Capacitor storage question...

[windstuffnow]

   I've been playing with some 1F capacitors and trying to calculate the amount of energy that could be stored.  I found a formula  E = .5xFxV^2 ... Is this total storage for 1 second?

   If the above is correct then it would deliver...

.5 x 1f x 5.5^2 = 15 watts/s or .25 watts/min. right? and a 1 hour rating would be 15/3600?

   Just trying to clarify in my mind how these things work and if they could actually be used in a very small storage system.

   I found some 2700 farad caps that look quite interesting but it seems like they really don't hold that much energy.   Their only rated at 2.5 volts but will surge 625 amps.   Total capacity at around 8400 w/s.   (if thats how their calculated ).  

   Is the above correct?   If not please explain what I might be doing wrong...

Thanks again in advance...

.

[willib]

Ed 1/2CV^2 is the total energy stored in a cap

where C is the capacitance  in Farads .

is the cap you are using a 12V audio cap?

[dinges]

The formula for storage (E=.5CV^2) is for total energy storage in the capacitor. Time does not enter into it. The unit is J, or, if you prefer, Ws (Watt x second; NOT watt per second!)

So if your cap holds 15Ws, it could deliver 15W during 1s, or 1W during 15s, or .1W during 150s, or... All this is assuming that there are no losses etc. Plus, as the capacitor discharges, voltage drops. It's got to do with the time constant of the system and the time:

Vt = Vo x (e^(-t/(R x C)))

Where V0=initial voltage

t=time (seconds)

R=resistance of the load

C=capacity of the capacitor (F)

Vt=voltage after t seconds.

(time constant, tau = R x C)

Yesterday I saw similar capacitors to yours: 2600F, 2.7V; apparently they're used as a starter aid for starting the motor of trucks. I was amazed when I heard this, and saw these capacitors. If I hadn't seen them myself a few days ago, I would have thought you had made a typo. 2600F... And I was impressed by those 'tiny' 1F/5.5V supercaps. Wish I had a few of those 2600F ones too.

BTW, as far as energy storage goes, nothing so far beats good old accus for powerdensity. But those capacitors sure can be good fun. BTW, don't charge them by correcting straight to a voltage source; they will be like a complete short, till they get charged. Add a resistor or lightbulb in series to limit current.

I still remember when my high-school physics teacher told our class that 1F capacitors don't exist nor ever would. And now (15 years later) we have 2600F ones. I feel old.

Peter.

[willib]

http://www.maxwell.com/pdf/uc/app_notes/BOOSTCAPs_Start_ICE-1007240.pdf

maxwell's site is a favorite of mine , but i allways have to clean the drool off the keboard , very messy

[windstuffnow]

  I was impressed myself when I ran into them.  I had read an article about someone converting a GM EV1 into a 1/4 mile racer and using the PC2700's made by Maxwell ( I believe in Austrailia ).   Pretty impressive and they even set some records.   I purchased these 1 F super caps some time ago and have used them for running LED's and playing around.   I'd love to have a few hundred of the large ones laying around.  

  I heard of another company that is working on a system that could/would replace the battery altogether for hybrids... don't remember the name.  

  Anyway, I became curious and dug out the 1 F to try to learn more about them.   I soon found out when you run them in series for a higher voltage it doesn't work out very well but when their in paralell at the lower voltage they will hold a charge for a very long time.   I ran an UB LED from 18 of them in parallel for 22 minutes.   Start voltage was 5.4 volts and the end voltage was 2.5 when the LED quit.  

  Lots of fun learning and tinkering... Thanks for the explantion and formula...

.

[dinges]

I misunderstood, thought you had some of the 2600F ones...

As far as putting capacitors in series; it's not as simple as that. You should put some resistors parallel to them, to make sure they see equal voltage.

I know that for a HV (1000V) power supply I once made, with caps of, say, 100u/400V, I used 'equalizing' resistors of 100k. I don't know what would be a good value for those low-voltage/high capacity capacitors of yours. Maybe someone with more knowledge than I can help you.

(my favourite trick: charging such a cap and letting a white led light on it for 30+ minutes)  

No, wait a minute; those 2600F supercaps I saw a few days ago (for that truck-starter/battery replacer) had the caps simply in series, NO equalizing resistors... Strange.

Utterly puzzled.

[dinges]

excellent link, Willib! Will forward that link to a mate, he'll be interested too.

Yeah, I'm wiping drool off the chin too. But no worries, in 5-7 years time they'll find their way to the dumpster (everything does, in the end) and guess who will be there to fish them back out again ?

Peter.

[willib]

you really have to be careful when using supercaps , mine has an ESR (equivalent series resistance) or  internal resistance of only 0.003 ohms at 110 Farads & 2.5V , so when fully charged it can discharge quite a few amps , initially

[Ungrounded Lightning Rod]

Your physics teacher was out of date when he said that.

I remember that was the word in the '50s.  And some time I think in the '60s seeing an add in Scientific American showing a hand holding two 1/2F, 1.5V caps, each the size of an oversize soup can, crowing about how they said it couldn't be done and now here's a farad at a useful voltage that you could hold in your hand.

I think these new supercaps actually use protons pushed in and out of a membrane by an electric field's force, which makes them ALMOST a battery with a linear charge/voltage curve rather than a cap, instead of using electrons as the charge carriers.  But I'm not sure about that...

[dinges]

"Your physics teacher was out of date when he said that."

Hmmm... Think I'll ask my money back then.

[terry5732]

It's very tough to store much in caps at low voltage. Your one Farad cap fully charged is something 1/10,000th of a AA cells' charge. But when you get into the high voltage boys you can really store some potential - enough to kill.

[richhagen]

Yeah, I look at it with longing for some of there capacitors too.  Until I see the prices at suppliers.  I would love to get my hands on some to play with, but they are still a bit rich for my blood.  Rich

[richhagen]

That shouldn't be a problem for most of the 'button' cell 5.5 volt 1 Farad capacitors like the ones I think he has, they were designed for memory backup, and have quite high internal resistance, directly shorting them won't hurt them, or generate significant heat.  Rich