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  #1  
Old 11-04-2009, 02:18 PM
Jake Jake is offline
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Is a Battery a Capacitor?

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

Last edited by Jake; 11-04-2009 at 02:18 PM..
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  #2  
Old 11-04-2009, 02:24 PM
beowulff beowulff is offline
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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.
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  #3  
Old 11-04-2009, 02:27 PM
pan1 pan1 is offline
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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)
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  #4  
Old 11-04-2009, 02:30 PM
Whack-a-Mole Whack-a-Mole is offline
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Originally Posted by pan1 View Post
And a modern battery is designed more to store electricity.
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).
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  #5  
Old 11-04-2009, 02:39 PM
beowulff beowulff is offline
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Quote:
Originally Posted by pan1 View Post
A capacitor and a battery are basically the same thing.
Completely untrue.
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  #6  
Old 11-04-2009, 02:39 PM
AskNott AskNott is offline
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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.
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  #7  
Old 11-04-2009, 03:05 PM
Hari Seldon Hari Seldon is offline
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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.
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  #8  
Old 11-04-2009, 03:16 PM
Stathol Stathol is offline
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Originally Posted by beowulff View Post
Completely untrue.
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).

Quote:
Originally Posted by Whack-a-Mole View Post
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).
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.
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  #9  
Old 11-04-2009, 03:19 PM
Mangetout Mangetout is online now
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What about electrolytic capacitors - in what way are they similar (or different) from batteries?
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  #10  
Old 11-04-2009, 03:30 PM
DocCathode DocCathode is offline
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Originally Posted by Stathol View Post
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.
.
Could you please explain power density and energy density?
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  #11  
Old 11-04-2009, 03:35 PM
beowulff beowulff is offline
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Quote:
Originally Posted by Mangetout View Post
What about electrolytic capacitors - in what way are they similar (or different) from batteries?
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.
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  #12  
Old 11-04-2009, 03:36 PM
Chronos Chronos is offline
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Quoth Hari Selden:
Quote:
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!
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:
Quote:
Could you please explain power density and energy density?
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.
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  #13  
Old 11-04-2009, 03:37 PM
Mangetout Mangetout is online now
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Is there any actual electrolysis taking place in there then?
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  #14  
Old 11-04-2009, 03:38 PM
beowulff beowulff is offline
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Quote:
Originally Posted by DocCathode View Post
Could you please explain power density and energy density?
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.
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  #15  
Old 11-04-2009, 03:41 PM
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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.
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  #16  
Old 11-04-2009, 03:42 PM
Machine Elf Machine Elf is offline
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Originally Posted by beowulff View Post
Completely untrue.
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.
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  #17  
Old 11-04-2009, 03:49 PM
beowulff beowulff is offline
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Quote:
Originally Posted by Joe Frickin Friday View Post
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.
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  #18  
Old 11-04-2009, 03:51 PM
beowulff beowulff is offline
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Originally Posted by Mangetout View Post
Is there any actual electrolysis taking place in there then?
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.
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  #19  
Old 11-04-2009, 03:57 PM
pan1 pan1 is offline
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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.
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  #20  
Old 11-04-2009, 04:01 PM
Magiver Magiver is offline
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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.
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  #21  
Old 11-04-2009, 04:07 PM
beowulff beowulff is offline
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Quote:
Originally Posted by pan1 View Post
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.
First of all, a "Battery" is an array of cells. What we Modern folks call a "Battery" is usually just a single cell. So, Franklin was correct in calling his device a "Battery" - it was a battery of condensers (or capacitors).
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  #22  
Old 11-04-2009, 04:08 PM
johnpost johnpost is online now
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Quote:
Originally Posted by pan1 View Post
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.
a battery means a collection or group, for example an artillery battery is a group of cannons. a capacitor battery is a group of capacitors wired together.

in common use an electrochemical battery is a collection of galvanic electrochemical cells though single galvanic cells are also called a battery.
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  #23  
Old 11-04-2009, 04:12 PM
Chronos Chronos is offline
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Quote:
Is there any actual electrolysis taking place in there [an electrolytic capacitor] then?
The electrolysis occurs in the manufacture of the device, not in its operation.
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  #24  
Old 11-04-2009, 04:21 PM
Stathol Stathol is offline
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Originally Posted by Mangetout View Post
Is there any actual electrolysis taking place in there then?
Not unless something has gone horribly wrong.

