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Posted: Tue, 26 Oct 2010, 19:25
Has anyone had any experience with regenerative brakes?! I drive downhill 50% of the time and keep thinking I should somehow use that energy. Not sure about the costs and if there is any off the shelf options?
Posted: Wed, 27 Oct 2010, 00:53
EVnovice wrote: Hi
Welcome to the forums!
Has anyone had any experience with regenerative brakes?!
Well, anyone who has an AC drivetrain has regen. On his SwiftE
, TomOfReno reports a 10.4% range extension. There are other benefits too, such as less wear and tear on the mechanical brakes. There have been reports of Prius reaching 350,000 miles (in the US) and never having brake pads replaced (I hope I'm not misremembering/misquoting there). The pack sees lower overall depth of discharge, so it lasts longer.
Many conversions with separately excited DC motors (so-called sepex motors) also have regen.
... if there is any off the shelf options?
Alas, the majority of conversions are still using good old series wound DC motors. There are various schemes to add regeneration to such conversions, but I would not recommend any of them.
It's almost a Frequently Asked Question as to whether it is worth adding an alternator to capture regenerative energy, e.g. Generator to direct-power EV for distance
. (Happens to be another DIY electriccar link). It's tempting, because usually an alternator would come with the donor vehicle. Generally, I'd say no; alternators are inefficient and only generate a small amount of power (about 1 kW; you'd want 10 or even 30 kW for good regen). Various other schemes to reverse the series DC motor's field and boost the back emf all require a high current reversing contactor, so it won't be smooth, and the potential for things going wrong are too great (imagine getting full reverse power by mistake; it's not the same thing as power goes from the pack to the motor).
So time and time again the answer seems to be: regen is not worth attempting on conversions using a series DC motor. As a result, there isn't anything available off the shelf to "add regenerative braking" to such vehicles.
Posted: Wed, 27 Oct 2010, 03:42
EVnovice wrote:I drive downhill 50% of the time and keep thinking I should somehow use that energy.
Thats alotta hill!
do you use the brakes presently to hold speed?
or down shift?
trying to get an idea of recoverable energy on your route.
I presume were talking about a car here?
if motorbike, almost all have regen easily (as almost all are at least BLDC or full AC as with Vectrix).
the drivetrain www.bev.com.au
uses has good regen.
Posted: Wed, 27 Oct 2010, 05:24
Regenerative braking relies for best operation on a number of factors....
1. Low rolling resistance
EV must not just consume its kinetic energy by gearbox, brushes dragging on armature (DC) or dragging brakes etc. Try an ICE in neutral for comparison, though still some gearbox drag.
AC induction motors have the lowest free turning resistance.
2. Low aerodynamic drag
Fourwheel drives and vans (not !)
Clearly a slippery body will not waste kinetic energy.
3. Efficient generating ability of traction motor and controller.
Regen efficiency should be as good as drive from traction motor.
AC induction and BLDC are good at that. In AC induction, motor efficiency typically equals generator efficiency.
4. No added complexity, cost or weight to motor or control system.
Again, AC and BLDC rule here.
5. Ability to regen efficiently at all motor RPM
Induction motors offer regen to the lowest RPM. Approx 1% of rated RPM i.e. EV to nearly full stop still returns power to battery pack.
Shunt DC regen falls down badly here as it requires reasonable RPM to generate sufficient voltage to feed the battery pack. Some DC regen systems actually drive motor to halt at lower RPM (plug braking, as used on forklifts and electric drills) This is not regen.
6. Battery system ability to accept regen energy in short bursts.
Regen can be for minutes on a long hill but typically seconds in city driving. Currents can be up to equivalent of full traction current but typically limited to max recharge rate for battery type/capacity.
Lead acid is the worst here, with lithium LiFePO4 a lot better and ultra caps by far the best.
7. Practical, convenient and smooth driver operation of the regen.
Regen (in part) can be akin to ICE engine braking on throttle back off and then additional regen applied by start of brake pedal. Or some other method. Driver may be oblivious to what is happening in the background, though smooth steady regen is more likely to recharge to battery.
