Clueless take 2

Technical discussion on converting internal combustion to electric
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Post by coulomb » Thu, 10 Feb 2011, 18:33

T1 Terry wrote: How long is the run time for peak power? The controller says it can handle 2 mins @ 550amps
These all depend on cooling. You can cool the controller with a cooler plate with water running through it to a separate radiator; Jack Rickard has done this (see http://www.evtv.me.

[ Edit: forgot to add: You can keep the motor cooler with forced cooling of the fins using a thermatic fan, or even better with liquid cooling. ]
Keep in mind I'm trying to slow 10 tonne down hill.

Ah. I'd say cooling is quite important for you.

Usually, regen is a smallish fraction of traction power, so it doesn't heat up the motor or controller all that significantly. However, you will possibly be pushing things (downhill :-) a bit harder than most. It could well be that even the 650 A output model is too small for this.
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Post by EClubman » Thu, 10 Feb 2011, 19:30

How long?

About the same as a piece of string.

It all depends.

What is the ambient temp and humidity?
What sort of air flow into/around the motor?
What is it attached to that could act as a heat sink?
How overspecced are the ratings for continuous duty?
Do you care that running the motor at X amps for Y seconds will reduce the continuous life of the motor by Z hours?

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Post by T1 Terry » Thu, 10 Feb 2011, 19:38

Can the regen be limited to the max capacity of the motor or controller? Is this regen operator variable between zero and full to the max limit set via a hand control? I'm thinking the limit would really be the max acceptance level of the battery pack, the regen current has to go some where, as to whether a single large capacity cell pack or a number of parallel smaller capacity cell battery packs is the better option I'm still trying to decide. Leaning toward the multiple small cell capacity batteries due to single cell high voltage causing a shut down so not all cells are off line and better ulitisation of space.
AC or DC?

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Post by Richo » Thu, 10 Feb 2011, 20:52

The max regen will need to be limited to the charging current of the battery.
Otherwise either an overvoltage problem will occur or the batteries will cook.

For exmaple a 10kWh pack with 1C charge will accpet 10kW of regen.
But a 10kWh pack with 0.1C charge will only accept 1kW of regen.
A 10kWh pack with 5C (headway or sim) will be 50kW.
So a 90HP motor would be too much regen for all of them.

The controller will need to have some control over regen to limit max current and max voltage.
So the short answer is NO but the long answer is YES.
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Post by Tritium_James » Thu, 10 Feb 2011, 22:44

Those power limits for regen are only when the motor is at max speed.

So if you're only at (say) 20% of full speed, and your pack can accept a 100A charge rate, then your motor and controller could be doing up to 100A/0.2 = 500A regen current without stressing the batteries.

As you go faster, the battery current will rise to match the motor current, and at 100% (full speed) they will be the same.

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Post by coulomb » Thu, 10 Feb 2011, 22:57

T1 Terry wrote: Can the regen be limited to the max capacity of the motor or controller?
Yes, the controller will do that. For a controller like the Tritium Wavesculptor, you just ask it for negative torque and give it a current limit (all in one CAN packet).

For something like a Curtis 1238, you can have a regen pot as well as an accelerator pot, so you can hook up the regen pot to the brake pedal, or to some sort of dashboard or on-floor control. You can also limit the amount of regen that "full on" on the pot requests.
Is this regen operator variable between zero and full to the max limit set via a hand control?
Yes, hand control if you want, or pedal, or anything.
I'm thinking the limit would really be the max acceptance level of the battery pack
Yes, that's the most important thing. There is also passenger comfort; some passengers don't appreciate nose-against-the-glass regen   Image
Whether a single large capacity cell pack or a number of parallel smaller capacity cell battery packs is the better option I'm still trying to decide.
Yes, but that's independent of regen considerations. If you have say 3 60 Ah cells in parallel or one 180 Ah cell, they'll both act much the same; if some [ in the parallel set of 3] want more charge than others, they'll shunt current between themselves, but their voltage will be the same (assuming sensible wiring). So from the outside, the 3x60 Ah and the 1x180 Ah look the same. As you say, smaller cells are easier to pack, but there are more connections to make, and the extra plastic and battery cage (of the 3x60) makes the pack slightly heavier.
AC or DC?
Religious!   Image

Actually, it sounds like you really want regen for your application; if that's the case, most would agree that AC is the easiest way to go. You can get regen with DC, usually with sepex motors, but that limits your choice of motor and controller. (Well, at the moment, the choices of AC controller are quite limited too.) What you probably can't do is take your choice of the many series DC motors, and expect to painlessly get regen from them. See for example DC Regen from practical and personal experience.

