Discussion in 'General Electronics Chat' started by arthur92710, Jan 24, 2008.

1. ### arthur92710 Thread Starter Active Member

Jun 25, 2007
307
1
Is it possible to make a battery out of capacitors. But that must output a constant voltage over a long period, about 5 hours.
Can you use a lot of electrolytic capacitors?
How many are needed? 2000? 5000? (well thats a bad question)
How much capacitance is needed?

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Yes, the question is better defined by: how much voltage at what current? I have seen ultracapacitors up to 22 farads, but at 5.5 volts.

3. ### Audioguru New Member

Dec 20, 2007
9,411
896
The voltage of a capacitor begins to drop quickly the moment it has a load. The voltage of a battery stays high for a long time.
A capacitor has a very small capacity to power anything. A battery is made to power things.

Use the formula R times C to determine how long a capacitor's voltage will drop to 37% of the supply voltage. C is Farads, R is the resistance of the load and the result is in seconds.

Look at the very steep voltage discharge of a huge 643 Farads capacitor with a starting voltage of 9V, a termination voltage of 3.33V and an average load current of 50mA.
Then look at the fairly flat voltage of four small AAA lithium cells with a starting voltage of 6.1V, a termination voltage of 5.3V and an average current of 50mA.

The clock on my old 486 computer has been powered by a lithium coin cell battery for 14 years and the clock is still running.

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4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
That's a good point - capacitors have no ability to hold a voltage when they start to discharge.

5. ### arthur92710 Thread Starter Active Member

Jun 25, 2007
307
1
What if you dont use one but a few thousand. and some extra circuit to keep the output voltage constant.

In all of the examples one only one capacitor is used so I think a few thousand would work.

6. ### mik3ca Active Member

Feb 11, 2007
189
0
Why are you trying to use capacitors as batteries?

I'm sure better solutions are available.

If your circuit is in a constant location, try a DC adapter.

You can also try connecting a few solar cells in series.

The problem with using a capacitor as a voltage supply is if the two capacitor leads touch each other, the charge that the capacitor had will no longer be in it. This means, the capacitor will act as a ridiculously high-valued resistor.

If you did this to a a DC adapter or a few solar cells, the same effect wouldn't happen.

so if I were you, I would invest in some solar cells.

7. ### kubeek AAC Fanatic!

Sep 20, 2005
4,687
805
what voltage and what current and for how long do you need?

8. ### Søren Senior Member

Sep 2, 2006
472
28
Hi,

It's quite simple to calculate: C = I * T / U
Where:
C is the capacitance in Farad
I is the current drawn in Ampere
T is the time in seconds
U is the allowable voltage drop in Volts during T

Example: You need to supply 0.5A for 30 seconds. Starting voltage is 5V and the end of discharge voltage should be 4.5V

Then C = 0.5 * 30 / (5-4.5) = 15 / 0.5 = 30 [Farad]

So you'll need a 30F cap capaple of a bit more than 5V AND some mean of regulating the output (and remember, even a LDO regulator will eat around half a volt to do it's stuff).

9. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
That is exactly what Audioguru shows in his PNG picture. 643 Farads = 64300 each 10,000ufd capacitors in parallel. Cost would be about \$15,000.00 USD compared to \$12.00 USD for the four lithium atteries. Capacitor bank mass would be roughly 63.5 pounds (not counting solder or circuit board) compared to just over an ounce for the four lithium batteries. Four AAA batteries will fit in a man's fist. 64300 25mmx16mm capacitors will fit in a pair of 50 gallon drums.

But, yes, it would "work."

10. ### Audioguru New Member

Dec 20, 2007
9,411
896
I wonder how many weeks it will take to charge 643F??