Ultracap's in eV's

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Richo
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Ultracap's in eV's

Post by Richo » Sat, 07 Mar 2009, 06:14

I imagined a BMW E30 with Ultracaps used for inital acceleration upto 100kph (10secs).
If I used 3 strings of 300 x BCAP0650 (650Farad caps) [6.5F total] it would work.
But at USD$68 a pop that is USD$61,200 worth of capacitance.
For 10seconds of acceleration.
But you could do that 500,000 to 1,000,000 times Image
Assuming you kept charging them back up again.
Which is possible with an AC system using regen.
So that's using them for traction purposes.

But they could also be used to replace the 12V battery.
As there is no starter motor to have a high drain they may work quite well.
However the lights etc must still work for a while as per the NCOP14.
So the caps would have to be sized accordingly.
The advantage is that peak demands can be met.
With the low cyclic use is likely going to last longer than you.
This would be about USD$400. (depending on size)
If the caps did run flat while out (left light on etc) you could use a "BOOST" button in the engine bay the start the DC-DC converter to recharge the caps from the traction battery.

Any other ideas on using Ultracaps on your eV?
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Post by Taffy » Sat, 07 Mar 2009, 06:34

Mount then on the side of the car but with the wrong polarity. When someone looks like there doing to nudge you or wash ur windows when you dont want them too. Press a switch and bang!
... umm ok maybe not the most practical use.

How about have say 24V worth that is attached by a couple of contactors to the circuit. The charge via the dc-dc and then when you need an extra 24V press a button and there you are. Pack goes from 120v to 144V for a few sec.
I dont see that being very useful either, why not by 24V extra of batteries to start with?

I really like a bank in series with the batteries allowing you to slow release from batteries but able to stress the caps when you need to quick draw. Space and $$ issues there.

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Post by Richo » Sat, 07 Mar 2009, 07:00

At 1mR impedance it is doable.
Yeah batteries are still smaller and cheaper.

I was wondering of the benefits of using them to stiffen cheap batteries.
So you could buy say 10C batteries or 1C batteries + caps.
I think the price would still have to drop quite a bit before that would happen.
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Post by acmotor » Sat, 07 Mar 2009, 07:26

The problem is getting hold of enough ultracaps to actually make a difference for a reasonable price.

Having said that..

I feel it is a poor use of caps to simply place them in parallel with batteries as the caps need to be cycled from full volatge to zero and all that energy extracted from the cap for it to be worthwhile. (unlike batteries where you would prefer not to change their voltage).

Since the energy stored (joules) is 1/2 CV^2 , then if there is little voltage change as when caps are in parallel with batteries, then there is a lot less useable energy going in and out of the cap compared to cycling the cap over its full working voltage range. i.e. a lot more useable energy for your dollar.

Taffy, the 24V module in series with battery pack is an idea I have toyed with.
In my ACIM case, I would diode isolate the battery pack from the DC bus (K to bus +, i.e. DC to controller) and back feed from the bus to a cap bank sitting above the batteries so regen power can go into the caps (in series with the battery bank so both get charge but less regen stress on batteries).
This way the 600VDC pack + say 250V of caps can be used. caps would go through 100% cycle and for same pack current the kW would be between Amps x 850V down to Amps x 600V as the caps discharged. once back to 600V (minus diode V) the controller would run off battery pack alone and caps would sit there for next regen event.
Caps could also be charged by an inverter off battery pack while waiting on the grid for takeoff.

This way the caps could be fully utilised and would provide up to 40% boost and provide good regen functionality.

Cap size only needs to be 40% of the energy required to accelerate for 10 seconds or so.

Another way would be to have 2 controllers in a DC system feeding the one motor so the caps could be on one controller, charged up ready to dump to motor and the other controller could take over when the caps voltage has dropped to the voltage the battery pack controller was to supply.
This would really only work at low motor volts, high current time but that is what is needed anyway.
Once again, the caps voltage cycle range would be more than if simply placed across the battery pack.
The PWM buck function of the DC controller doing the work as the caps discharged.

Does this make sense ?    
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Post by acmotor » Sat, 07 Mar 2009, 07:46

This is what I mean...

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Voltages are examples only for my system.
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Post by Richo » Sat, 07 Mar 2009, 08:06

The diode would only have to be rated to 250V?
In which case a mosfet maybe used (depending on availability) instead with polarity monitor.
So when diode is conducting mosfet turns on.
Similar to SR in DC-DC converters.

