* at 114 MPH and after the car slowed down using the regenerative braking *.6 kWh* was put back into the battery. This results in a net energy usage of *.5 kWh* for the 1/4 mile pass. Pretty amazing that the car can recover just over half of energy used and put it back into the battery using it’s regenerative braking."


from this ref: 
http://www.dragtimes.com/blog/tesla-model-s-drag-racing-energy-usage-and-cost

Which agrees with this small amount of data I took:
http://www.diyelectriccar.com/forums/showpost.php?p=280765&postcount=14

Of course as stated above, the amount of energy recovered with regen as a proportion of total energy used on a given trip will strongly depend on the driving conditions. It will be quite small for long highway driving (high energy/mile expended) with few stops (little opportunity for regen), and much better for trips at lower speeds (lower energy/mile) and lots of stops (lots of regen).


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## DavidDymaxion (Dec 1, 2008)

Coulombic efficiency is very high (Ahr in = Ahr out). Battery energy efficiency can be very high for low currents, but falls for larger currents. The typical currents of driving and regen are large enough that charge and discharge energy efficiencies aren't huge.

Suppose we have a lithium cell of 3.2V and 0.002 Ohms resistance. Upon regen charge, it takes higher voltage to charge it, but you draw that energy out at lower voltage. Let's say you accelerate at 400A for 10 s and regen at 200A for 20 s

Vout = 3.2 - 400A * 0.002 Ohm = 2.4V
Vin = 3.2 + 200A * 0.002 Ohm = 3.6V

Power = Volts * Amps, and Energy = Power * time.

Efficiency in and out = (2.4V * 400A * 10 s) / (3.6V * 200A * 20 s) = 67%



Siwastaja said:


> This is an old post but it needs to be corrected. When the numbers add up like here, it's important to get the input data right, otherwise it is garbage in garbage out. These numbers are clearly incorrect.
> 
> Typical efficiency for a motor is about 90%, and for a controller, it's about 95%. (These are typical, conservative numbers, not optimistic; there are motors that can perform at 95% eff and controllers that get 98%. But it's pretty extreme.)
> 
> ...


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## Siua (Mar 14, 2014)

I am trying to find a good motor with regen capabilities to complement a 150cc engine. Our controller design is easier implemented with a DC motor. Can a PM DC motor offer similar efficiency in regen capabilities to an AC? And when looking for one can any PM DC motor work well with no extra wear on the motor or do I need to find one specifically designed for regen capabilities?


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## major (Apr 4, 2008)

Siua said:


> I am trying to find a good motor with regen capabilities to complement a 150cc engine. Our controller design is easier implemented with a DC motor. Can a PM DC motor offer similar efficiency in regen capabilities to an AC?


Pretty close, yes.



Siua said:


> And when looking for one can any PM DC motor work well with no extra wear on the motor


There will be additional wear on the brushes and comm from regen vs without regen.



Siua said:


> or do I need to find one specifically designed for regen capabilities?


Is there such a thing 

Strange questions from someone who has designed a controller for it. And what is meant by 150cc engine? Is it a hybrid vehicle?


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## Siua (Mar 14, 2014)

Thank you for your help. I'm an engineering student, I guess we haven't gotten to the electric motor part yet  The 150cc's is the displacement on a gasoline engine, it comes in both two and four strokes.


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## major (Apr 4, 2008)

Siua said:


> The 150cc's is the displacement on a gasoline engine, it comes in both two and four strokes.


You miss the point. How is this relevant to regenerative braking? Or to do-it-yourself electric cars?


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## evmetro (Apr 9, 2012)

It is interesting how much people like to focus on how much energy is available with regen, and how people like to factor regen into range calculations, but hardly anybody ever addresses how difficult it is to properly utilize it. Just because a car has regen capability does not mean that it extends the range. Depending on how regen is triggered, it could actually decrease range.


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## McRat (Jul 10, 2012)

The more aggressive regen is, the better the results.

Aero is a very good brake at high speeds. You want to slow the car down quickly with regen to avoid aero losses.

If you drive like granny, you won't see a lot of regen. You'll use the kinetic energy to overcome rolling resistance and aero, instead of recovering it.

If you drive like an animal in the city with an EV, watch what happens. This is where an EV shines. An ICE vehicle pulling that crap will get tragically poor fuel economy, yet an EV with aggressive regen will get excellent mileage.

It's hard to imagine, but jack rabbit starts in an EV don't affect mileage. It's going to take you MORE energy to get up to 50mph if you do it slowly since you have the aero to fight.

