capacitors as batteries?

[donkey]

ok I will make the first question very simple but there are heaps more. if i was to use a cap as a battery do I need a special circuit?
second is how the discharge is calculated. I know in a battery you use the Mah rating but how do you get that from a cap?
3rd if I use a 1.5volt power source to charge a 16volt cap would the cap output 16volt or 1.5volt?
4th if I leave the cap charged how long til it self discharges... if it does?

thats the start hope I can get some answers

 

[Harald Kapp]

if i was to use a cap as a battery do I need a special circuit?

There is one noteable diffference between capacitors and batteries:
A capacitor's voltage will fall continuously when current is drained from the capacitor.
A battery's voltage will stay at a (more or less) fixed level for some time until the capacity is drained. Then the voltage will fall rapidly. So yes, you may need a special circuit (Google SEPIC converter).

second is how the discharge is calculated.

The capacitor equation: C=Q/V
Therefore Q=V*C which will give you a value usually in fractions of AmpereSeconds (As).
Since 1h=3600s, 3600As=1Ah Or calculate backwards to get the required C for a given As and Volts rating.

3rd if I use a 1.5volt power source to charge a 16volt cap would the cap output 16volt or 1.5volt?

Honestly: as stated, that's a silly question. Consider any electronic circuit that has almost certainly at least one capacitor across the supply voltage rails. If that circuit is operated by a battery, would the voltage rise above the battery's voltage? Why should it?
You can charge a capacitor to more than Vbat using a step-up regulator or charge pump, but that is way more than just connecting a battery to a capacitor.

4th if I leave the cap charged how long til it self discharges... if it does?

It does. How fast depends on the type and quality of the capacitor. An electrolytic capacitor will discharge faster than a foil capacitor.

 

[donkey]

ok cool all except one point Q=V*C
could you be so kind as to define eachletter for me. c is amp seconds i guess v=volts I guess and q is how long it will take for the item to discharge?
the question I have is how to calculate farad to ah... unless q is farad...

 

[Laplace]

Q - coulombs (1 coulomb is the electrical charge of approx 6 quintillion electrons)
V - volts
C - capacitance in farads

I - current in amperes (one ampere is the flow of one coulomb per second)

So the more useful equation is I*T=V*C or

I*(delta T)=C*(delta V)

The rate of capacitor charge or discharge (volts/sec) = I/C and is applicable over any interval where the current is relatively constant or can be averaged to a constant.