# How to start developing a Regenerative brake system for an EV race car?



## Duncan (Dec 8, 2008)

Hi Synergy

Why do you want re-gen?

In a race car you are hard on the brakes - a light set of discs can absorb energy at the limits of the tires - this will amount to many hundreds of Kw in energy absorption

Re-gen is for gentle braking - you don't do that on the track!!


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## major (Apr 4, 2008)

Synergy said:


> I am trying to develop an Regenerative brake system for an Electric race car,
> The powertrain for the car has already been developed, it consists of:
> 
> 2 20kw PM DC brushless motors from 2 vetrix scooters
> ...


Hi Syn,

The "inverters" or controllers should be fully capable of regeneration with PM BLDC motors. Just program them to do so.

And Duncan,

Regeneration need not be "gentle". I've used very aggressive regen, up to breaking traction of tires on a bus. Typically, you can get as much braking torque as motor torque from the electric propulsion system, if not a little more. But you do have to have some place to put that energy. I would think his battery pack would be capable after the first lap.

Regards,

major


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## Duncan (Dec 8, 2008)

Hi Major

Maybe gentle was too strong a term but on the track you will be dissipating many times as much power in braking as you have available for acceleration
example 
500kg vehicle?
112 mph - 180 kph - 50m/sec - not fast
1 g deceleration - with racing tires I would expect to beat this
a fairly modest brake setup will dissipate 250Kw, 333Hp, 500x10x50 watts
- and should have at least 50% headroom " just in case" -500Hp!!

a lot more than the motors and batteries can cope with

You will need the brakes to get a decent lap time, if you can get re-gen for no additional weight then do so but it will not buy you any time -

It's not as if you could save weight by charging the batteries less!! 

If I was racing I would be more worried about the re-gen braking (presumably on two wheels) unsettling the car before I applied the main brakes and making the whole brake ballance issue more complicated

A road car is different - most of the braking is gentle - the main brakes are only needed for emergency situations

On the track every braking event is at emergency braking levels - or you are simply giving time away


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## Bottomfeeder (Jun 13, 2008)

Check the charging limits for your batteries. I think you'll find that you'll have to dump much more than the 3C or 10C limits that are typical. I remember reading about an EVer who used a bank of supercaps to store up his regen and slowly bleed it back into the battery, or quickly dump it into the motor again, as the situation requires. I think it was Metricmind.


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## toddshotrods (Feb 10, 2009)

I'm with Duncan. I was determined to preserve the regen capabilities of my big SepEx motor, and eventually realized that the extra cost was completely unjustified. My project is a pure street-legal race car and regen just didn't make a lot of sense when I really looked at it objectively. For really aggressive race-type driving, the complexity of balancing the varying loads of regen (based on how much charge is in the pack, the rate the pack can take the charge, how all that affects my rear braking bias, etc) just aren't worth it to me.

You're approaching a turn and drive in hard, then brake for a fraction of a second to a couple seconds, then jump back into the accelerator pedal at the apex. The time you had to capitalize on regen from when you lifted off the accelerator, until you got back in the accelerator, is too short - if you plan to drive competively. A fast racer in a good car is going in full-boil, counting on his brakes to catch it the last second. If you're finding time for regen you're probably losing. 

Maybe the occasional downhill entrance into a tight turn would provide a nice regen opportunity, but from what I understand (drag racer learning about serious road race driving) the ICE downshifting that would produce electric regen is usually to be in the right gear coming out of the turn. When I watch the in-car videos of fast drivers the times they actually downshift, and you hear engine braking, is way too fast and too limited to go through any trouble to replicate. Many times, I see the downshifts but don't hear the engine until after the apex. My guess is it's easier to maintain the balance with the brake pads.

I decided on massive brakes that would stop three times my weight (1300lbs with me on board) easily for a whole race. As far as wasting energy as heat goes - it's racing, we're wasting energy anyway!


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## peggus (Feb 18, 2008)

Why are you guys trying so hard to discourage him from adding regen braking? 

He didn't say what type of race car it is, maybe he's into endurance racing and is looking for every tiny edge when it comes to range? And, regen can indeed save weight if it means he can use a smaller pack. 