It's called an electrolytic capacitor simply because they contain an electrolytic fluid (i.e. a fluid that conducts electricity). And, as others have pointed out, they use electrolysis to make them. Electrolytic capacitors are really quite clever. All capacitors have the same general form:

Code:
conductor | dielectric (insulator) | conductor
A traditional capacitor would do this:

Code:
metal plate | dielectric | metal plate
An electrolytic capacitor looks like this:

Code:
aluminum | aluminum oxide | electrolytic fluid
Physically, it looks like only two layers. The aluminum conductor simply gives way to a thin outer coating of aluminum oxide (i.e. aluminum "rust") that acts as a dielectric.The electrolytic fluid can store and release charge much like a metal plate. So you still have all the components of a traditional capacitor, they just take a somewhat unusual physical form.

Quote:
Originally Posted by beowulff View Post
However, Google "capacitor plague" if you want to learn about poorly made caps that have unintentional electrolysis occurring.
Haha, I was just about to mention that in the same context. It's been a (thankfully) long time since I opened up a computer case and saw crusty orange goo.
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  #25  
Old 11-04-2009, 04:41 PM
engineer_comp_geek engineer_comp_geek is online now
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We misuse the term "battery" today. A AA battery isn't a battery. It's a cell. AA, AAA, C, and D are all cells. A 9 volt is a battery. Open it up and you'll find six cells inside of it. Good luck trying to convince your average person on the street that a C cell isn't a battery though.

A capacitor and a battery both store energy, but they are only superficially similar. A battery tries to be a constant voltage source, but in reality ends up being more like a constant voltage source in series with a resistor.

A capacitor is nowhere near a constant voltage source. The voltage and current relationship in a capacitor is a simple integral/differential type relationship. There are plenty of dopers here who understand calculus and get this right away, but for the benefit of those who only know algebra, basically if you keep feeding current into a capacitor the voltage is going to keep increasing. Contrast this to a battery where the voltage is going to stay mostly constant. Likewise, when the battery discharges, it is going to keep a relatively constant voltage until it gets almost completely discharged, at which point the voltage suddenly drops like a rock. A capacitor discharges on an exponential curve.

Typical capacitor discharge curve:
http://www.sunstoneengineering.com/s...arge_curve.jpg

Typical battery discharge curve:
http://www.analog.com/library/analog...een/FIG_07.gif

As you can see, they are dramatically different.

You can use a capacitor to store energy and power something, just like (well, more like similar to) a battery. To do this, you have to stay up on the high end of the capacitor discharge curve so that your voltage doesn't drop too low. VCRs in the old days had a battery to backup the clock (or else they just flashed 12:00 if the power blinked). More modern VCRs and now DVD players and the like typically use a capacitor backup. There are two reasons for this change. The first is a fairly dramatic improvement in capacitor technology in the past twenty years, which allows a fairly small capacitor to store a lot more energy than before. The second is the use of lower and lower power circuits as technology in general has improved. These two factors combined have allowed electronic device manufacturers to use cheaper capacitors where batteries used to be required.

Note however that even though capacitors are sometimes used like batteries to store and release energy, they are doing it in different ways. Batteries store the energy chemically. Capacitors store the energy in an electric field. It's completely different ways of accomplishing the storage and release of energy.

