EDLC capacitors - any good as "battery supplies" in DACs etc?

[nat8808]

Just came accross these in the Maplin catalogue and then checked the web a bit..

Web says they are faster at power delivery than many batteries and hold a fair amount of energy.

Has anyone ever tried using them as battery supplies in low power gear like DACs?

[technobear]

A capacitor stores charge which is measured in Coulombs.

1 Farad = 1 Coulomb per Volt

1 Amp = 1 Coulomb per second

From these two equations we can deduce that even the capacious 360F supercapacitor only holds less than one tenth the charge of a couple of NiMH AA's.

Say we charge the 360 Farad cap to 2.4 Volts, that's 864 Coulombs.

We could extract that as 864 Amps for 1 second or 1 Amp for 864 seconds.

But 864 seconds is only 14.4 minutes or 0.24 hours so the capacity can be stated as 0.24 Amp hours.

A typical NiMH AA has a capacity of 2400mAh or 2.4 Amp hours. That's ten times as much. You would need two to get 2.4 Volts.

There is another problem though. The voltage of a capacitor is linearly proportional to the amount of charge it contains. As you start to draw current, the voltage drops linearly. When you've extracted half the charge in the above example, the capacitor voltage will be only 1 Volt, not 2.

Batteries behave somewhat differently and maintain their voltage to much lower levels of charge.

So supercapacitors are useful where you want to release large currents which a battery could not supply but for very short periods of time so as not to remove too much charge and diminish the voltage below a useable level.

As such they could be useful in a power supply capacitor bank, but not as a battery for a DAC.

[nat8808]

In a sense I guess I equate capacitors in a bank in a power supply as the same energy supply as a battery, just that one is being constantly "re-charged".

With these, they would need less charging and so could be a kind of half-way between, able to supply energy faster than a battery, like a capacitor can, whilst also being able to be isolated from the mains for longer periods. Quite how that would work, I'm not so sure - perhaps some kind of voltage controlled switch to connect them to another bank of similar caps which are part of a more conventional supply.