Try it. My daughter gets worse range in our EV than I do. And I drive it like I stole it.


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## McRat (Jul 10, 2012)

evmetro said:


> It is interesting how much people like to focus on how much energy is available with regen, and how people like to factor regen into range calculations, but hardly anybody ever addresses how difficult it is to properly utilize it. Just because a car has regen capability does not mean that it extends the range. Depending on how regen is triggered, it could actually decrease range.


When you race production cars, you wear out a set of iron rotors and pads every 30-90 minutes. Think of how much energy it takes to do that. You pull into the pits and the brakes are smoking. 

Regen is going to be a huge advantage to race cars, and is what will probably doom ICE engines in roadracing. Brakes will work INSTANTLY at the start of the race, and work the same the whole race.


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## Duncan (Dec 8, 2008)

I agree that you need to drive like a hooligan to get the most benefit from re-gen

Don't agree about the track
You do brake like a hooligan on the track BUT
You also need 
More braking than you are likely to have re-gen available
Disc brakes can absorb an awesome amount of power
Braking on all four wheels
Total control of your brakes

Making the discs last longer is not important - so long as you complete the race

Re-gen is good for moderate braking - but all of the cars that have re-gen have friction brakes sized for the emergency situation


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## McRat (Jul 10, 2012)

Duncan said:


> I agree that you need to drive like a hooligan to get the most benefit from re-gen
> 
> Don't agree about the track
> You do brake like a hooligan on the track BUT
> ...



Friction brake systems are good. But regen braking with digital control will beat them. AWD. Better control, more powerful braking, less weight. No brakes at the wheels. When a car slows down, the kinetic energy falls rapidly. With friction brakes, either the driver or electronics must stop locking up the tires. With regen, it naturally reduces the available braking effort.

In 10 years, we will see.


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## RIPPERTON (Jan 26, 2010)

You cant drive like a hooligan with regen.
The fact is most regen systems arnt powerful enough to process all the kinetic energy in an aggressive or emergency stop.
Regen only works when you drive like a retired accountant, then you have to drive very foresight fully and spread you braking out over as long a distance as possible meaning you have to look way ahead for red lights and anything that gets in your way. Then you can get as much as 27% extra range as I have proven in my Mira.
It for this same reason that regen wont work in racing. At least not alone.
You might get a tandem system to work but kinetic energy in racing is just untouchable spikes of energy thats best dealt with by wasting it as heat.


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## Duncan (Dec 8, 2008)

If you drive carefully you will hardly use the brakes in which case you will get zero benefit from re-gen

Any energy conversion is "taxed" by the gods of engineering
The best way to reduce your "tax bill" is to minimize your conversions - so once you have converted your power into kinetic energy leave it there as long as possible


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## McRat (Jul 10, 2012)

RIPPERTON said:


> You cant drive like a hooligan with regen.
> The fact is most regen systems arnt powerful enough to process all the kinetic energy in an aggressive or emergency stop.
> Regen only works when you drive like a retired accountant, then you have to drive very foresight fully and spread you braking out over as long a distance as possible meaning you have to look way ahead for red lights and anything that gets in your way. Then you can get as much as 27% extra range as I have proven in my Mira.
> It for this same reason that regen wont work in racing. At least not alone.
> You might get a tandem system to work but kinetic energy in racing is just untouchable spikes of energy thats best dealt with by wasting it as heat.


Regen can be as strong as the engineer wants it to be. Test drive a GM Volt or ELR, run in max regen (L, Sport). In fact, it's too strong in low traction or panic stops. Not unsafe, but twitchy as the tires brush the traction limit. 

Now, you will always be limited on RWD cars. You simply don't have shiit for rear braking. Good tires, big brakes, can make a light car go 85/15 FR/RR bias, more commonly 70/30. AWD would be the best for max regen.


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## tomofreno (Mar 3, 2009)

Thought I would try to quantify this a bit, to see how large the effect of acceleration/deceleration rate is. Bottom line is, it makes a difference, but not a huge one for typical range of rates. Those not interested in the details might want to skip to the examples at the bottom.

