## Practical Application of Capacitors

I was thinking that a car battery is a 12v source that delivers between 300 and 900 cold cranking camps. That'd put the cranking wattage at between 3600 watts and 10,800, right?

Here's a car battery 'helper': http://goo.gl/Roo2l

This is hypothetical at this point but what I was wondering was since a car only pulls the 3600 to 10,800 watts for about 2 or 3 seconds, would it be possible to safely use a capacitor based system to deliver this power instead of the chemical battery?

I know, I know, be careful.
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 Recognitions: Gold Member Science Advisor yes possibly, but what are you going to use to charge the capacitor ? the car battery ? the booster thing you linked to which appears to probably have its own battery inside it ? so you are going to complicate things by having some battery charge a capacitor for engine starting, instead of just starting the engine direct from the battery ? Dave

 Quote by davenn yes possibly, but what are you going to use to charge the capacitor ? the car battery ? the booster thing you linked to which appears to probably have its own battery inside it ? so you are going to complicate things by having some battery charge a capacitor for engine starting, instead of just starting the engine direct from the battery ? Dave
At this point it's a theoretical exercise. Yes I agree it would be weird to have a way to charge the capacitors but not start the engine itself. You'd need a secondary generator somewhere. Which brings me to another question, how long do capacitors typically hold their charge?

## Practical Application of Capacitors

Sort of related.

When a car battery dies is it the voltage that goes or the amperage or both?

Since we are taught to hook the jumper cables up in parallel I assume it's the amperage that is lost when the battery is drained. Can you guys chime in on this?

Recognitions:
Gold Member
 Quote by mearvk ....... Which brings me to another question, how long do capacitors typically hold their charge?
That would depend on the capacitor type and manufacturing. It ( a large value one) would still hold a charge after some days or so probably. It would slowly leak across the dielectric.

Dave

Recognitions:
Gold Member
 Quote by mearvk Sort of related. When a car battery dies is it the voltage that goes or the amperage or both? Since we are taught to hook the jumper cables up in parallel I assume it's the amperage that is lost when the battery is drained. Can you guys chime in on this?
well you could say both. but principally its the voltage that goes. measure a good battery and the same type of flat battery what will you see ? .... the flat one has very little voltage

Thats a simplified way of looking at it without going into how the battery ( whatever type) produces a voltage for a start .... google will give lots of answers there :)

Just remember a battery doesnt store a voltage. It generates a voltage by a chemical reaction
So when a battery is "flat" it means that the chemical reaction is no longer occurring

cheers
Dave