# Why do we use capacitors when batteries can very well store charges?

Can batteries be used instead of capacitors? I am trying to figure out a basic, superficial and any obvious difference between the two.

• Um...have you compared the amount of charge in a usual capacitor and that of a battery of the same size? – ACuriousMind Apr 14 '15 at 9:57
• No. I have read somewhere that the basic difference is that a battery cannot release the stored charges as quickly as a capacitor does. So, if I am to create a flash of light, as in a camera, I would use a capacitor and not a battery. Is that it? – Swami Apr 14 '15 at 10:03
• You ask "Can batteries be used instead of capacitators?" The answer strongly depends on the intended application/purpose. Maybe you can restate your question with some more specifics. – Cornelius Sicker Apr 14 '15 at 11:04

Why do we use capacitors when batteries can very well store charges?

There's an important point that, so far, I don't see in other answers.

Neither of these devices store charge!

A "discharged" battery or capacitor contain the same net quantity of electrical charge as a "fully charged" battery or capacitor.

What they are "charged" with is energy, not electrical charges. The verb "charge" here is used in the same sense as when you are invited to charge your glass with champagne at a celebration. The verb "charge" and the noun "charge" are not, in this context, as related as their spelling suggests.

## Actual use

Batteries are used for storing energy over long periods of time (typically hours, days, months or years) and for then supplying that energy to a device for a period of operation that may be minutes but is more likely hours.

Capacitors are more typically used for purposes for which batteries are unsuitable

• filtering.
• smoothing.
• oscillating.
• etc.

These generally involve much shorter timescales and much higher frequencies.

There are some, relatively small, areas of overlap. For example Supercapacitors can be used in some automotive applications in place of batteries.

• But if you time-integrate the current going into a capacitor, and coming out the other side, can't you call that "charge"? $I = dQ/dt$ – Mike Dunlavey Apr 14 '15 at 16:14
• @mike: since as much flows out as in, you can't call it "stored". – RedGrittyBrick Apr 14 '15 at 16:17
• I see your point. – Mike Dunlavey Apr 14 '15 at 16:49

While a capacitor can be used to store charge, usually we are interested in other properties. Most notably, it has a voltage proportional to the amount of charge stored ($Q=CV$) which means it acts as an integrator of current. There are many circuit applications where you use this property - which incidentally also means that the apparent impedance of a capacitor is a function of frequency - at high frequency it has lower impedance according to

$$Z =\frac{1}{j\omega C}$$

The latter property is used all the time in filters, radios, and any circuit that had time dependent behavior. On the other hand a battery has a voltage that is mostly driven by the chemistry (a good battery keeps its voltage as it loses power).

Second, polarity. While some high energy density (large value) electrolytic capacitors need to be polarized in a specific way (or they can explode spectacularly - don't try this at home!) many types are suitably for AC. When you make an LC resonant circuit (for example in the receive circuit of a radio) that is essential.

Third - the current a battery can produce is limited by the rate at which electrolyte can diffuse. While some multilayered structures have allowed significant drops in the internal resistance of a battery, capacitors are orders of magnitude better in this regard. For example the JET (fusion tokamak) needed very large currents through its magnet during a pulse - and used a giant capacitor bank to provide that instantaneous current.

These are just three reasons. I am sure others can come up with more.

Practically we use capacitors when we require a large amount of charge to be flown within fractions of seconds.. Battery provides a nearly uniform voltage and effective in long use, but when it comes to discharge a large amount of charge in a fraction of second, battery is ineffective.. How ever by a building a capacitor with large capacitance we store a bulk of charge (large potential difference) to be flown within seconds.. E.g flash of a camera, Z-machine at nuclear reactor..

I've to make an electronic circuit 'RC' and the relation between current and tension between two nodes must be fulfilled by a capacitor 'C' (it integrates the current; see the relation in the WP). I can use a battery with a constant tension to power the circuit ($V_{in}$in the second image) but not to model that relation.

batteries are a much more efficient at storing electricity but in circuits, it makes much more sense to use capacitors in circuits as they are much more efficient for the short term storage of electricity. batteries are a lot more bulky and to work as a capacitor they would need to be rechargeable. it would not make sense to have two batteries in a single circuit anyway because then you would have two possibly conflicting power sources. furthermore batteries cannot be fitted as easily into microchips and cannot be used in all the same applications as capacitors. capacitors can also be used to smooth the irregularities in corrected DC current.

a capacitor acts as an dam for electricity that resists changes in potential difference (voltage).

a battery acts at a supply of electricity due to the potential difference across it.

A capacitor stores charge on a pair of plates. A battery generates charge through chemical reactions that break neutral atoms into positive and negative ions.

Both store energy. A battery stores chemical energy. A capacitor stores potential energy in the separated charges.

Sometimes a capacitor has an electrolyte between the plates. This is a molecule that is polarized and aligned by an electric field.

This is is sort of equivalent to bringing the plates very close together. It reduces potential energy because the separation is smaller. It permits more charge to be stored for the same energy.

A very large 1 Farad capacitor can run a small electronic device for a minute or so.

In other ways, they are not interchangeable. The voltage across the terminals of a capacitor is proportional to the stored charge. The voltage across the terminals of a battery is constant - determined by the chemicals in it.

Charge can flow in and out of a capacitor. Some batteries are rechargeable, but others are not.

## protected by Qmechanic♦Apr 14 '15 at 16:02

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).