# A practical KERS concept ?



## madderscience (Jun 28, 2008)

We don't outright dismiss a KERS (kinetic energy recovery system, aka regen, or regenerative braking) on DC systems because of the 3rd law of thermodynamics. (you can't get out more than you put in) We dismiss it because in most highway-capable EVs that run DC, they are using series wound motors that are difficult to regulate when operated as generators. Among other things, when being operated as a generator the brush timing needs to be reversed on these motors to avoid commutator arcing. In the case of AC systems, along with shunt wound and permanent magnet DC systems, regenerative braking is much easier and doesn't need to require any additional major components in the system.

I actually thought about building a system similar to what you are talking about with the hope of providing some minimal regen. (to the point where I collected some of the components) Here is what I concluded though:

1) A typical car's 12V electrical system will provide a load of up to about 500 watts (based on a typical automotive 40A alternator's capacity. 

2) A typical DC/DC converter is around 90% efficient and weighs a couple of pounds, if that. It has no moving parts except (in mine's case) a small fan, no friction losses, and can be mounted pretty much anywhere in the car.

3) A typical 40A automotive alternator (including belt drive, etc) is probably around 70% efficient, and will weigh 10 or 15 pounds. A typical air conditioning compressor clutch is also pretty heavy (5 to 10lbs) , and they usually pull around 4 or 5 amps just to engage their electronic clutch. They are designed to be mounted on the "driven" device which means even when disengaged, you have efficiency losses in the drive belt. So the mechanical weight of this system would probably be at least 30 pounds, probably closer to 40 or 50 by the time you design all the brackets, etc.

So right away, you are carring around significantly more weight to try and do regen to power the 12V system. 

The next question, since this is actuated only when on the brakes, is how much time you actually spend on the brakes. Surely you would spend absolutely no more than 50% of your drive time on the brakes, but probably it is more accurate to say 10% to 25% of the time depending on your driving situation but it's just a guess. So let's assume you are on the brakes 25% of the time, and the system can produce a full 500 watts during that time (overly optimistic, but just getting to the logical point here) This means that on average, you are able to collect 125 watts of power from this system during the drive (25% of 500 watts). This would be equivanent to having a 10 amp DC/DC converter running full blast, which most everybody would suggest is too small for a highway vehicle. (most are 30 amps rated or larger) This would mean you still need a DC/DC, or an auxilary charger for the 12V system.

So my conclusion was while there is nothing physically impossible about the approach of using an alternator and A/C clutch for brake-only 12V regen, it probably isn't worth it. Your mileage may vary, but here are some other ideas:

So for the same extra weight, what else could you add to the system that might work better? How about just adding another traction battery? If you were running lead acid, you could basically add another golf cart battery (about 60lbs) for roughly the same weight. This battery could (accounting for peukert, DOD, etc) provide another 500 watt hours of power, or 500 watts if discharged over the course of an hour which in an EV would probably be 30 or 40 miles of driving. All this without any additional complexity, just more storage capacity.

If your car chassis is amicable to it, you could also integrate a 40 or 50 watt solar panel into the roof of the chassis (to avoid aerodynamic losses) and add a solar charge controller to keep the 12V system topped off. It would still weigh less than the alternator regen approach but of course be heavier than just running a DC/DC. The big problem with using solar to augment the 12V system is that when you need it the most (in the dark when all the lights are on) it is nothing but dead weight. 

Good luck.


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## Max57TBird (Mar 22, 2010)

Madderscience,
Thanx for your reply to my poost; I will try to address your post item for item:
First, I didn't mean that we dismissed all DC Regen systems as perpetral motion and I have tried to read and consider all the arguements. And I agree with your points 1 & 2.
As to #3, yes, it will add weight, but it is a toy I want to pursue. I will be mounting the clutch "Backwards" so that the belt won't turn until the clutch is engaged.
Now, your next point, that it is only activated when the brakes are applied is where I probably didn't make myself clear enough. It would be activated by a microswitch on the pot box The m'switch would have 2 detents, one for coast, and another that engages the clutch and provides about 5 HP braking action for the vehicle. Now the system is activated every time you let off the gas as in comming up behind a slower car, or approaching a stop sign (without brakes applied), or go down a hill. I don't have any % of time or energy recovery estimates, and only experimenting with a prototype system will tell. I've had a lot of wacko ideas that didn't pass the proof of concept test.
Interestingly, there was another thread on this same subject: http://www.diyelectriccar.com/forums/showthread.php?t=46885
and "Coley" replied that he only charges his aux battery once a week. That sounds like the coup de gras there.
Lane


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## madderscience (Jun 28, 2008)

If it is purely for experimentation purposes, go for it. I tend to look at things from a practicality viewpoint so I'm sorry if I sound like a buzzkill.

