# Capacitors/batteries
**URL:** https://boards.straightdope.com/t/capacitors-batteries/4443
**Category:** Factual Questions
**Created:** [November 3, 1999, 2:18pm UTC](https://boards.straightdope.com/t/capacitors-batteries/4443 "1999-11-03T14:18:00Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@Earl\_Snake-Hips\_Tucker](https://boards.straightdope.com/u/Earl_Snake-Hips_Tucker)
#### Post date: [November 3, 1999, 2:18pm UTC](https://boards.straightdope.com/t/capacitors-batteries/4443/1 "1999-11-03T14:18:00Z")
What I (think I) know is this–both of them are doohickeys that charged so that they can generate an electric current–but that’s all.
The questions:
What’s the difference?
What makes one more practical for an application than the other?
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### Author:  [@AWB](https://boards.straightdope.com/u/AWB)
#### Post date: [November 3, 1999, 2:26pm UTC](https://boards.straightdope.com/t/capacitors-batteries/4443/2 "1999-11-03T14:26:00Z")
A battery generates its charge at a slower rate, and usually by some chemical chemical reaction.
A capacitor collects a charge applied to it, but will release it suddenly either when its contacts are shorted out, or the maximum charge is achieved. If set up correctly, a capacitor will charge and discharge at a constant rate, producing an oscillating current. This is used in electronic alarms, helping make the trilling alarm sounds.
P.S. - A capacitor can also lose its charge if the current is disconnected. It will slowly leak charge until it is neutral.
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### Author:  [@putrid](https://boards.straightdope.com/u/putrid)
#### Post date: [November 3, 1999, 4:34pm UTC](https://boards.straightdope.com/t/capacitors-batteries/4443/3 "1999-11-03T16:34:00Z")
Yeah. To put it another way, the main purpose of a battery is to store energy while a circuit is off. The main purpose of a capacitor is to _store_ energy at a specific voltage while a circuit is on (and powered by another source.)
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### Author:  [@Undead\_Dude](https://boards.straightdope.com/u/Undead_Dude)
#### Post date: [November 3, 1999, 4:35pm UTC](https://boards.straightdope.com/t/capacitors-batteries/4443/4 "1999-11-03T16:35:00Z")
A chemical battery generates a voltage, which is something that a capacitor cannot do.
Batteries in general are designed to release electricity at a nearly constant voltage over a long period of time. If a battery is rechargeable, the recharge is (at least with existing batteries) a fairly long time.
A capacitor (by its lonesome – no other circuitry) discharges with an exponentially decaying voltage, usually much faster than a battery, and recharges just as quickly when a higher voltage is applied to it.
Generally speaking, capacitors are not that good for supplying a consistent power source. They are more often used in conjunction with inductors to create electric oscillations. I think they would also be useful for regulating a voltage (as in a surge suppressor).
> [@](#):
>
> but will release it suddenly either when its contacts are shorted out, or the maximum charge is achieved – AWB
If it is shorted, it will definitely release all of its charge, but it will also release a part of its charge if the voltage that is has been charged up to is reduced. Actually, if a capacitor is overcharged, it will burn out, which doesn’t return any power to the circuit. The charge “crosses the gap” so to speak. There is no “trigger” level that would make a capacitor (by it’s lonesome) release a charge.
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### Author:  [@system](https://boards.straightdope.com/u/system)
#### Post date: [November 4, 1999, 6:09am UTC](https://boards.straightdope.com/t/capacitors-batteries/4443/5 "1999-11-04T06:09:00Z")
Capacitors also pass the effects of AC and block DC.
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### Author:  [@Arjuna34](https://boards.straightdope.com/u/Arjuna34)
#### Post date: [November 3, 1999, 7:41pm UTC](https://boards.straightdope.com/t/capacitors-batteries/4443/6 "1999-11-03T19:41:00Z")
A battery stores energy in the chemical bonds of the goop inside it. When it delivers energy, a chemical reaction is taking place in the battery. The specific chemistry determines the battery voltage (i.e. about 1.5 volts for alkaline, about 1.2 volts for nicad, 3 volts for lithium batteries). To get a higher voltage, you have to connect multiple batteries together (technically, you are connecting multiple cells to form a battery).
A capacitor stores energy in an electric field. The typical example is a capacitor formed by two parallel plates. The plates are charged with opposite polarities. The electric field between the plates stores the energy. Shorting the plates together completes a circuit and discharges the plates.
Batteries have a much higher energy density than capacitors- normally batteries are used to store energy, and capacitors are used in electronic circuits. There are exceptions of course 🙂
Arjuna34
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### Author:  [@Stephen](https://boards.straightdope.com/u/Stephen)
#### Post date: [November 3, 1999, 8:35pm UTC](https://boards.straightdope.com/t/capacitors-batteries/4443/7 "1999-11-03T20:35:00Z")
> [@](#):
>
> Capacitors also pass the effects of AC and block DC.
Well, that really depends on how you use them. One popular use of capacitors is to reduce the effects of AC after it’s been rectified to DC. By passing the output from the rectifying diodes to one or more “filter caps” you can eliminate much of the AC “ripple” left over (if you want to be even more thorough, you can then pass this filtered current through a choke coil).
So, filter caps reduce the effects of AC and pass DC, see? Si?
* * *
Stephen
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