Rolling resistance force = Fr = Crr*ev weight*cos(theta), assume level ground so = Crr*ev weight
Drag force = Fd = Cd*A*v2

For 2001 Suzuki Swift ev (my car): Cd = 0.32, A = 1.8 m2, m = 1030 kg, Crr = 0.014 (determined by roll down test)

*Acceleration, 0 to 60 mph:*

Consider two acceleration rates: 
a1 = 6 mph/sec (2.7 m/s^2), i.e. 0 to 60 mph in 10 sec
a2 = 2 mph/sec (0.9 m/s^2), i.e. 0 to 60 mph in 30 sec

Assume initial vehicle position = initial vehicle velocity = 0
Then velocity v = a*t, distance x = a*t^2/2 = 135 m for a1, 405 m for a2

The work done against drag and resistance forces during acceleration to 60 mph is:

Wd = Fd * x = Fd * a*tf^2/2

Wr = Fr * x = Fr * a*tf^2/2 

where tf = time to accelerate to 60 mph

Fd depends on velocity, so integrate velocity over the acceleration time and divide by this time, tf, to find the average velocity:

vave = a*t^2/2*tf evaluated from 0 to tf =a*tf/2
v1ave = 2.7 * 10/2 = 13.5 m/s
v2ave = 0.9 * 30/2 = 13.5 m/s
It is independent of the acceleration rate because the vehicle spends half of tf under 30 mph and half over 30 mph due to constant acceleration rate.

Then the average drag force for either acceleration rate = Fdave = Cd*A*vave^2 = 14.4 lb = 64.1 N. 
Rolling resistance,Fr = 31.6 lb = 140.6 N 

Wd1 = Fdave * 135 = 64.1 * 135 = 8654 J = 2.4 Wh

Wd2 = Fdave * 405 = 64.1 * 405 = 25961 J = 7.2 Wh

Wr1 = 141 * 135 = 19035 J = 5.3 Wh

Wr2 = 141 * 405 = 57105 J = 15.9 Wh

Vehicle K.E. at 60 mph = mv2/2 = 1030*26.82/2 = 3.7 e5 J = 103 Wh

Percentage of K.E. lost to work against rr and drag forces during acceleration:

For acceleration at a1: 100*(2.4 + 5.3)/103 = 7.7%

For acceleration at a2: 100*(7.2 + 15.9)/103 = 23.1%

Difference of about 15.4 Wh.

*Deceleration, 60 mph to 0:*

The work done on the vehicle by the rolling resistance and drag forces is the same as the work done by the vehicle against them during acceleration with the assumption that acceleration rate = deceleration rate:

Wd1 = 2.4 Wh

Wd2 = 7.2 Wh

Wr1 = 5.3Wh

Wr2 = 15.9 Wh

So at the fast deceleration rate, 6 mph/sec, 7.7% of the vehicle K.E. is lost to work against these forces, and at the slow deceleration rate, 2 mph/sec, 23.1% is lost.

If we assume a combined motor/controller loss of 20% and drive train loss of 10%, then for deceleration rate a1:
100 – (20 + 10 + 7.7) = 62% goes into the battery pack. 

For deceleration rate a2:
100 – (20 + 10 + 23.1) = 47% goes into the battery pack. 

Measurements with my car at an acceleration rate in between these two have indicated about 50%.

*Net energy used per acceleration/deceleration cycle*: 

Over one cycle of acceleration to 60 mph and deceleration to 0 mph the vehicle kinetic energy plus energy due to all the losses must be expended during acceleration, and during deceleration the battery gets back the vehicle K.E. minus the losses, so net energy used is: energy out of battery – energy into battery = (K.E. + losses) – (K.E. – losses) = twice the losses (assuming zero distance driven between accelerating and decelerating). 

For a1: 103 * (1+ 0.077 + 0.2 + 0.1) – 103 * (1 - 0.07 - 0.2 - 0.1) = 141 – 65 = 78 Wh, versus 141 Wh without regen

For a2: 103 * (1+ 0.212 + 0.2 + 0.1) – 103 * (1 - 0.21 - 0.2 - 0.1) = 156 – 50 = 105 Wh, versus 156 Wh without regen

*Percentage of expended energy driving on level ground that is regained through regen:*

The percentage of the energy expended driving the car on level ground that is regained through regen = 
energy gained by regen/(energy used at constant speed + energy used to accelerate), or:

n*(K.E. – L)/ [(d – 2*n*x) * (Fd + Fr) + n*(K.E. + L)], for 2*n*x < d

Where: 
n = number of acceleration/deceleration cycles, L = sum of average losses (need to average if travel at different constant speeds between stops, i.e. loss at 35 mph, loss at 50 mph… just assume one constant speed between stops for simplicity) 
d is total distance traveled 
x is the distance required to accelerate or decelerate to/from speed, so total distance driven at constant speed is d – 2nx assuming the same rate for acceleration and deceleration, and same constant speed between each acceleration and deceleration for simplicity.