The interesting thing with reasonable regen in an EV is it makes little difference how hard you drive the vehicle (in terms of acceleration and deceleration) as the power/regen sorts out the energy. Even the humble prius demonstrates this with its good stop/start city economy.
Coming up to a red light with regen enabled EV is actually quite satisfying ! Not to mention a downhill run when the battery pack is nearly empty.
Posted: Wed, 27 Oct 2010, 06:30
What about if your trip down hill is carrying a boot full of the groceries or other load. So going on the down hill trip may get extra energy than the trip back up the hill.
Yeah its sounds a bit silly. But just trying to think about the problems faced by heavier cars.
When you see all those trady's going to work with their UTEs, VANs or Wagons full of gear.
If your fuel bill is big because the load is heavy. Then an EV Conversion with regen will have a bigger impact than when you have a single person vehicle.
Energy = 1/2 Mass X V^2 so tripple the Load then its tripples the acceleration energy needed.
Also Hill energy due to Mass x Gravity x Height so same as for change in speed.
Of course the Drag Loss Energy CdA x V^2 has nothing to do with a heavy load.
Except the rolling resistance and if the load changes the Air resistance because it change the shape of the Car.
Of course use the smallest sleekest car you feel safe in, but if you need a bigger car to carry 5 kids and luggage don't eliminate the EV conversion option if your trips are 30 - 50Kms, that's not too big a battery.
Posted: Wed, 27 Oct 2010, 14:23
I appreciate your comments. and yes Matt, I live halfway up a mountain and driving back I have to use my brakes to stay within the 50 km/h speed limit. That's about 5-7 km for every trip. I have a converted Dihatsu shrade with a DC motor. It is probably over a tonne or so (with its 14 lead acid batteries), but I hardly ever load it more that what is necessary i.e. myself (albeit unnecesarily heavy at 96kg!!).
I will try to work through some of the equations you have posted. But thanks again'
Posted: Wed, 27 Oct 2010, 16:18
Few assumptions (albeit reasonable ones) if you drive 50% downhill I think it is fair to assume the other 50% is up hill (unless you have found a way to make a journey greater than a 100% of the distance)
You mentioned that the return journey is downhill so that makes it fairly safe to assume that the “to” journey is uphill.
Since the energy is being spent in the “to” journey that leaves plenty of capacity for regen energy to be captured in the “from” journey (downhill).
As Matt and others have said AC is very efficient in its regen capability. I can’t see why if you used batteries capable of capturing a high rate of regen energy that you couldn’t almost travel for free.
I don’t mean literally for free, by that I mean since you would be getting such a high regen percentage return to the batteries, it should require nowhere near the charging time it would if the same distance was flat.
Posted: Wed, 27 Oct 2010, 16:27
Your assumptions are correct except that currently I don't have regen capability (or AC motor) and I was wondering how much it would cost to retrofit those.
Posted: Wed, 27 Oct 2010, 16:33
EV2Go, I think you have the energy use thing backwards. If the vehicle had regen, travelling 50% uphill then 50% downhill would be the SAME power as if the car drove that distance on the flat, ie the amount consumed by aero/rolling/efficiency losses.
If you don't have regen, the up/down scenario would be much MORE power than the flat situation, because you use the same aero/rolling/efficincy power, PLUS the power to climb the hill, which you then didn't manage to get back on the downhill, because of no regen.
Posted: Wed, 27 Oct 2010, 16:36
unfortunately I don't have the regen capability at the moment.
Posted: Wed, 27 Oct 2010, 16:38
Perhaps you could provide more details on your set up. What motor and controller are you using? Did you do the conversion yourself?
What other stuff does the motor power (aircon, steering pump)?
Given your hilly situation there may (MAY) be some experimental stuff the forum could work through.