Since he's using two 20kW scooter motors I don't think we need to presume he's going to go into F1 racing. Further, regen braking is not an all or nothing deal like some people would have you believe. You can still use the mechanical brakes to absorb the power that the E drive train can not handle, in this case anything over 40kW. 

Since he is using PM motors with inverters already, it is just "a simple case of software" (TM), and integrating a regen pot with the brake pedal.

I don't know anything about the inverters he's using so I can't tell him how, exactly, to go about doing this. If he's using the Vectrix inverters then maybe someone over on "V is for voltage" may know, they tend to be more scooter oriented. http://visforvoltage.org/

Good luck.


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## toddshotrods (Feb 10, 2009)

In my case, I am not necessarily trying to discourage him. It's more a matter of presenting the other side of the coin. The best way to start developing a regen system is to understand the pros and cons, and inherent limitations, to determine whether it's even practical for the application. Usually people that start seeking regen (myself included) see it as a sort of magic bullet that can do more than it's really capable of. We don't know much about his exact plans, so all we can do is present a lot of information for him to use to make good decisions. I'm not an expert, but I have been helped by some, and just presented one case where regen proved to not be the best solution. I have a SepEx motor and all it would take for me to have regen is "a simple case of software" as well...


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## quicker240 (Jul 12, 2010)

So you put a small belt pulley on your drive shaft, mount a small 100a alternator above it with an electric clutch ala a/c compressor. Wire the clutch into the brake light switch. output of the alternator to the battery pack, and BAMM! regen charging!


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## ewdysar (Jun 15, 2010)

quicker240 said:


> So you put a small belt pulley on your drive shaft, mount a small 100a alternator above it with an electric clutch ala a/c compressor. Wire the clutch into the brake light switch. output of the alternator to the battery pack, and BAMM! regen charging!


Yep, 1200W of regen, or less than 2 hp of decelerative force. I don't know his planned voltage, but at 120V, thats 10A of regen. If you stay on the regen for 36 seconds (that's a long time for braking) you get 0.1Ah or 12Wh back into your batteries. 

If he's looking to slow down, this amount of regen won't do that much. If he's looking for extended range, again it won't amount to much. If he traded in the extra weight of the alternator and electric clutch for a couple of extra Lithium cells, he'd get more additional range than the regen could provide. 

But yes, your suggestion would work.

Eric


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## toddshotrods (Feb 10, 2009)

Regen makes the most sense on the street in normal "city" driving. Long slow stops are commonplace, and you can rack up a lot of them in a day of commuting. If you have long downgrade stretches, hills, or mountain roads, all the better. If you have some type of racing that simulates these conditions, maybe at a more brisk pace, it could make sense. In a fierce, tight-turn, road race style competition - I would go with more battery.

The fact that your motors are actually capable of regen helps. From there it's a matter of what it takes to get a controller to take advantage of it. I am not familiar with PM. I know for AC and SepEx the inverter or controller have to be programmed for the motor. You need to figure out how much energy you can recapture and what it will cost you to get that into your battery pack. Weigh that against how much energy you would get from additional cell, and what they would cost you.

I honestly see the greatest value for regen in making vehicles feel more like traditional ICE powertrains - for people making the transition. I started experimenting with my manual trans ICE driver, to see if I would like _*not*_ having regen in my street rod, and actually realized that I drop my car in neutral and stop with the brakes a lot anyway. Good brakes work fine.


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## quicker240 (Jul 12, 2010)

quicker240 said:


> So you put a small belt pulley on your drive shaft, mount a small 100a alternator above it with an electric clutch ala a/c compressor. Wire the clutch into the brake light switch. output of the alternator to the battery pack, and BAMM! regen charging!


 
^^^^^^^ Admittedly bad joke...


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## DawidvC (Feb 14, 2010)

Synergy said:


> Hi all
> 
> I am trying to develop an Regenerative brake system for an Electric race car,
> i was hoping to get some feedback on how i can go about starting this project and the considerations i should be making.
> ...


