Turning supercapacitors into batteries

[seza]

Hi All,

I have been pondering this for a while ...
If once can make or buy a very beefy supercap say few 100farads,
1. can one treat a supercap with that much storage cap as a battery?
2. if so what circuit can be used to allow the SC to be used as a battery for prolonged discharge

thanks for your help

 

[dorke]

1.yes.
2.generally,you need to charge it and let it supply it's charge to the load

What do you want to use them for?
What load?
What voltage?

 

[seza]

Thanks - Dorke for your prompt reply,

to be honest - I am thinking aloud at the mo, I have been reading about making SC using activated charcoal in an aqueous solution,
apparently these can be pretty powerful ... me thinking why can't these be turned into batteries,
then I read about the fast discharge properties of caps in general ...
anyway - I though may be someone here can give me some ideas

 

[Colin Mitchell]

Do you realise how many farads a simple 3.7v 18650 cell is?

 

[GPG]

http://batteryuniversity.com/learn/article/whats_the_role_of_the_supercapacitor

 

[BobK]

The other thing that distinguishes a capacitor from a battery is the discharge curve. A capacitor will lose voltage linearly as discharged. A battery holds its voltage, or declines at a slow rate up to a point then falls off rapidly. This makes the battery more suitable to power a device requiring constant voltage. But you can get around this with capacitors by using a boost converter, which allows you to maintain constant voltage while extracting a good percentage of the energy from the capacitor.

That said, they are still not practical as batteries is most circumstances since the energy density is far lower. A capacitor that had as much energy as the 18650 cell Colin refers to would be bigger than a breadbox,

Bob

 

[seza]

Thank you all guys - very constructive input,
I will have to read bit more - I guess.

 

[AnalogKid]

Do you realise how many farads a simple 3.7v 18650 cell is?

No. How many?

ak

 

[AnalogKid]

Years ago, Coleman (of all people) introduced a cordless screwdriver with supercaps in the handle instead of nicads. It didn't have as long a runtime as the standard Skil model, but it went from dead to full charge in 90 seconds.

ak

 

[BobK]

No. How many?

ak

Depends strongly on the max voltage of the supercap.

Assuming a 5V one, which might be useful for this purpose, and are commonly available:

For the battery, a 18650 cell with 4000 mAH has

4 * 3.7 * 60 * 60 = 53280 Joules of energy

A supercap will have

1/2 C V^2

For 5V we have:

1/2 C * 25 = 53280
C = 53280 * 2 / 25 = 4262 Farads,

And that is if you can extract all the energy from the supercap, which you cannot.

Bob