# Types of Capacitors

Discussion in 'General Electronics Chat' started by emoy020, Oct 26, 2012.

1. ### emoy020 Thread Starter New Member

Oct 23, 2012
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Can someone help me to classify capacitors that i can use in dc motors...
And capacitors which discharges in a long period..?

2. ### Audioguru Expert

Dec 20, 2007
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A capacitor cannot power a DC motor. A battery is used instead.

3. ### BillO Distinguished Member

Nov 24, 2008
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If you were using a very efficient motor, such as something that would draw about 10ma @ 2.5V and used a 3000F super capacitor charged up to 2.5V with a leakage current of 5ma, you could get the motor to run fairly well for about 55 hours or so in which time the voltage would drop to about 1.5V.

The motor would need to be something like this, and the capacitor would need to be like this for that performance. You can determine the performance for any motor and any capacitor over a 1 volt drop from the following:

Run Time [in seconds] = (Size of capacitor [in Farads]) / (Current draw of motor [in amps] + leakage current of capacitor [in amps])

You need to keep in mind that, unlike batteries, the voltage across a capacitor will drop linearly over time.

SPQR likes this.
4. ### Austin Clark Active Member

Dec 28, 2011
412
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I assume that you mean to have a capacitor in parallel with a battery or some other power source, right? Or do you mean to power the motor off of a charged capacitor only? Either way, the higher the Farad rating, the more the capacitor can store, so long as the voltage rating is at or higher than the maximum you intend to use. If the motor will draw a whole lot of current, you may also want to consider capacitor ESR, or equivalent series resistance, as this will limit the amount of current the capacitor can supply at any given time (caps in general have low ESR, so it likely won't matter, just be aware of it).

5. ### Audioguru Expert

Dec 20, 2007
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A capacitor is not a battery. A fairly small battery can power a motor for about 10 minutes if the motor is not working hard.
A huge capacitor can power a motor for a few seconds.

6. ### BillO Distinguished Member

Nov 24, 2008
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Hi AG, my calculations are not wrong. Just do the math.

7. ### Audioguru Expert

Dec 20, 2007
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My motors draw hundreds of mA.
I have never seen a motor that draws only 10mA at only 2.5V. What can it do?

If the supercapacitors cost \$25.00 and they are used then I wonder how much they are when new?

8. ### storken New Member

Jan 25, 2012
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A 3000F 2,5V supercap can store about 9kJ of energy. A single cell lipo 3,7V1ah battery stores 13kJ. Weight of supercap: 615g. Weight of lipo cell: 20g. Volume? Supercap: 450ml. Lipo cell: 8ml.

What would you pick?

9. ### Audioguru Expert

Dec 20, 2007
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The voltage from a discharging capacitor drops exponentially which is very steep.
But the voltage from a discharging LiPo battery is almost flat and stays up for most of the discharge.
A super capacitor is VERY expensive but a LiPo battery is not.

What would you pick?

10. ### BillO Distinguished Member

Nov 24, 2008
992
139
I am surprised at you gentlemen. I would pick whatever is best for my situation. For instance, if I needed near instantaneous re-charge rate or very high charge cycles or huge current capabilities I'd choose a capacitor.

I'd even choose lead-acid batteries in some cases and disposable alkaline in others.

I can probably even think of a situation where it might advantageous to have capacitor, Lithium and lead-acid all at one time. Perhaps an electric car would benefit from having all three.

11. ### BillO Distinguished Member

Nov 24, 2008
992
139
Exponential decay is neither steep or not steep. It depends on the parameters and application.

True, but can you charge your LiPo battery in 5 seconds, or discharge it to 0V, or re-charge it 100,000 times?

For what use? There are use cases (stated above) where using LiPo batteries will not only get very expensive, but would be dangerous too.

See my other post.

12. ### storken New Member

Jan 25, 2012
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Stop using strawman arguments.

13. ### BillO Distinguished Member

Nov 24, 2008
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What in Volta's name are you talking about?

