Soapbox on regenerative braking

Technical discussion on converting internal combustion to electric
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vince
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Post by vince »

It may have been noted somewhere but I for one would like to know just how much regen is possible.I know 100% is what we all ideally wish for.What percent is presently known to be possible?I seem to be picking up an echo!
Last edited by vince on Tue, 21 Jul 2009, 06:43, edited 1 time in total.

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Post by Tritium_James »

Goombi wrote: Tritium_James,
One has to ask a question what is causing all the heat that could melt the wiring??   ...chopped...
Sorry you've lost me completely here. I wasn't talking about water cooling for the motor/wiring. I was talking about a further point from what Zeva mentioned, about having a water cooled load dump resistor to adsorb excess regen energy when the pack is full.
Goombi wrote:Wherer did you get these figures? 70%?
From our own vehicles & controllers & simulations. Also http://www.teslamotors.com/blog4/?p=58 quote 64%, but that includes aero losses at higher speeds. Around town it is better.
Goombi wrote:The only way you could possibly ( but not likely) overcharge your batteries if you were to live on Mt Fuji and work in the gully below is this the case?
No. The simple case is accelerate gradually over say 10 seconds, then regen quickly (say 2 seconds). Yes, you have put less energy back into the batteries than you took out, that is correct. But the rapid recharge will cause the battery voltage to go above maximum limits, unless you have somewhere else (a load resistor) to dump the power.

Here's my drive to work this morning. 7 minutes (the time scale is in samples, at 2Hz) a few hills, several sets of lights, moderate traffic. Whenever the voltage goes above 350-360V, I'm regen braking. You can see the motor controller cuts back the regen to keep the pack voltage from exceeding ~380V. Whenever this is happening, then I'm getting less regen than I'd expect. This is disconcerting unless you're used to it, and not good for a vehicle to be driven by the general public, and why a load dump resistor would be a good idea. Image


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You've torn it now - Goombi hates spreadsheets...

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Post by Tritium_James »

vince wrote: It may have been noted somewhere but I for one would like to know just how much regen is possible.I know 100% is what we all ideally wish for.What percent is presently known to be possible?I seem to be picking up an echo!
Vince, with an AC system 100% regen is possible, if you have somewhere to put the energy. Most battery packs can supply more power than they can adsorb, and can't usually accept 100% regen power. But if you have a load dump (resistor) setup, then the motor and controller are perfectly happy to do 100% regen. Note that this may certainly cause issues with brake biasing and wheel lockup.

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Post by Tritium_James »

Johny wrote: You've torn it now - Goombi hates spreadsheets...
Heh. Well it's up-to-date real-world data, only an hour old! Image

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I appreciate it. I like data. Image

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Post by Electrocycle »

I think people are mixing up regen efficiency with regen range extension.

Even if you have 100% efficient regen (perfect motor, controller, and batteries) you'll only regain the energy used to accelerate the car - not energy used to keep it moving.

So, with a 100% efficient regen system you might extend your range by 20-25% depending on the overall efficiency of the car.
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Post by Johny »

Tritium_James wrote:Whenever this is happening, then I'm getting less regen than I'd expect. This is disconcerting unless you're used to it,...
Yes I can see in the first 4/7 ths of the trip the voltage flat tops a fair bit then toward the end of the trip it just makes it to the maximum. Driving "feel' must change a lot over that reasonable small period of time.

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Post by Tritium_James »

Electrocycle: Yes, perfectly correct.

Johny: Yes. In this car the amount of regen is controlled by the lift-off on the accelerator pedal, fully off = max regen. But then I can also set what max regen is with a slider control elsewhere. For this trip it was only set about halfway, because if it's turned all the way up the shift in regen as the pack discharges is even worse and it's not that fun to drive.
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Post by weber »

Electrocycle wrote: I think people are mixing up regen efficiency with regen range extension.

Possibly. But when Tritium James wrote: "with an AC system 100% regen is possible", he could not have meant either of those things, but must have been talking about regen power and/or torque as a percentage of driving power or torque.
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Post by vince »

There are some diehard evers that try for regen with a dc motor by hooking up an alternator or bldc motor to the end shaft.What say you/yous to this setup on an ac motor end shaft if one exists?While also utilizing the regen from this ac motor.
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Post by weber »

vince wrote: There are some diehard evers that try for regen with a dc motor by hooking up an alternator or bldc motor to the end shaft.What say you/yous to this setup on an ac motor end shaft if one exists?While also utilizing the regen from this ac motor.

Sounds completely pointless to me. You get regen for free with AC drive systems. As Johny said (in effect) way back in this thread: With AC, regen just happens.
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Post by vince »

I guess there is no point getting greedy- or-am i just in a sci-fi mood where/when evs are/become self sustaining!?!Maybe we could ask assimov-is he still alive?
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Post by coulomb »

TJ: sounds like your ride to work is pretty fun; there are some steep angles in that graph!

