charging capacitor

Discussion in 'General Electronics Chat' started by ecjohnny, Jul 6, 2012.

1. ecjohnny Thread Starter Senior Member

Jul 16, 2005
142
0
Hey.
I recently found charging capacitor and using it as battery interesting.
I tried using a electrolyte capacitor (forgotten the value) and discharge it and lit up an led for a moment(resistor). This is just a simple experiment, nothing much.

However i am curious if it works with those tiny smd/smt capacitors(ceramic?). Is it still possible to store charge and discharge it as "very-temporary-short-life" battery?

2. wmodavis Distinguished Member

Oct 23, 2010
739
151
All capacitors have the same characteristic as you observed - they will store electrical charge.

The amount they can store depends on the capacitance value and the applied voltage. Many will have too low of capacitance to store enough charge to show up with an LED as in the experiment you did but that is what they do.

3. MrChips Moderator

Oct 2, 2009
18,205
5,720
That's the catch. The value of the capacitor is essential, along with the current draw.

The charge Q stored on the capacitor is Q = C x V
where C is the capacitance and V is the voltage on the capacitor.

Since you are not going to discharge the capacitor completely, suppose you took 1/10 of the charge, an approximation of the time taken is

Q/10 = I x t

where I is the current and t is the time.

Super caps are now readily available with values of 1F and greater.

Here is an example.

Suppose C = 1F and we charge it to 5V.
Q = 1 x 5 = 5 coulombs

If we took 1mA from the cap

t = Q/(10I) = 5/(10 x 10^-3) = 5000/10 = 500 secs

The voltage would have fallen from 5V to 4.5V

Last edited: Jul 6, 2012
4. steveb Senior Member

Jul 3, 2008
2,432
469
In principle yes, but you would need to have very low current draw, as MrChips points out.

You will have more luck with supercapacitors, such as those made by Maxwell. You can get thousands of Farads at a couple of volts.

http://www.tecategroup.com/ultracapacitors-supercapacitors/maxwell.php

One issue is that the voltage will drop faster than a battery (as shown with MrChips formulas), which is usually not desirable for a voltage source. So, you probably want to have a switching regulator that converts the varying input voltage to a constant output voltage. Realistically, you might only discharge to half the full rated voltage, which translates to 75 % energy utilization.

I was once told that someone makes a military/police flashlight using supercapacitors. The advantage is that they can be very quickly charged (a couple of minutes, rather than hours) since capacitors can be charged at much higher currents than batteries.

5. WBahn Moderator

Mar 31, 2012
23,868
7,384
One place that caps are frequently used as short-term batteries is in battery-powered circuits that have volitile memory that can't be stored to non-volitile memory before changing batteries. Many old programmable calculators were this way. So there was a capacitor that could feed the memory for a couple of minutes while you put new batteries in.