Supercapacitors are used in product designs for all the reasons I gave. Don't let your ignorance be used to judge the arguments of others. Either refute what I said with something verifiable or keep your jug corked.

Thanks, mate.

14. ### storken New Member

Jan 25, 2012
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Using supercaps for long term energy storage are rarely used. They had their era in HEV's (some busses), but they are surpassed by modern lipo technology. Do you see any cellphones using the 'awesome' supercaps?

You didn't refute any of my arguments. I proved that lipo batteries are way more suited in the terms of volumetric and gravimetric energy content.

Lipo batteries are just as safe as supercapacitors, don't give me the old batteries are hazardous crap - everything is hazardous if you treat it wrongly.

Thread starter is obviously not familiar with batteries. Neither are you. You're the one that should hop off the supercapacitor bandwagon.

- An actual EE.

15. ### BillO Distinguished Member

Nov 24, 2008
992
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Right...

For an 'actual EE' you are pretty limited in your imagination. It's either buses or cell phones, eh? First, I did not say they were for long term storage. Not ever, and certainly not here. Just to anticipate...you are the EE, so I am not going to rhyme off all the places they are used. This is something you should be able to look up yourself. I'm not a librarian and I refuse to cite things that can easily be found using google.

I didn't try to refute any of your arguments. I simply supplied some valid cases for the use of super caps. That's it. You're the one that somehow managed to see that as a straw man argument, when it was clearly not. You might want to look up the colloquial meaning of 'straw man argument'. Perhaps you meant something different?

Either you can't read or you have trouble following simple thoughts. What I said was,

which is an accurate statement. What it means (and I'll take you to the obvious conclusion here as you seem to have trouble with this sort of thing) is that you would not use LiPo batteries for applications where it is better to use capacitors. Is that plain enough for you? I quoted 3 such cases where LiPos would not be safe, or would fail early. I re-itterate

• They cannot be discharged to 0V
• They cannot be charged in a few seconds
• They cannot be cycled 100,000 times
Capacitors can do all those,
sooooooo.... if you need to do one or more of those things....
You use a capacitor!!!!
You really are full of yourself, aren't you? I am not on any super cap band wagon. Why don't you read what i wrote again. I say it in simple words for you one more time. All I said was, there are times you should use a capacitor and there are times you should not. I further implied that the times you would use a capacitor are not the times you would use a LiPo. Okay?

Annnnd... You have absolutely no clue what the OP's application is. All you know is that he wants a capacitor suitable to run a motor for a long period. I answered his question. You tried to dissuade him without having an idea of his application. Now that sounds like a real engineer.

Good for you!

Does that mean you can make immediate assumptions about another's qualifications by reading a few lines on an Internet forum? You have no idea who I am, yet you directly accuse me of having no knowledge of batteries and for some reason you imply I am less qualified than you!?!

Well, well, there's probably no fixing that.

16. ### Austin Clark Active Member

Dec 28, 2011
412
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This thread got awfully fun awfully fast.

Supercaps have their place, as do batteries of all sorts. Lipo batteries are best in most every-day uses. Supercaps can supply enormous amounts of power in a small amount of time, whereas lipo batteries cannot, let alone safely.

Storken, I have to say, you came in a little hot for no real reason. It seems you think he was attacking you personally or something, which is not at all the case. In fact, nobody here was even disagreeing with you, just bringing up a few points to make things clear for others. If lipo batteries were better than super caps in every single way, they wouldn't exist.

17. ### Audioguru Expert

Dec 20, 2007
10,899
1,245
Canada Post refuses to carry "dangerous" LiPo batteries (but they carry products that have them inside). They don't know about how much energy is inside a charged supercap so don't tell them.

18. ### storken New Member

Jan 25, 2012
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Sorry, my rudeness was more the product of direct, cut-through-the-bullshit communications style that is natural to people who are more concerned about solving problems than making others feel warm and fuzzy.

Thank you BillO for the clear summary of how I screwed up. I'm over it, in case you wondered.