Capacitors and batteries can both be used as filters. Although they end up accomplishing the same goal, just like above, they are using two different methods to do it. Car batteries have been used for years to smooth out the output of a typical car alternator. Capacitors in power supplies perform exactly the same function. However, even though batteries and capacitors are doing the same function, they are doing it in different ways.

Power factor correction highlights how batteries and capacitors are different, though. Inductors and capacitors are both energy storage devices. Inductors store energy in magnetic fields, and capacitors store energy in electric fields. Because of this difference, if you are on an AC circuit, capacitors are charging while inductors are discharging and vice versa. Your typical home tends to run a bit inductive, due mostly to motors (vacuum cleaner, refrigerator compressor motor, etc). The power company balances this out with large capacitor banks, since if they can get the capacitances and inductances to exactly balance then the power transmission becomes much more efficient. You can't stick a battery in place of a capacitor in a power factor correction circuit. Batteries just don't function the same way.

Similarly, in an AC circuit you can use a capacitor in series to cut down the voltage going into a device without generating heat like a series resistor would do. A battery wouldn't work for this. You could do a similar thing with a battery on a DC circuit, where a capacitor would simply stop all current from flowing.

So there are clearly some applications where capacitors and batteries behave completely differently.

You can use them similarly in certain types of applications, but batteries and capacitors are not at all the same thing.
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  #26  
Old 11-04-2009, 05:04 PM
Francis Vaughan Francis Vaughan is offline
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Ha! - simultaneous post with the above.

Between them Joe Frikin Friday and Stathol have it complete.

Just to reitterate. A capacitor (also once called a condenser) is a fundamental electrical device. It stores energy with an electrostatic field. As Joe said, it has a defined charge to voltage law. The more charge you put into a capactitor the more voltage appears across its terminals. This is so until you reach the physical limits of the construction, when typically the dielectric fails.

What we call a battery (and has been mentioned is actually an electrochemical cell - but a group of them is a battery - in the same way that a group of guns firing together is a battery.) Cells store energy by a chemical process. The reduction potential of the chemicals chosen in the cell determine the voltage the cell provides, and it is fixed. Second order efects in the construction of the cell result is some small variation in voltage, but through the cell's working range the terminal voltage is essentially the same.

Although Electrolytic capacitors contain water and a chemical cocktail, no chemical reaction occurs in operation. The fluid is simply one plate of the capacitor. The fluid is chosen so that it conducts well, and doesn't degrade with heat too much. The trick with the electrolytic capacitor is to get a huge surface area, which is done by etching the surface of the aluminium foil. Over time the manufacturers control of the etching process has allowed them to produce more and more fine grained and deeper etching, and has led to substantial gains in effective surface area, and thus smaller capacitors for the same capacity. This etching is also why the opposite plate needs to be a fluid - it is required to flow into the etched surface to make intimate contact with the dielectric (aluminium oxide) created on the surface.

Supercapacitors take the idea of the electolytic capacitor and use a 3D structure to make one electrode. This yields phenominal capacity.

You can't interchange batteries and capacitors in a circuit. It simply doesn't work. Capacitors have other interesting properties - in particular they are used in alternating current circuits where the phase angle between current and voltage is important.

It is worth noting that one of the other fundamental circuit elements - the inductor - also stores energy. It uses a magnetic field to do so. Nobody suggests that it looks like a battery. (A superconducting inductor makes a perfectly good storage device, if a little unweildy and hard to manage.)

Last edited by Francis Vaughan; 11-04-2009 at 05:05 PM..
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  #27  
Old 11-04-2009, 06:00 PM
tetranz tetranz is offline
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Originally Posted by Chronos View Post
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.
I remember when those big value, small size capacitors became readily available. It was pretty weird see a capacitor labeled in farads when most people working in electronics had never seen anything bigger than maybe a few 1000 microfarads.

