# DC regen tangent (moved from Prototype Controller Thread)



## JRP3 (Mar 7, 2008)

I should point out that some lead AGM, like Hawker/Odyssey, can handle high discharge and recharge. One other benefit to regen is saving wear and tear on braking components. It might save you a few sets of pads and some rotors over the life of the vehicle. I agree that if most of your driving is fairly flat and steady it's not really worth much, but I'm always driving up and down hills and could use the extra 10-15%, which is one of the reasons I'm going with AC on my current build.


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## ElectriCar (Jun 15, 2008)

*Re: New controller prototype*



Tesseract said:


> Sure, but it's not as simple as just wiring some super caps in parallel with the battery pack. You have to feed them the regen output current separately, then use another converter to "parcel out" that stored charge at a slower rate to the batteries. This is a lot of extra circuitry just to accommodate the Peukert effect in lead-acid batteries. The LiFePO4 chemistry, in contrast, can absorb huge peak charging currents just fine.
> 
> If you live in hilly terrain, and regen is important to you, then you pretty much need to go with LiFePO4... if it's too expensive, then so is regen


This all makes me sick. I am converting to lithium at some point and you just made it all the more difficult to keep dragging all that lead around. I want the new MIT lithiums though with the super fast recharge ability. Those things will accept a charge faster than you can supply it from anything other than a cap bank.


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## dimitri (May 16, 2008)

*Re: New controller prototype*



ElectriCar said:


> This all makes me sick. I am converting to lithium at some point and you just made it all the more difficult to keep dragging all that lead around. I want the new MIT lithiums though with the super fast recharge ability. Those things will accept a charge faster than you can supply it from anything other than a cap bank.


Even current ThunderSky's can accept up to 3C charging current, I don't think your regen will ever produce 3C current, especially given Tesseract's efficiency numbers.

I think the biggest issue is complexity of the battery management system with regen. If you start with 100% charged pack and you live on top of the hill, then by the time you reach the bottom of the hill, your battery will be way overcharged. Although its a hypothetical situation, it may destroy your pack rather quickly.

I think the simplest approach is to block regen based on SOC, which still requires BMS capable of such feedback, and not all of them have it.

Having said that, I would simply use the Regen on/off switch manually if I had regen and lived in hilly area. I would start with it being off and turn it on when SOC came down to say 80%. At that point it highly improbable that you will ever get enough regen energy back into the battery to reach 100%, unless somehow you go down hill most of the time


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## Qer (May 7, 2008)

*Re: New controller prototype*



JRP3 said:


> One other benefit to regen is saving wear and tear on braking components. It might save you a few sets of pads and some rotors over the life of the vehicle.


Where I live not using the brakes could actually turn out being more expensive in the long run. People that use engine brake a lot in Sweden actually learn the hard way that salt slush + engine braking easily sums up to stuck brakes. Having to replace the whole caliper isn't uncommon, although that usually takes a year or two of total neglect.

Of course, usually you can avoid that problem by braking hard once in a while to exercise the system...


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## Qer (May 7, 2008)

*Re: New controller prototype*



dimitri said:


> I think the biggest issue is complexity of the battery management system with regen. If you start with 100% charged pack and you live on top of the hill, then by the time you reach the bottom of the hill, your battery will be way overcharged. Although its a hypothetical situation, it may destroy your pack rather quickly.


That's not that hard to avoid. If the controller count the amount of Ah coming out of the pack it'll know how many Ah it can push back. The losses will give a margin that guarantee that you're never going to be even close to overcharge the pack.

Of course, if you want to be REALLY eco-friendly and only charge your pack to, say, 80% and charge the last 20% as you go the situation becomes much more complex. I'd prefer to not even consider handling that situation...


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## JRP3 (Mar 7, 2008)

*Re: New controller prototype*

The Curtis AC controller I'll be using automatically cuts back on regen if it gets too close to the controller cutoff so there shouldn't be a problem. Since lithium cells quickly sag to around 3.2 volts, and since you don't normally charge them 100%, even if you start off on a long hill regen probably won't put enough voltage back in to cause concern even if you don't have auto cutback.