Would the current limit during regen be the batteries?
ie 40Ah 3C -> 120A even though caps can handle hundereds
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Post by Richo » Sat, 07 Mar 2009, 17:18

Sorry, forgot that the batteries are split.
So Diode will have to take full rail Voltage at some point.
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EDIT:Image upload. Image
Last edited by Richo on Sat, 07 Mar 2009, 06:36, edited 1 time in total.
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Post by Richo » Sat, 07 Mar 2009, 18:08

If using 650F caps then there would be 92 total.
So 46 on each +/- voltage rail.
So to last 10secs the average current would be 167A.
Cost USD$6256.

If using 310F caps then there would be 100 total.
So 50 on each +/- voltage rail.
So to last 10secs the average current would be 72.5A.
Cost USD$2000-2200.
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Post by acmotor » Sat, 07 Mar 2009, 21:03

Mosfet would not be an option as most have reverse diodes across their channel. Unless you use their internal diode and drive the gate to control regen current to the battery pack.

So this is doable ? and would get the full 'capacity' of the caps up and working ? $2,000 not bad for up to 40% increase in kW.

Regen current limit of 120A is a serious amount !

Regen efficiency would be better than with just battery pack.
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Post by coulomb » Sat, 07 Mar 2009, 22:23

Richo wrote: Sorry, forgot that the batteries are split.
Well, that makes for nice symmetry and all, but really except for safety while driving, I see no need to split the capacitor bank and use a second diode. Just put it on the positive or negative sides, but not both.
So Diode will have to take full rail Voltage at some point.
Huh? The diode is across the capacitor. If the capacitor is limited to 250v, then the diode will only ever see 250v across it (when reverse biased, obviously). Am I missing something here?

I actually like this idea. What are the weight of ultra caps compared to LiFePO4 batteries? With the MX-5, we'd prefer to get away with fewer cells and hence pack weight, but we'd want to be able to get to reasonable speeds for overtaking on the highway. If we can't afford the (weight of) the full pack, this might be a way to do it, albeit only for small periods of time. Though if the DC-DC that charges the ultracaps from the main pack was rather large...

I suspect that the ultra caps are almost as expensive as small lithium cells, though.

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Post by acmotor » Sun, 08 Mar 2009, 00:55

The ultra caps will be there long after the car has rusted away and definitey long after the battery pack needs replacement although this will not be as soon as normal as the caps will have taken some stress off the batts. The lithium salesmen will understand the value for money long term.

Coulomb,
The battery pack split is a higher voltage AC thing. Breaking the pack up serves several purposes as discussed elsewhere in the forum.
A 300V AC system (asure etc) probably only needs the single sided arrangement.

A DC motor system can use the same concept although a large part of the advantage is lost as there will be no regen. (please nobody open that topic again).
This does require the DC controller to be reasonably higher voltage than the battery pack. e.g. 200V controller on 100V pack.

The DC-DC (if used) charging the caps only needs to charge at a reasonably low rate. e.g. to charge the caps in say 1 minute depending on your expectations.
If say the caps provided 500kW seconds of energy, they could be charged over 60 seconds at 8kW rate. Still a reasonable DC-DC. Maybe come back to 2kW rate and reduce how often you go 0-100 in 5 seconds.
Mind you, this is still really ideal regen territory.

Richo, put some more numbers to this lot for us !

Keep in mind that EV batteries may all end up ultra caps and no chemical batteries anyway ! Just not yet. Image
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Post by antiscab » Sun, 08 Mar 2009, 00:56

i see another problem,
the diode would have to see *full* pack current *all* the time.
were talking a whole DC controller power stage worth of FETs here.
where is the heat going to go?
are the extra losses worth it (we are still talking Kj right?)

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Post by acmotor » Sun, 08 Mar 2009, 01:04

Matt,

I only need a 60A diode ! The advantages of higher voltage system.

True, a 600A diode costs 10 times as much and has 10 times the power loss. Perhaps just use a contactor. Or bypass diode with a contactor. Or rip it all out and go AC.
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Post by Johny » Sun, 08 Mar 2009, 02:15

You are actually going to need more control over when the ultra-caps discharge. Otherwise they will discharge while running normally before you even want to use their extra "surge".
I would assume that once charged, you would want to keep them handy rather than use them up on the first take-off.

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Post by acmotor » Sun, 08 Mar 2009, 02:23

You are right, we are just musing the concept. It is similar to class H in audio amps. Some management of the caps may be desireable, although if just used for regen then no further managment would be required ?
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Post by coulomb » Sun, 08 Mar 2009, 07:17

acmotor wrote:Coulomb,
The battery pack split is a higher voltage AC thing. Breaking the pack up serves several purposes as discussed elsewhere in the forum.
A 300V AC system (asure etc) probably only needs the single sided arrangement.
Sure, I agree on all that. But why does it have to be split exactly symmetrically, if (hopefully) the extra voltage from the caps is gone any time the bonnet is open? You can still split the battery and earth the centre point, just only put the cap on one side (positive or negative, it doesn't matter). Sure, you'll have 550v on one side and 300v on the other, instead of a neat 425 each side. Does that really matter, as long as when the motor is off, there is no more than 300v with respect to the chassis?