Edit: make the 3x60 vs 1x180 Ah comparison clearer.
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Post by Richo » Fri, 11 Feb 2011, 01:04

Tritium_James wrote: Those power limits for regen are only when the motor is at max speed.

True.
At some point an eV will be at max speed and go into regen.
And you will have 90HP available to dump into the batteries.
Assuming the wheels don't lock up Image
So the short answer is NO but the long answer is YES.
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Post by T1 Terry » Fri, 11 Feb 2011, 01:24

I've had a deeper look at the choice of batteries and it seems it will need to be Thundersky over CALB, higher charge voltage and higher charging amps to maintain a reasonable cycle life. 2 x 100ah or 1 X 200ah have the same charge acceptance, still unsure if 2 battery banks of 36 x 100ah cells or one bank 72 cells connected series/parallel. Either way I will have a battery bank that can accept 78kw or regen with a small margin of safety so that exceeds the capability nof both the AC50 and Curtis controller I think.
The thread about practical DC regen was what opened my mind to the possibilty of DC regen and a Kelly controller but I remember talk previously regarding DC motor suitable for regen having weak low down torque. Then there was talk of regen with series wound DC motors and my eyes rolled back in my head so I asked the question hoping for more clarification.
With the Kelly controller it appeared a progressive regen could be had with the operator in control of when and how much regen is applied, I'm guessing the Cutis controller has the same function for an AC application. The series wound DC has me thrown though, is it also controllable by the operator as to the amount of regen at any given moment or is it on or off with no grey areas in between? I would imagine that a sudden application of 50kw of regen would develop a level of passenger discomfort even with 10 tonnes pushing it along and an auto transmission to smooth it out a bit. I would imagine the fruit bowl sliding off the table and sailing through the air into the back of the head being a tad disconcerting too.

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Edited for poor key strokes
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Post by coulomb » Fri, 11 Feb 2011, 04:19

T1 Terry wrote: Then there was talk of regen with series wound DC motors and my eyes rolled back in my head ...
The series wound DC has me thrown though, is it also controllable by the operator as to the amount of regen at any given moment or is it on or off with no grey areas in between?

The system that Alex_Brooy proposed uses a small controller for the extra field excitation, so it's somewhat variable. There is an actual pot with a knob in his prototype.

However, he prototyped it on a 72 V system and small motor. When he tried it on a larger Warp 11" motor, he found that he didn't get enough regen to make it at all worthwhile. See Series DC + Regen.... at DIYelectriccar.com. So there is no successful series DC regen system that I'm aware of, for other than quite small EVs. (That is, assuming I read that post correctly.)
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Post by Richo » Fri, 11 Feb 2011, 20:46

72 x 3.2V x 100Ah = 23kWh
So 78kW/23kwh = 3.3C
This exceeds the "claimed" max charge of 3C on TS cells.
Considering the nominal is only 0.5C (11.5kW)
So the short answer is NO but the long answer is YES.
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Post by T1 Terry » Fri, 11 Feb 2011, 22:42

Richo wrote: 72 x 3.2V x 100Ah = 23kWh
So 78kW/23kwh = 3.3C
This exceeds the "claimed" max charge of 3C on TS cells.
Considering the nominal is only 0.5C (11.5kW)

But isn't the max charge voltage 3.65v? That would make it 72 x 3.65 x 100 = 26.28kwh. 78kw/26.28 = 2.96C, just inside the 3C limit. They actually claim a peak charge voltage of 4v = 28.8KWh = 2.7C
I have no idea about the maths on this, I just copied yours Image