You are right I didn't read your original post carefully enough. So it's "off pedal" as well as braking regen. So you would be able to get more power out of it than I calculated. It is also clear from your goals that you would need to use a much larger alternator than I was figuring on. (a 40A alternator can produce about 2/3 HP or about 500 watts). You are talking about 5HP, or 300 amps at 12V, or 3600 watts. I know truck alternators go up to a couple of hundred amps so it might be possible. Of course, they weigh a lot more than a 40 amp alternator would as well. Are there any 12V batteries that can actually handle being burst charged at 300 amps? That don't weigh 100lbs? (I am running a 12V, 7AH gel for my SLI battery, it would explode if I tried to charge it in bursts like that) You would need some sort of charge regulation as well.

If you are really going to have that much regen, it would probably be better to rewind the alternator for higher voltage and recharge the main pack. This would keep the amps down, and the safety up and probably negate the need for as much charge regulation as you would need for the 12V charging. Of course this would mean you still need a DC/DC which is what you want to get rid of.

As for running the clutch backward, I wanted to do the same thing because you are right, theoretically it could be set up for zero or nearly zero friction load when disengaged. I was concerned however that due to the design of these magnetic clutches (notice how the flat springs are normally tensioned under load) that running it backward would cause it to chatter or otherwise not work properly. Pushing on a rope and all. Also, an A/C compressor clutch probably won't be able to handle 5HP. It will be able to handle about 1HP, which is roughly what a normal automotive air conditioning load is I believe. But, if you are going to have 5HP of regen, maybe it will be enough that it will be a net positive even without a clutch. Just directly connect the alternator to the motor. That would also eliminate the problem of a considerable amount of inertia that you would be trying to engage when the motor is screaming along at 5000rpm as you descend a hill. Clutch smokin' time.

Seems like a working system would now have considerable complexity and likely well over 100lbs of weight. All to replace a 90% efficient, 3 pound little box.

Perhaps a better approach (buzzkill alert again) if you are going to put this much effort into it, would be to get ahold of a car-sized shunt or compount wound DC motor, and invest your efforts into building a roadgoing EV sized shunt motor controller. Nobody had done that to my knowledge and it would be a considerable benefit to the conversion community. You could start with a golf cart or forklift controller and figure out how to scale it up in voltage. Such a system would weigh no more than a standard series wound DC system and have no extra major components.

Good luck.


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## kek_63 (Apr 20, 2008)

Look outside of the automotive world for the clutch. My Toro lawn tractor has an electric clutch mounted to the engine which free wheels until it is engaged (for powering the mower deck).

Keith


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## peggus (Feb 18, 2008)

http://www.waynesev.com/ev/regeneration.html


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## madderscience (Jun 28, 2008)

Wow. Yup, it can work. Yup, its complicated. I guess he tries to avoid engaging the regen system at speed to avoid excess starting load on the clutch. So if he knew he was going to head down a long hill, he'd turn on the clutch and the alternator fields at the top.

(note: 169V at 8 amps is about 1.4KW or almost 2hp of regen)


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## jehan12413 (Feb 4, 2010)

It seems every time I hear about someone designing a system like this they have a switch to turn the alternator on for regen. The problem with that is the braking can be significant , something you certainly don't want on a slipperly surface. When I disigned a system like this I used no clutch and used a # 41 chain and sprocket to connect the alternator. With the field deenergized there is virtually no resistance to turn it. As far as the regen you simply use a potentiometer on the brake pedal connected to the field through an amplifier so you can vary the regen with brake application. The main output lead then stays connected to your battery (either main or auxilary). One other point is the current rating on an alternator is continuous, a 61 amp GM alternator can put out 300 amps intermittantly without overheating. If you want more there are many different field coils available so I wouldn't even consider rewinding it.


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