The system you have specified is already capable of regen, you just need to set it up in the invertors - check the programming of the inverters. Your main problem will be to dump the excess energy that regen will be generating somewhere, which usually means a resistor with high wattage to handle the currents you will be generating. Maybe Major can assist you in finding the right values for these resistors, if you were to ask him nicely.

Remember that you can regen as hard with your motors as you can accelerate, so that puts limits on how fast you will decelerate using just regen. Check what you need re decel, and figure in conventional brakes as well.

Regards
Dawid


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## Bottomfeeder (Jun 13, 2008)

Why would you want to dump the energy into a resistor? It seems like a lot of work to go through just to heat up a resistor instead of a rotor.

You'll need something that can accept high current during these aggressive braking moments. As far as I know super/ultra caps are the only game in town.

Of course, you'll also need some controls so the standard brakes kick on during hard stops. Perhaps the standard brakes on your car don't kick on until the second half of the throw of the pedal. You could put a variable resistor (much like a throttle pot) on the brake pedal, and use that as a control signal for your regen for the rest of it. So for the first half of the pedal throw, you'll be braking by wire. For the second half, you'll have old fashioned hydraulics as well.

The rest is software.


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## toddshotrods (Feb 10, 2009)

IIRC, super/ultra caps are prohibitively expensive and also get heavier as their capacity increases - like batteries. So, if a resistor is the most economically feasible method of dumping the regen energy, how efficient and reliable would it be at quickly dissipating the heat into the atmosphere?

Brake rotors are designed specifically for that purpose. To convert energy into heat, quickly shed it into the atmosphere and be ready to do it all again without notice. An extreme example would be watching an F1 car's ceramic rotors glowing red when the driver is on the binders, then return to their normal condition quickly. To a lesser degree that's what even cast iron rotors are designed to do.

Are there resistors that are designed for the same purpose? How expensive are they?



Bottomfeeder said:


> Why would you want to dump the energy into a resistor? It seems like a lot of work to go through just to heat up a resistor instead of a rotor.
> 
> You'll need something that can accept high current during these aggressive braking moments. As far as I know super/ultra caps are the only game in town.
> 
> ...


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## DawidvC (Feb 14, 2010)

Bottomfeeder, if you push more energy into the system than it will accept at any one moment, you can create a problem. Specifically, you can go overvoltage on your controller and / or battery system. Supercaps help, but they have limited capacity. Remember, with this specific setup, you can regen as much energy (minus losses) as you can take out, and any bldc is an effective generator. If your system can no longer accomodate the energy you have two choices - stopping regen - very difficult with this setup, or dumping the energy, hence the resistors.

Regards
Dawid


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## JRP3 (Mar 7, 2008)

My relatively mild AC35 system can put over 200amps into my SE100Ah pack on regen, which will slow you fairly quickly, and does not overvolt my controller. The batteries can handle burst of 300 amps, larger cells could handle more, as could higher C rate cells, Headway, A123. You should be able to set up regen to give your brakes a nice assist and maybe allow them to perform better for a longer period of time. I would think you could set it up similar to the feel of a down shift on a high compression motor, without having to downshift. Since acceleration always takes more energy than deceleration you'll have plenty of room in the battery pack to store the regen energy as soon as you start the race.


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## peggus (Feb 18, 2008)

Limiting regen on over voltage is a minor controls issue and something his inverters are probably already capable of. And, it is far less of a problem than it is made out to be, unless you start out at the top of a hill you'll likely never run into it. 

Batteries can take short bursts of higher than normal charging current without issue, generally they can absorb as much as they can dish out.


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## TX_Dj (Jul 25, 2008)

peggus said:


> Batteries can take short bursts of higher than normal charging current without issue, generally they can absorb as much as they can dish out.


"Generally"? Seriously?

The batteries that I use in my race scooter can pump out 60A for up to 10 seconds before they self-destruct.

They can only take 1.8A "initial current" during charge.

I think you'll find this is true for lots of batteries, which would "generally" be the case.


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## peggus (Feb 18, 2008)

Yes, generally. Depends of course on chemistry, SOC, what your cutoff criteria are and how much you're willing to abuse your pack. Do note that I'm talking about relatively short pulses here, not continuous recharge.

Part of what I do for a living is testing batteries, so my sample size is slightly larger than 1.


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