Examples: 
(1) My car accelerates at a1 to 60 mph, drives 10 miles, then decelerates at the same rate to a stop, estimated percent energy regained is: 2.7%

(2) Acceleration at a1 to 35 mph and deceleration at same rate 10 times in 10 miles:12.7% 

(3) Acceleration at a1 to 35 mph and deceleration at same rate 20 times in 10 miles:20.7%

A more typical acceleration/deceleration rate might be 3 mph/sec, or 1.34 m/s (0 to 60 mph in 20 sec). Then the same three scenarios give 2.4%, 12%, and 20%, so a factor of 2 slower rate doesn’t change the result that much. At a2 = 2 mph/sec acceleration/deceleration rate the three scenarios give 2.2%, 11%, and 19%.

Increasing total miles traveled, d, to 30 in scenario (1) gives 0.9%. Increasing stops in this scenario to 3, with 30 miles total, gives 2.7%.

Increasing vehicle mass increases the percentage of energy recovered, but it’s a small effect for larger number of stop/starts. For example the first scenario goes from 2.7% to 3.8%, second goes from 12.7% to 14.7%, third goes from 20.7% to 23.2% if vehicle mass is doubled.

Decreasing losses in the motor/controller and drive train of course increases the energy recovered. For example, decreasing motor/controller loss to 15% in scenario (2) increases the energy gained from 12.7% to 16.5%.

Increasing drag coefficient or area, A, reduces percentage of energy recovered, but it too is a small effect. Seems numbers for most cars will be close to the above. These calculations agree well with data I've taken on my car.


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## gunnarhs (Apr 24, 2012)

mikep_95133 said:


> I hope it's ok if I updated this part of the wiki on regen. It's very valuable. Soleq's use sep ex to generate regen. I've personally measured 20-40% regen return on my vehicle.
> 
> Mike


Hi Mike, did you get that values of 20-40% with the SepEx or other motor?
My values are something like this (when full regen is turned on)
3% with Sepex
5% with AC-induction
10% with PMSM (OEM car)

But I am not driving much in heavy city traffic, though terrain is somewhat hilly.


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## MaxB (May 23, 2014)

New poster here trying to figure the EV thing out. So if you do live in hilly terrain, with the kind of descents where you have to step on the brake or risk going off the road, would regen then be much more desirable? Thanks very much and great forum.


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## major (Apr 4, 2008)

MaxB said:


> So if you do live in hilly terrain, with the kind of descents where you have to step on the brake or risk going off the road, would regen then be much more desirable?


Absolutely


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## MaxB (May 23, 2014)

If you wouldn't mind also clearing one more thing up for me: As I think I understand it, a DIY EV with an AC motor and the right controller will give you some regen but not as much as an OEM product like a Leaf or Volt. Is this also correct? Thank you again!


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## dedlast (Aug 17, 2013)

MaxB said:


> If you wouldn't mind also clearing one more thing up for me: As I think I understand it, a DIY EV with an AC motor and the right controller will give you some regen but not as much as an OEM product like a Leaf or Volt. Is this also correct? Thank you again!


I don't think this is correct. With a DIY EV that has access to the programming of the contoller, you should be able to get somewhat better regen output than the OEM's will provide. At the least, you can fine tune it to your particular situation.

B


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## major (Apr 4, 2008)

MaxB said:


> a DIY EV with an AC motor and the right controller will give you some regen but not as much as an OEM product like a Leaf or Volt. Is this also correct?





dedlast said:


> you should be able to get somewhat better regen output than the OEM's will provide.


I don't know of any reason why you would be unable to get as much, or any reason why you could get more.


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## dedlast (Aug 17, 2013)

major said:


> I don't know of any reason why you would be unable to get as much, or any reason why you could get more.


I was just thinking that a person could adjust the regen to a more aggressive setting if they wanted to. But then again, at this point all my knowledge is "book learning" from the forum, not hands on experience.

B


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## major (Apr 4, 2008)

major said:


> I don't know of any reason why you would be unable to get as much, or any reason why you could get more.


I am addressing fundamental principle. Of course there is adjustment for lower levels in either case. Also possible differences on how the electric braking interfaces with the service brakes and maybe even ABS.