Just a point though. Even with 100% regeneration efficiency (not likely) your range will not be any greater than it would be on flat and level, constant speed driving. The rolling and wind resistance has to be overcome with power. Good regen braking, at best, can smooth a start/stop or hilly drive to more resemble the range of flat and level driving.
Posted: Wed, 27 Oct 2010, 16:42
BTW there is quite a large discussion on regenerative braking here
Posted: Wed, 27 Oct 2010, 16:49
I’m the first to admit I don’t understand these matters as well as you guys but here’s my thinking.
Distance covered 14 klms (7 up, 7 down) first 7 klms requiring power from the motor (understandable more than if it was a flat road) second 7 klms require zero power from motor but instead return maximum possible back to the batteries.
Effectively it is a heavy 7klm journey with 7 klms of regen. Even if it used double the power it would on the flat to go up the hill, that’s only the same as a flat 14klm journey with the added benefit of 7 klms of regen.
Posted: Wed, 27 Oct 2010, 16:59
EV2Go wrote:....., that’s only the same as a flat 14klm journey with the added benefit of 7 klms of regen.
Right except for the last bit. It's the same as a 14km journey. The regen only put back what the extra energy involved in going up the hills took out.
Once more thing. With lead acid regen will have to held in check - you can't put all available power back into the pack - the batteries won't handle it.
Finally. This is all going to have very finite (realistically quite low since it will be a "tack on") efficiencies.
Posted: Wed, 27 Oct 2010, 17:10
Agree definately wouldn't work with current batteries and motor, would need high efficency AC and good lithium or LiPo.
I couldn't see a tack on regen being worth the effort, unless it was for help with the braking.
Edit: If we are only talking a genuine no throttle return trip I still figure it's better than a 14klm flat trip with no regen.
Posted: Wed, 27 Oct 2010, 18:11
EV2Go wrote:If we are only talking a genuine no throttle return trip I still figure it's better than a 14klm flat trip with no regen.
My apologies - you are right. I didn't realise that the return trip was what you were talking about. I was considering the whole journey.
Posted: Wed, 27 Oct 2010, 19:03
if were not talking about braking to a stop, but rather braking to maintain speed, regen on DC is *much* more doable.
a few more questions, has the gearbox been retained in the conversion?
what controller are you using?
the motor field could be rewound for lower current higher voltage and taken out of the high current circuit.
basically you end up with a sepex motor.
you could either run it with your present controller, and add a much lower power controller to run the field windings (and a contactor for bypass of your present controller during regen).
or you could buy a purpose built sepex controller.
Posted: Wed, 27 Oct 2010, 20:45
I suggest you read my post on a Zapi controller I'm using. It has regen as a built-in feature to be used with series motors.
Posted: Wed, 27 Oct 2010, 21:02
Corrected link here.
Posted: Thu, 28 Oct 2010, 06:23
So assuming 144V system and 100Ah LA batteries, recharge at 20A (2.9kW)(C/5, any higher rate would be wasted in LA ?) for 7km at say 50kmph (8 minutes) gives 0.39kWh.
If the EV were giving 2km range for this 0.39kWh in normal driving then that would be a return of 2/7 = 28%, ignoring the recharge efficiency.
Posted: Thu, 28 Oct 2010, 14:55
Just in case someone is not subscribed, PeterS just added a round up comment to his Zapi controller with regen.
Posted: Thu, 28 Oct 2010, 17:26
Not meaning to steal your thread here EV Novice but I have a question for the forum. Is it possible to build a DC generator that would also act as a power assist at nom 48vdc so regen would be up around 60vdc?
Posted: Sat, 02 Feb 2013, 05:50
hydraulic regen is twice as efficient, 80% vs 40% and very simple, motor and pump is one unit, pumps against an accumulator, drives from it back to the reserve tank. I got a suitable gear pump for 80 bucks on ebay. Hoses and fittings cost a lot but a friend has the gear to make them (works on heavy machinery) I wish I had got the cheapo aircraft accumulator when I had found it but I guess some springs and a ram would work too.