Is a Battery a Capacitor?

Or vice-versy?
Could we run automobiles on capacitors?

No, a battery is not a capacitor, although it probably exhibits some small amount of capacitance. A battery generates current by chemical means. A capacitor stores charge on an insulator. Capacitors typically have orders of magnitude less energy density than batteries, but they are getting better. So, it’s not practical to run a car on capacitors yet.

A capacitor and a battery are basically the same thing.

Usually when we talk capacitor we are discussing one particularly designed to regulate the electricity flow.

And a modern battery is designed more to store electricity.

But both are basically the same idea and stem from the same glass jars in the 18th century (or earlier if you believe in the baghdad batteries)

I thought they can both store electricity. Just a capacitor has less overall energy density for its size but on the flip side can provide pretty much all its charge near instantly (whereas a battery needs to unload its energy over time).

Completely untrue.

For at least one purpose, a car battery can serve the same function as a capacitor.

An old friend of mine has a 2-year degree in electronic engineering technology from Valporaiso Tech. One of his teachers had extensive experience with, and a fascination for, radios and broadcasting. Here’s one of the things he told me.

Getting good radio reception in a car can be very tricky, especially for short wave and CB radios. Under the hood is an assortment of whirring and buzzing stuff, and the grounded hood can only do so much to contain the interference. Sometimes, even the exhaust system can act as an antenna for the engine’s cackle.

If the usual fixes don’t work, and your radio still makes a racket, you can run its power wires in a shielded cable directly to the battery. The battery will have the noise canceling ability of a capacitor the size of your car.

A battery stores energy in a chemical medium, while a capacitor stores it in an electric field. Now you could argue on deep principles that the chemical medium is just storage in the position of electrons in the material, but as we commonly understand it they are quite different.

I recall that the Univac I had a 1 farad capacitor. Presumably it was a power supply filter, but I never found out. It was in a separate room and consisted of a bank of 2000 electrolytic capacitors, 500 microfards each. Quite impressive. It would have been even more impressive if the rectifier had somehow shorted. Kerbloom!

I sometimes wondered if storage batteries wouldn’t have been better.

I concur.

Yes, batteries and capacitors both store charge, but they aren’t “basically the same thing”. Batteries store or discharge energy by chemical reaction. A capacitor stores energy by the accumulation of an electric field between two plates (with an insulator between).

This is pretty much the way of it with modern battery and capacitor technology. Relatively speaking, batteries have high energy density, but low power density. Capacitors have high power density, but low energy density.

There are some hopeful developments on the horizon for capacitors in the realm of nanotechnology. One of the crucial elements of capacitor design, is that the closer together the charged plates are, the more energy you can store in the capacitor (because the electromagnetic force between two charged particles gets stronger with decreasing distance). Nanotech, of course, is all about making things smaller. Of course, with a smaller distance between the charged plates, you have a thinner layer of insulation between them. So shrinking a capacitor means you have a stronger EM field with a thinner dielectric layer between them. Current dielectric materials aren’t up to the task – they’ll just burn out and destroy the capacitor. But nanotech is also producing all kinds of new wonky ceramics and other materials that may provide much stronger dielectrics.

So we may eventually be able to produce “supercapacitors” that have the power density of a capacitor, but also the energy density of a conventional battery. Or conversely, we may be able to develop batteries with much higher power density.

What about electrolytic capacitors - in what way are they similar (or different) from batteries?

Could you please explain power density and energy density?

Electrolytic caps simply use a fluid to create an insulating layer on one of the electrodes. This layer is what keeps the charges separate. They are no more similar to batteries than any other cap.

Quoth Hari Selden:

Whereas today, a 1 farad capacitor costs a couple of bucks and fits easily in the palm of your hand. This really is an area of rapid progress.

Quoth the aptly-named DocCathode:

Basically, you can store more energy in a battery, but the energy you store in a capacitor, you can get out quicker. Consider a camera, for instance: The battery contains enough energy to set off the flash many times, but it can’t release the energy quick enough for the flash. So the battery slowly fills a capacitor, and then the capacitor quickly empties to run the flash. The capacitor can only hold enough energy for one flash, but it releases that energy all at once.

Is there any actual electrolysis taking place in there then?

Energy density is how much energy is stored per given volume.
Power density measures how fast you can extract the stored energy. Capacitors can usually discharge their entire energy very quickly. Batteries are limited by how fast the chemical reaction can occur, and how fast the reactants can be replenished, so they are not as good at providing bursts of high power.

both a battery and a capacitor store an electrical charge, the battery does so chemically and the capacitor electrostatically. in general batteries can provide a moderate amount of current for a long time, capacitors can provide a large amount of current for a fraction of a second.

there are special cases for both batteries and capacitors based on their type and application; for example a shorted nicad battery can weld steel and a capacitor can preserve digital memory for many months.

An ideal capacitor has a linear voltage-versus charge relationship defined by its capacitance, commonly measured in farads. Stuff charge in or take charge out, and the voltage increases or decreases by a corresponding amount.

An ideal battery presents constant voltage, regardless of the amount of charge you stuff in or take out. Technically you could describe a battery as a capacitor with infinite capacitance.

Yes, and I could describe my cat as a small dog that meows instead of barks, but that doesn’t make him one.

You can model a battery as a capacitor with infinite capacitance, but it wouldn’t be a very accurate model, and it would be pointless.

A capacitor is a capacitor, and a battery is a battery. They work on different principles and they perform different functions (although there is some overlap). To confuse them is not doing anyone any favors.

In a well made and correctly used device, not once the initial oxide film is developed.

However, Google “capacitor plague” if you want to learn about poorly made caps that have unintentional electrolysis occurring.

So the concensus is, when Ben Franklin referred to his rack of Capacitors as a Battery, the terminology was misadopted to mean the same as the Voltaic Jars of the early 19th Century?

So just because a Capacitor and a Battery were basically the same thing to Ben Franklin, doesn’t make it true today.

To answer the op, yes you can run a car on capacitors. There are already capacitor batteries on the market and they are very light weight. However, the energy per cubic volume is much lower than the batteries used in hybrids today. While it would be possible to stick them in every nook and cranny (fender wells, doors, bumpers, under the seats) it probably poses a safety hazard in an accident. The advantage of capacitor batteries is a near instant recharge rate in comparison to other batteries and that advantage is about to be lost to new anode technology where they are micro drilled to increase surface area for charging.