# Calculating "AH" of a super capacitor?

Discussion in 'General Electronics Chat' started by spinnaker, Dec 20, 2010.

1. ### spinnaker Thread Starter AAC Fanatic!

Oct 29, 2009
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I am looking at building a propeller clock. Most plans I see call for a .47f super cap. These are a bit more expensive than the .33f versions.

How do I calculate how long my PIC will sleep under the power of the super cap on a .33f cap? Is this rated in ampere hours or maybe micro amp hours?

How is it calculated?

2. ### #12 Expert

Nov 30, 2010
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Same way you calculate any capacitor. One farad delivering one amp for one minute will lose one volt. See how many volts you can lose before the brain goes stupid, how much current it draws, and what voltage the capacitor will be at when you start emptying it.

3. ### spinnaker Thread Starter AAC Fanatic!

Oct 29, 2009
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Thanks #12! That is what I needed!

So if my PIC can operate at 3.0V - 5.0V, is it better my vdd is 5.0V? That way the cap is charged to 5.0 V and I make better use of the available cap.

4. ### #12 Expert

Nov 30, 2010
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How much voltage could you afford to let the capacitor lose if the capacitor started out at 3 volts?

5. ### spinnaker Thread Starter AAC Fanatic!

Oct 29, 2009
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I need to check out the datasheet to be sure but I would think the most I can lose before the pic stops running is about .5 volt lose. So 2.5 minimum.

6. ### #12 Expert

Nov 30, 2010
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Funny..I thought you just said the brain operated from 5 volts to 3 volts...didn't you?

7. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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I think you meant second instead of minutes.

1 Farad = 1 Coulomb per Volt
1 Coulomb = 1 Amp-Second
1 Farad = 1 Amp-Second per Volt

Assuming no losses, and 100% use of stored energy, A 5V charge on 1F is a capacity of 5 Amp Seconds, or ~1.39mAH

Low clock speed and use of the nanopower sleep modes is suggested. What is the project?

--ETA: Saw post above about 0.47F or 0.33F. Then roughly 900uAH for .47F@5V, or ~465uAH for .33F@5V, again, no losses, discharging cap to zero. You would only calculate a 2V useful charge on the cap, resulting in 261uAH for 0.47F. 183uAH for a 0.33F cap charged to 5V, discharged to 3, discarding that half volt is a SWAG for losses.

Last edited: Dec 20, 2010
8. ### spinnaker Thread Starter AAC Fanatic!

Oct 29, 2009
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Like I said I am going off of memory. I need to check the datasheet. Plus my final choice of pics will have a lot to do with it.

9. ### #12 Expert

Nov 30, 2010
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Arrrgh! You caught me in an act of stupid!

I hate it when my brain slips a digit.