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## malith90 (Nov 17, 2015)

hii

I have a golden motor 10kw 72v motor and motor controller.whrn in braking the regeneration occurs and the battery terminal voltages increase more than 72 v. how can i capture this energy to a seperate capacitor bank

Thanks


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## major (Apr 4, 2008)

malith90 said:


> hii
> 
> I have a golden motor 10kw 72v motor and motor controller.whrn in braking the regeneration occurs and the battery terminal voltages increase more than 72 v. how can i capture this energy to a seperate capacitor bank
> 
> Thanks


Why would you want to do that? 

I suppose you could design a circuit using diodes, and/or contactors, possibly with a converter to do such. But that would likely add losses into your system making it less effective and probably less reliable. 

major


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## evBEDNER (Sep 14, 2014)

Does anyone know what the typical regen efficiency is for a separately excited DC motor (vs. a typical AC motor)?

I was planning to use an AC motor in my next build, but then I came across a nice separately excited 11 inch GE motor..... i think it's 72 volts. And I know that Curtis makes an 80 volt sepex controller (600a peak)... so I was thinking of going that route instead.

I'm sure that the GE 11 inch motor has way more torque than an AC35 or even an AC50.... at a fraction of the cost.


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## major (Apr 4, 2008)

evBEDNER said:


> Does anyone know what the typical regen efficiency is for a separately excited DC motor (vs. a typical AC motor)?
> 
> I was planning to use an AC motor in my next build, but then I came across a nice separately excited 11 inch GE motor..... i think it's 72 volts. And I know that Curtis makes an 80 volt sepex controller (600a peak)... so I was thinking of going that route instead.
> 
> I'm sure that the GE 11 inch motor has way more torque than an AC35 or even an AC50.... at a fraction of the cost.


Hi evB,

Both of those motor types, as well as all motors, and generators, are bidirectional, when properly excited and exhibit pretty much equal efficiency, motor or generating, all else being equal. So, the efficiency shown on the motor performance graphs would be what to expect when the machine is regenerating at similar speeds and loads. 

Regards,

major


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## evBEDNER (Sep 14, 2014)

In that case, I see no reason to go with AC at all... especially when I'm on such a tight budget. 

The sepex motor graphs I saw range from 89 to 92% efficiency (peak) which is not too far off from the AC kits. Then there is the issue of brush wear....

but my assumption is that, since the brushes on a sepex are only running the armature and not the four field coils, they will probably last a while....much longer than a series wound motor, correct?


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## major (Apr 4, 2008)

evBEDNER said:


> In that case, I see no reason to go with AC at all... especially when I'm on such a tight budget.
> 
> The sepex motor graphs I saw range from 89 to 92% efficiency (peak) which is not too far off from the AC kits. Then there is the issue of brush wear....
> 
> but my assumption is that, since the brushes on a sepex are only running the armature and not the four field coils, they will probably last a while....much longer than a series wound motor, correct?


No, that's not a good deduction, for a number of reasons. Hey, no offense intended here, but you don't seem too knowledgeable on motors/control. While a SepEx may be neat, it will require a level of expertise which I don't sense from you. The AC package may suite your needs with a whole lot less effort. I've used both types and like both. Just sayin'.

And with properly applied DC systems, brushwear should not be issue.

Regards, 

major


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## GizmoEV (Nov 28, 2009)

major said:


> The AC package may suite your needs with a whole lot less effort.


Based on the challenge I had with getting my Sevcon SepEx controller reprogrammed when I had to replace my motor in my Gizmo, I'm with Major on this one. I went with a longer armature which resulted in many parameters I had to figure out to get the performance I needed without overheating the motor under normal conditions. An AC motor with matched controller would be much simpler.

With the Sevcon controller I cannot set the max and min regen current. I can only set max and the minimum is 50% of that which means I either have smooth regen at the low end and not enough high current regen or I have good high regen current but it comes on a bit strong at the low end. Also, in my situation, once I get below 10-12mph it actually takes energy out of the battery to regen any further. My 2016 Kia Soul EV+ will positively regen down to 1-2mph, though it has a permanent magnet AC motor.


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## MrZion (May 4, 2016)

Hello everyone,

This is my first post so take it easy on me....

First of all, I am wanting to do this conversion on a 1989 VW transporter, I know this truck/car aerodynamically couldn't be worse, but it's only for my small island I live on.

Next,
I am starting my EV conversion process by starting by building a 200kW AC motor controller thanks to a very awesome instructable posted! Once this is complete I want to incorporate an AC (PMAC) regen braking system (at some point in the whole process), but I am confused as to how AC regen actually takes place. It would be greatly appreciated if someone could essentially do a component by component summarization of the process.

Thanks in Advanced,

W


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