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What was the maths to get the 78kw figure?
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Post by woody » Sun, 13 Feb 2011, 00:26

Just to make it more complex:
The 3C limit is for battery current, so 300Amps
Internal resistance of TS/CALB means about 0.2V/C drop from nominal - I.E. 3.2-0.6=2.6V @ 3C
So 2.6V x 72 cells x 300 amps = 780W x 72 cells = about 56kW electrical
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Post by T1 Terry » Sun, 13 Feb 2011, 04:27

Buggered if I could follow your maths path Woody but somehow we came to the same answer assuming the 2.6v bit is correct but I'm not sure how this figure was calculated either. 2 batteries of 36 300ah cells @ 2.6v = 56.16kw. Now I'm assuming this it the max amount of regen current the batteies will accept before the voltage starts to run away, have I got that part right?
From this chart for the AC 50 motor is there figures here that I can calculate the Max amps and at what revs the max. regen would be?
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Post by coulomb » Sun, 13 Feb 2011, 19:43

T1 Terry wrote: Now I'm assuming this it the max amount of regen current the batteries will accept before the voltage starts to run away, have I got that part right?
Not really. That's the maximum *discharge* rate. You'd think it would be the same as the maximum charge rate, but for whatever reason it is not.

[ Edit: My bad. Charge and discharge rates are in fact the same at 3C, as pointed out below, though with discharge you can briefly go higher. So it's basically a question of whether you have taken out enough charge so that 3C doesn't overvoltage the cell. ]

Prismatic LiFe cells are usually specified (for general charging) at about 0.3C; obviously you want more than that for regen. Basically, you can put in as much as you have available (assuming the driver has commanded a lot of braking), *as long as* the voltage on *each individual cell* is within spec. For TS/Winston, that's about 4.0 VPC. For TS/CALB, that's 3.6 VPC.

Unfortunately, that doesn't tell you how much regen you are likely to be able to accept. My guess is about 1C, perhaps 1.5 C.

[ Edit: It's actually 3C, as the datasheet says, with the proviso that no cell goes over-voltage. The BMS, in concert with the controller, should be enforcing that. ]
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Post by antiscab » Sun, 13 Feb 2011, 21:22

I have done 2C regen for a second or two on my Vectrix (120A into 60Ah cells) and voltage only gets too 3.4vpc average.

120A is the most a Vectrix can regen (and even then thats only when travelling at 110kmh).

thats unless I have taken 5Ah or less from a full charge, in which case ability to accept regen is reduced.

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Post by T1 Terry » Sun, 13 Feb 2011, 22:36

Not really. That's the maximum *discharge* rate
I've taken the voltage droop into account by adding 2 extra batteries to each 48v pack, 18 instead of 16 cells, as a bonus I don't need to recharge to a high cut off voltage to get the capacity but regen is an issue I still need to iron out.
Both the CALB and Thundersky battery specs give a 3C max charging rate although the normal rate is 0.3C and I guess this is the rate the cycle life is calculated at not 3C.
Do I need to look at Ultracaps to soften the regen load or am I better spending that sort of money on more cell capacity?
Regen braking is going to be a serious safety issue with my bus conversion as the 351 Cleveland won't have a lot of engine braking capacity and exhaust brakes don't work on a petrol engine due to lower compression and carbon problems stuffing up the butterfly valve. Without some form of electromagnetic braking service brake overheating and brake fade could/would become a major issue on long down hill runs.
I could blow the excess regen amps away in a resistor pack but I'd much prefer to reuse that energy rather than waste it.

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Post by coulomb » Sun, 13 Feb 2011, 23:17

T1 Terry wrote: Both the CALB and Thundersky battery specs give a 3C max charging rate although the normal rate is 0.3C and I guess this is the rate the cycle life is calculated at not 3C.
Yes, you're quite right, my bad. I've edited mu post as best as I can.
Do I need to look at Ultracaps to soften the regen load or am I better spending that sort of money on more cell capacity?
The consensus seems to be no caps, add extra cells instead.
Regen braking is going to be a serious safety issue with my bus conversion ... Without some form of electromagnetic braking service brake overheating and brake fade could/would become a major issue on long down hill runs.
As long as there isn't a long downhill run near the start of a journey, you should be fine. And if there is and you need to do this regularly, just charge to a lower voltage, or a lower SOC using coulomb counting, so the cells have more capacity to absorb energy.
I could blow the excess regen amps away in a resistor pack but I'd much prefer to reuse that energy rather than waste it.