I assume that this is the 911 with an aging lead acid pack? It would be great to compare with a similar vehicle with lithium. I assume that lithium will take the regen better, although if we are stuck at a 3.60 V limit, I wonder how it will go. Maybe in half a year or so the lead acid pack will die, the pack will get replaced with lithium, and we can get a very fair comparison.

I hadn't considered before that batteries do seem to want to take charge slower than they can dish it out, hence the need for the controller to cut back regen in the first ~ 4 minutes in this case. Thanks for that!
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Post by woody »

weber wrote:
Electrocycle wrote: I think people are mixing up regen efficiency with regen range extension.

Possibly. But when Tritium James wrote: "with an AC system 100% regen is possible", he could not have meant either of those things, but must have been talking about regen power and/or torque as a percentage of driving power or torque.
This is a good thing to think about.

I'd say that "Perfect" regen would give you the same range in stop/start traffic up & down hills as you'd get on the flat, constant speed. Lets call that 100% regen efficiency.

In the real world, you lose some power through your motor, (10%), controller (5-10%), wiring, and your batteries don't charge efficiently at a high rate, so you're down to 64% as Tritium_James said for Tesla, less for lead-acid batteries as the BMS has to burn the extra power.

So if most of your driving is on the flat, constant speed, or your driving style can only be described as "hypermiling" then regen won't help your range much at all.

If you're living in a hilly area and/or have lots of stop-start traffic then you'd normally be wasting energy with braking, regen could in theory extend your range by maybe 20-30%, but your regen efficiency cuts that down to 10-15%.

It also saves your brake wear.

I had an interesting chat to a mechanic friend who had an old customer who was hypermiling his kingswood before it was cool. One year he had to fail him for rego because it failed the stop test:
a) the brakes had seized as they'd hardly been used
b) the tyres had no grip as they'd gone hard from being at 60+PSI for a long time
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Post by woody »

vince wrote: I guess there is no point getting greedy- or-am i just in a sci-fi mood where/when evs are/become self sustaining!?!Maybe we could ask assimov-is he still alive?


Ask Newton, he's dead. You can't get something for nothing.

I guess you're asking what % range extension you'd get with regen, it is highly dependent on terrain, traffic, driving style. Next to nothing if you're hypermiling or on the flat with no stop/start. Possibly 10-15% if your in a hilly area with lots of stop/start.

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Post by coulomb »

vince wrote: What say you/yous to this setup on an ac motor end shaft if one exists? While also utilizing the regen from this ac motor.

No need for that. The AC motor will convert some 90% of the mechanical power supplied to it into electrical power. The rest of the mechanical power is converted to heat in the motor, so there is no power "left over" for the second AC motor on the shaft to turn into electricity.

The other losses that bring the "round trip" efficiency to about 70% are in the controller and battery. There are also losses in the drive train, and of course you never recover the energy needed to push the air out of the way at speed. But in city traffic, the air displacement power is low to quite moderate. It starts getting significant at 80 km/h, and is dominant at 100 km/h and above.
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Post by vince »

Ask Newton, he's dead. You can't get something for nothing.

I'm still laughing!

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Post by Tritium_James »

coulomb wrote: TJ: sounds like your ride to work is pretty fun; there are some steep angles in that graph!
Yes. I note around T=480 there is a 0 - 72.3km/h acceleration in 4.5s. Image
coulomb wrote:I assume that this is the 911 with an aging lead acid pack? It would be great to compare with a similar vehicle with lithium. I assume that lithium will take the regen better, although if we are stuck at a 3.60 V limit, I wonder how it will go. Maybe in half a year or so the lead acid pack will die, the pack will get replaced with lithium, and we can get a very fair comparison.
Yes, again. And the plan is a 100 cell 100Ah high-rate Kokam pack in the next 12-18 months. Half the weight, 3x the real-world capacity, and higher power too. They're capable of 7C for quite some time, and have a 0.5mR internal resistance, measured at 500A and room temp.
coulomb wrote:I hadn't considered before that batteries do seem to want to take charge slower than they can dish it out, hence the need for the controller to cut back regen in the first ~ 4 minutes in this case. Thanks for that!
It is very noticeable when driving, and not really in a good way. What makes it worse is that, when voltage limiting, the regen gets progressively stronger as the car slows down. This is because the pack voltage limit is effectively a power limit, and what you are trying to control with the pedal is torque. So for constant power, torque goes up as speed goes down, and you get this effect where the regen isn't strong enough at 60km/h, but then is too strong at 10km/h. This goes away as the pack gets discharged a bit, then it's more even over the speed range. A load dump would solve this issue.

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Post by acmotor »

If I can try to clarify the % regen number ...

There are two numbers being thrown around.
% regen and % regen efficiency.

On an AC system (mine at least), the regen is set in torque as a % of the nominal motor torque. i.e. on my 11kW 4 pole in red suzi 100% regen = 72Nm of torque. I can set from 0 to ~ 400% regen torque.