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## JRP3 (Mar 7, 2008)

*Re: New controller prototype*



Harold in CR said:


> Attach a pulley to the front of the Differential, or on the driveshaft, in a RWD vehicle, install an Alternator on a bracket, with an electric clutch. Put a switch on the steering wheel, so it's easy access ???
> 
> Going downhill, activate the clutch. I'm thinking a 200 amp Alternator would give a decent amount of current back to the batteries, and the motor controller should not be activated, to have a problem with over voltage or current surge ???


A number of people have used the front output shaft on their motors with an A/C clutch and belt.


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## dimitri (May 16, 2008)

*Re: New controller prototype*

OEM alternator is only 12V, not good to charge main pack. If you rewind it to higher voltage, then it'll be much less current. Also, lets look at energy, not amps. Even if its 200 Amp at 12V and assume no loss, you only get 2400Watt, you said 3 miles hill, say 45 mph for example, takes 4 minutes to travel, so you got back 160Watt/hour of energy. That will extend your range half mile on flat road. Now the question is, is that worth the hassle? IMHO, hell no 

That's because alternator is too small to generate any serious power. Now, if we use main motor, that's a different story, but as Tesseract pointed out, even that is probably not worth the trouble.


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## dimitri (May 16, 2008)

*Re: New controller prototype*



JRP3 said:


> The Curtis AC controller I'll be using automatically cuts back on regen if it gets too close to the controller cutoff so there shouldn't be a problem. Since lithium cells quickly sag to around 3.2 volts, and since you don't normally charge them 100%, even if you start off on a long hill regen probably won't put enough voltage back in to cause concern even if you don't have auto cutback.


Its not about how they sag, its about their SOC. They will jump back to 4V in a blink of an eye if you attempt to charge them when they are already 90-100% SOC. And "probably" is not good enough when $10,000 battery is at stake


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## JRP3 (Mar 7, 2008)

*Re: New controller prototype*

You shouldn't be charging your cells to 100% if you want them to last as long as possible anyway. Plus I don't think you'll be putting all that much energy back into them even on a long run. Let's say you've got a 200wh/mi. vehicle, how much of that will actually make it back into the pack on regen? Another way around that would be to have a resistance heating element, like your cabin heater, to kick on and disperse that energy as heat if it gets to high.
It wouldn't run long since it's not going to be that much energy.


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## dimitri (May 16, 2008)

*Re: New controller prototype*

That's exactly my point, if there isn't much energy in regen, then why bother with it in DC system, adding cost and complexity. Now, if you are building AC system and regen happens to be available at no additional cost, then go for it, just watch the SOC carefully.


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## ElectriCar (Jun 15, 2008)

*Re: New controller prototype*



Harold in CR said:


> This is one of the reasons I am very slow at getting a project started. I have a 3 mile long hill, with small leveling off spots, and it starts right where I live. Coming back up, will be the challenge.
> 
> After mentioning the "Nascar" type Alternator setup, several months back, I still have not seen anyone post why it would not be a good idea.
> 
> ...


Harold, here's a thread I started about this a few weeks ago. There was one guy who rewound his alternator to output like 180VDC or such and turned it with his motor. I calculated 200A @ 13.6V= almost 3kw. I think that would work well but you'd have to invert the 12V to 120V then either rectify it back to DC to charge the pack directly (less efficient) or energize a 120V charger (least efficient). That or go with the HV alternator and pump it right back into the pack (most efficient).


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## dimitri (May 16, 2008)

*Re: New controller prototype*

Sorry, we really took this thread off course here. Regen via controller only applies when you use main motor to regen, not separate alternator. Maybe this discussion can be picked up in another thread?

Tesseract, back to BMF 750 and BMF 1500 controllers. Any more pictures?


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## John (Sep 11, 2007)

*Re: New controller prototype*



Harold in CR said:


> I will be going with a Hybrid of my own design. I will have to climb that 3 mile long hill at the END of my trip.


More importantly how high is the hill? Potential energy = mass x gravity x height. If mass is in kg, gravity is 9.81N/kg and height is in meters, PE will be Watt seconds or Joules (Ws = J) so divide by 3600 to convert to Wh. Factor in efficiency of your system and energy to cover the horizontal distance and you should be able to estimate energy requirements.