Absolutely. You already have energy dissipating devices; they're called brakes. It's a pity that they aren't rated for continuous worst-case use.
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Post by coulomb » Sun, 13 Feb 2011, 23:25

T1 Terry wrote:I've taken the voltage droop into account by adding 2 extra batteries to each 48v pack, 18 instead of 16 cells, ...

Presumably you have two such nominal 48 V packs (really 58 V nominal with 18 cells) to drive the Curtis controller.

Just be aware that 130 VDC is the absolute limit for the Curtis controller; it will start to throw error codes and the like at 130 VDC. With 36 cells (and people do run 36 LiFe cells), the absolute maximum average VPC is 130 / 36 = 3.61 V, so you will have to honour the 3.6 V limit pretty closely. With regen, any resistance in the cables and links translates to the controller seeing a higher voltage than the sum of what the cells see, so you may be limiting to a bit less than 3.60 VPC. But that's OK; the Curtis should be able to limit regen to protect itself. I have not heard of people having problems with over-voltage due to excessive regen.
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Post by T1 Terry » Mon, 14 Feb 2011, 02:13

coulomb wrote:the absolute maximum average VPC is 130 / 36 = 3.61 V, so you will have to honour the 3.6 V limit pretty closely.
Good point, hadn't thought of that one, at least I won't have an issue with over charging the batteries.
If the AC50 motor draws a max of 500amps @ 96v what would the max regen voltage and amps be?
I would imagine the motor wouldn't be as efficient as a generator, there would be heat losses at the motor and controller... just trying to get an idea of how many amps max the batteries would see. I'm assuming the over all voltage would be governed by the battery current acceptance rate.
Worst case scenario even at a peak regen output of 500amps, for 10 mins @ 130v into a 108v battery pack is 100ah, half the batteries total capacity. Hmmm.... if everything can handle the heat generated it's a doable with enough prior planning, that's if I've got the basic understanding correct.

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Post by coulomb » Mon, 14 Feb 2011, 02:47

T1 Terry wrote: If the AC50 motor draws a max of 500amps @ 96v what would the max regen voltage and amps be?

I would say 499 A @ 130 VDC. The losses in a controller are mostly voltage losses; it would not take more than an ampere to run the whole controller and main contactor (assuming it is fed from the controller). If the IGBTs can take current that is equivalent to 499 A from the battery, then they can handle current equivalent to 499 A going into the battery.

My understanding is that the motor losses and controller voltage losses, and the difference in voltage from discharged pack to charged pack are all supplied by the vehicle, i.e. losses appear as *useful* regenerative force on the vehicle. (For once, losses are useful in a sense, if I have got this right).

I assume you realise that the 1238-7501 is rated at 550 A RMS *output per phase* (peak, for about 1 minute from memory), which translates to √3/√2 x 550 ~= 675 ADC from the pack, if the output is at maximum voltage.
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Post by T1 Terry » Mon, 14 Feb 2011, 03:27

The controller specs say it's rated for 2 mins @ 550amp but I'm assuming that's a heating issue so if I water cool it I should be able to extend that to the 10 min max in either drive or regen I hope. I have no idea regarding the motor and heat generation at peak drive amps or regen amps, a thermo fan and maybe some sort of copper tubing around the outside to give some water cooling. All that heat generation must eat amps surely?
The 675amp from the pack @ 550 motor amps has thrown me a bit, I thought I had a safety margin with a 3C max discharge of 600amps but now I need more... bugga Maybe the 550amps RMS can be dialed back at the controller, another 36 x 100ah Thunderskys would solve all the problems but my Lotto numbers haven't come up for a while and selling my first born is no longer an option. I'd certainly have a lot of house power storage though Image I'd get my monies worth on a powered site at a caravan park recharging them.

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