This is not on/off, bang/bang regen, rather it is just a max regen torque the drive will go to as per the % setting. So depending on the accelerator setting, there is simply a smooth slope from max accelerator forward to max regen retard of vehicle motion. If you are a smooth driver, you may only see a few % regen even though the max has been set high.
TJs example of accelerate gently for 10 seconds then take your foot off and have the regen pull you up in say 2 seconds can equally be remove foot slowly over 10 seconds and little regen will occur since vehicle motion losses will use up most of the kinetic energy so not much regen.

In fact the petrol heads will enjoy the fact that best regen comes when an EV is driven hard and fast. Image
A point to remember that when the Tesla motor got hot in JC's test, it was heated from regen work as well as acceleration.

Now 40 to 60% regen (i.e. some 30 to 45Nm of torque on red suzi) is quite a driveable number.
400% locks the back wheels and in fact anything over 60% tends to exceed battery reccomended recharge current and at higher SOC, take pack voltage too high.

Now the regen efficiency is a different number.
This is a measure of the efficiency of the system to recover the kinetic energy (motion energy) of the EV.

There are several parts to what is 'regen efficiency'
There is the total energy efficiency of the EV. i.e. for acceleration.. power point to,charger,battery,controller,motor,gearbox(yuk),differential,tyres(RR),wind drag etc i.e.road.. then back via tyres,diff,gearbox,motor,controller,battery when in regen.

But perhaps the point of interest with regen is the centre part.. battery to road to battery.

I'll try some numbers... (don't nit pick !)
Acceleration...
battery 95+%
controller 97%
motor 90%
diff,bearings,tyres 90%

So battery to road ~ 75%

Now going back the other way from vehicle Kinetic energy..
diff,bearings,tyres 90%
motor 90%
controller 97%
battery, Now here's a difference, the higher the reacharge rate the greater the loss but say 95+% for 1C regen. This is also battery chemistry dependent.

So, similar 75% on return path. This is what TJ was suggesting Image

Now 75% of 75% is 56%. This would seem a (rough) theoretical limit to battery to battery regen efficiency. But also the nominal efficiency and I consider that I get something around that number in red suzi.
The let down in its case is the recharge efficiency of lead acid batteries. The Rodeo will be better with the Lithium.

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Post by weber »

Regarding braking resistors: I'm not so sure a 10:1 duty cycle will save us. You Perth folks don't have much in the way of mountain ranges within EV range do you? South East Queensland motorists will probably be familiar with coming down off the range at Toowoomba or Cunninghams Gap, Mt Tambourine, Mt Nebo or Maleny). These take many minutes.

In an industrial situation they are expecting to deal with significant kinetic energy, but they probably aren't expecting hundreds of metres worth of gravitational potential energy. And if your braking resistor gets too hot halfway down the hill and your drive has to stop providing regen braking, that's not much fun.

Water cooling is clearly a good idea. But another way is to turn the energy into electromagnetic radiation and beam it away. i.e. incandescent lamps as I mentioned earlier.

40 or more 240 Vac 500 W floodlamp tubes in an appropriate series/parallel arrangment may not be so crazy after all. We could maybe arrange them under the floor of the cabin in such a way that they don't come any lower than the chassis rails. We would first glue a thin layer of reflective aluminium foil under the floor, but over the wiring for the lamps. Then mount the insulated and heat resistant lamp holders. Then have the sheet of 1 mm polycarbonate (impact-resistant) underbody streamlining we were planning on mounting to the under-floor chassis rails anyway (going all the way from front to back of the whole vehicle).

It would look pretty cool at night. A huge glow coming from underneath the car. UFO reports? Image
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Post by acmotor »

Must be something in the water. Image Oh you meant cooling water.

You could use the kW to drive a massive sound system with a recording of a jake brake. Image
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Post by Tritium_James »

Weber, with that setup most of the load would be dissipated as heat anyway, halogen floodlamp tubes are only something like 6 or 8% efficient at electricity -> light, the rest is heat. So you might as well just save the expense and delicate glass tubes and use a resistor element instead. Coming down the range at night you'll still get some glow happening anyway (think hotplate element)!

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Post by Johny »

weber wrote:South East Queensland motorists will probably be familiar with coming down off the range at Toowoomba or Cunninghams Gap, Mt Tambourine, Mt Nebo or Maleny). These take many minutes.
I was going to post some comment about air-cooling the brake resistors with a fancy opening grill like early Mac trucks when I realised that the best way to hold speed on an incline is a lousy drag profile.

For the MX5 you have to reinforce the soft-top to be able to hold the car at 60k/h without ripping off.Image

Maybe there is some serious way to use air drag???

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Post by coulomb »

Parachutes are well tested. Unfortunately, somewhat one-off, and annoy / surprise the heck out of the driver behind   Image

TJ, ordinary resistors won't be all that radiant. What we need is a few oven elements (especially ones in a long U-shape) in series. They'd only glow if they're needed for a long time. But I think even polycarbonate would get baked...
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