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## John (Sep 11, 2007)

*Re: New controller prototype*



Harold in CR said:


> Without knowing exactly, I'm going to guess nearly a ½ mile in elevation change from the bottom, where it's pretty flat, could be more ???


Thats about 800m. If your car weight is about 1500kg (3306 lb) P.E. = 1500 x 9.81 x 800 = 11772000Ws = 3.27kWh. Energy available for regen would be this minus energy required to cover the three miles about 0.6kWh leaving 2.67kWh. If you drove the distance in 5 minuites or 1/12th of an hour (36mph) average power becomes 2.67 x 12 = 32kW. Note greater speed equals greater power. Even with 90% alternator efficiency power is still 28.8kW. With a 7kW alternator you would recover less than 1/4 of this. 800m seems quite high. It gives you a fairly steep grade over a three mile distance.


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## Mesuge (Mar 6, 2008)

Interesting train of thoughts these calculations, I think that Aptera, Tesla S, and few others are utilizing similar approach in their onboard nav software. It's is also worth including in your own simulated/datalog gps spreadsheets, +/- some additional losses and inefficiencies, it's working rather fine as guesstimative tool. Obviously this is mostly usefull for EVs/people with limited range and challenging terrain around. For those >100mi range folks this is just Wh pennies anyway..


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## engineer_Bill (Jun 24, 2008)

Not to rain on anyones parade, but using a Series motor to regen is probably not worth it. I am using a sep-ex which is much easier to add regen. I'm still not completely happy with it. and get a lot less from regen than I expected.


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## Tesseract (Sep 27, 2008)

engineer_Bill said:


> Not to rain on anyones parade, but using a Series motor to regen is probably not worth it. I am using a sep-ex which is much easier to add regen. I'm still not completely happy with it. and get a lot less from regen than I expected.


I totally agree that regen is probably not worth the extra complexity required in the controller if the only criterion considered is energy recovery, but it does have one other not-to-be-underestimated benefit: it allows the electric vehicle to behave similarly to an ICE "off throttle". We are all used to our ICE vehicles decelerating when we let off the accelerator, while DC EV's simply coast. Now I'm not arguing that getting "engine braking" back is worth the extra complexity, either, just that it is another reason to want regen.

That said, the hardware for the new controller I designed is capable of regen with a series wound motor (it can seamlessly switch from buck mode for motoring to boost mode for regenerating). Whether a feedback loop can be implemented in software to tame the infinities in the series motor's transfer function is a whole 'nother can o' worms. But we are exerting some serious effort to see if we can pull it off.


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## JRP3 (Mar 7, 2008)

Tesseract said:


> That said, the hardware for the new controller I designed is capable of regen with a series wound motor (it can seamlessly switch from buck mode for motoring to boost mode for regenerating). Whether a feedback loop can be implemented in software to tame the infinities in the series motor's transfer function is a whole 'nother can o' worms. But we are exerting some serious effort to see if we can pull it off.


Even if you get the controller side worked out doesn't brush advancement make it nearly impossible to get regen without arcing problems, since the brushes become retarded during regen?


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## Tesseract (Sep 27, 2008)

JRP3 said:


> Even if you get the controller side worked out doesn't brush advancement make it nearly impossible to get regen without arcing problems, since the brushes become retarded during regen?


Yep - regen (or reverse, which is the same thing from the motor's perspective) is murder on an advanced brush-timing motor, but not all motors use brush advancement to tolerate a higher voltage. Granted, the most popular motors (currently) do (i.e. - WarP and ImPulse motors by NetGain) but others like those from D&D and Kostov do not. 

That said, there is so little hard data out there on the behavior of series DC motors in regen that we expect to have to induce a few catastrophic commutator fireballs to find out what's really going on.


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## JRP3 (Mar 7, 2008)

Tesseract said:


> That said, there is so little hard data out there on the behavior of series DC motors in regen that we expect to have to induce a few catastrophic commutator fireballs to find out what's really going on.


I hope you'll be video taping


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