That saves the cost and losses of a 60A diode. (Well, 150A for more performance).

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Post by acmotor » Sun, 08 Mar 2009, 07:49

No argument there.

Let me see, I'll think of something....... Ok what about this excuse :-

Detecting earth leakage faults by earthing the centre of the battery pack (and measuring current) relies on symetry of the voltages around the earth point otherwise the fluctuations of the cap voltage (if one sided only) will make it difficult to detect 20mA or less leakages. This is due to the capacitive coupling from the traction system components to chassis.
(BTW, I am still waiting to see a viable leakage detector on a DC series motor system in operation)

How was that ? Image
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Post by coulomb » Sun, 08 Mar 2009, 17:36

acmotor wrote:How was that ? Image

Pretty good Image,    if the capacitive coupling thing is real Image .    I agree that a good leakage detection system is very important on a high voltage system.

The other thing is that it might take a while for 6 farads of capacitance to bleed away its voltage. If so you'd want the balance for safety while working on the vehicle.

Edit: capacitance -> capacitive coupling
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Post by acmotor » Sun, 08 Mar 2009, 19:20

Ummm, yes.
With ESRs in the micro ohms you would want to be adopting some good electrical safety practice ! Breakup of battery pack and caps to less than 72V when not operating the vehicle would be a good idea.

You would not wish to throw away the energy in the caps every time you turned the vehicle off. It can stay there, just like a battery.

Note. As I have said many times, the earthing of the centre of the pack can be via a resistor. 5k seems a good number. Check the discusion elsewhere on the forum on this.

Anyway, even a small qty of ultracaps can be incorporated in an EV with this 'stacking' concept. Image
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Post by Richo » Sun, 08 Mar 2009, 21:30

coulomb wrote:But why does it have to be split exactly symmetrically, if (hopefully) the extra voltage from the caps is gone any time the bonnet is open? Sure, you'll have 550v on one side and 300v on the other, instead of a neat 425 each side. Does that really matter?

That saves the cost and losses of a 60A diode. (Well, 150A for more performance).


It matters from controller design point of view.
There are still DC bus caps in the controller which are quite expensive.
So having a lop-sided DC filter caps is far from ideal.

Also as I point out a mosfet replaces the diode and uses synchronous rectification (SR). So the diode loss doesn't really come into it.
And you are correct it does only see the 125V in the balanced system and 250V in the unbalanced system.

The discharge on the caps are ~1mA.
So the charge will be there for some time.

However using the SR approach the caps can be discharged on damand.
ie switch on bonnet detect bonnet opened so caps discharge for safety.
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Post by Richo » Sun, 08 Mar 2009, 21:38

antiscab wrote: i see another problem,
the diode would have to see *full* pack current *all* the time.
were talking a whole DC controller power stage worth of FETs here.
where is the heat going to go?
are the extra losses worth it (we are still talking Kj right?)

Matt


Yeah sorry Matt this arrangement is more suited to AC which regens the caps during braking.
So not much use on DC high current systems.

Otherwise I would suggest a management system that isolates the caps and recharges them during low current times and engages the caps during high current times (until depleted).
Last edited by Richo on Sun, 08 Mar 2009, 10:40, edited 1 time in total.
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Post by acmotor » Sun, 08 Mar 2009, 21:54

Yes, sorry Matt. Are you still playing with DC motors ? Image
So many limitations ! Image
BTW, did you check out Mal's vid.
mal's vid

edit: no charge for the extra spaces in the URL !
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Post by Richo » Sun, 08 Mar 2009, 23:38

That's a great vid - he does look happy Image
Mal's Vid Sorry Acmotor you have a couple of spaces in the URL.

Would have like a longer one of Mal showing around the car pointing out the various bits...
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Post by antiscab » Sun, 08 Mar 2009, 23:46

acmotor wrote: Yes, sorry Matt. Are you still playing with DC motors ? Image
So many limitations ! Image
BTW, did you check out Mal's vid.
mal's vid


haha ooer what hit me Image
yes if you only have to worry about 60A, losses arent much of a problem.

i was actually considering for a 288v 3-ph AC system with a 415v vfd to allow for headroom (i just need to find a nice long thin induction motor rated for big power *grins*)
i agree, caps are completely useless without regen.

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Post by acmotor » Mon, 09 Mar 2009, 00:19

Hey Richo, did you like Mal's torqometer ??
Neat, I want what one in the Rodeo !

Matt, maybe metric mind ?
Hey are you going to join AEVA on facebook? Image
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