I think they were (still ?) used as the backup "battery" for the clock in VCRs etc.
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  #28  
Old 11-04-2009, 06:00 PM
Kevbo Kevbo is offline
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Originally Posted by AskNott View Post
F
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.
This is not the reason it works. It works because the various items supplied from the fuse box in the car normally share one of 1-3 runs between the fuse box and the alternator or battery. This couples noise into the radio power leads. Dedicating a run to the battery decouples the radio from these glitches. Among the glitches may be alternator noise related to the battery charging current, which such a dedicated run also eliminates.
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  #29  
Old 11-04-2009, 08:04 PM
Snnipe 70E Snnipe 70E is offline
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Saying a battery and a cap are the same becoust they both can store energy would be lik saying a house and a high rise building are the same because people go into both.
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  #30  
Old 11-05-2009, 07:12 AM
Superfluous Parentheses Superfluous Parentheses is offline
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Also: some videos of blowing up capacitors
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  #31  
Old 11-05-2009, 09:27 AM
Hari Seldon Hari Seldon is offline
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Saying a cell is not a battery is like saying that 1 is not a number.
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  #32  
Old 11-05-2009, 09:43 AM
sweeteviljesus sweeteviljesus is online now
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Quote:
Originally Posted by Jake View Post
Could we run automobiles on capacitors?
Ultracapacitors powerful enough to run cars are an active area of research if Technology Review is to be believed.
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  #33  
Old 11-05-2009, 10:55 AM
Stathol Stathol is offline
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Quote:
Originally Posted by Hari Seldon View Post
Saying a cell is not a battery is like saying that 1 is not a number.
I know I'm being totally pedantic here, but:

Quote:
Originally Posted by The OED
III. (from 4) A combination of simple instruments, usually to produce a compound instrument of increased power; applied originally with a reference to the discharge of electricity from such a combination.

9. Electr. An apparatus consisting of a number of Leyden jars so connected that they may be charged and discharged simultaneously.

10. a. Galvanism. An apparatus consisting of a series of cells, each containing the essentials for producing voltaic electricity, connected together. Also used of any such apparatus for producing voltaic electricity, whether of one cell or more.

b. attrib. and Comb., as battery-receiver, -set, -unit; battery-operated, -powered adjs.
There is a distinct use of plurals

Modern usage has corrupted the word "battery" to refer to what is actually just a single cell, but that doesn't really make the usage correct. 1 is certainly a number; however technically speaking, you cannot have a "battery" of one cell any more than you can have a "flock" of one bird.

A car battery is indeed a battery because it contains multiple lead-acid cells. But a "size AA battery" is actually a "size AA cell". Energize, et al. are bad about perpetuating this confusion, but if you ever shop around for non-consumer NiMH cells, or similar, you'll find that the vendors always refer to them as cells and only use the term "battery" to refer to several cells working in conjunction.
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  #34  
Old 11-05-2009, 11:17 AM
PlainJain PlainJain is offline
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Originally Posted by beowulff View Post
Completely untrue.
It depends how you define "basically".

If I were showing them to an alien I could describe each as an energy storing device for later use. Sure, one stores a lot and releases it slowly, the other stores less but releases it quickly but I don't think our green friend would have trouble with the 'basically the same' concept. Or so it seems to a lay person like myself.

Much like one could argue that all religions are basically the same. While it could be equally countered, "completely untrue". Depending on context, both are true.

Last edited by PlainJain; 11-05-2009 at 11:21 AM..
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  #35  
Old 11-05-2009, 11:33 AM
beowulff beowulff is offline
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Quote:
Originally Posted by PlainJain View Post
It depends how you define "basically".

If I were showing them to an alien I could describe each as an energy storing device for later use. Sure, one stores a lot and releases it slowly, the other stores less but releases it quickly but I don't think our green friend would have trouble with the 'basically the same' concept. Or so it seems to a lay person like myself.

Much like one could argue that all religions are basically the same. While it could be equally countered, "completely untrue". Depending on context, both are true.
Once again, untrue.

"Basically" means that at the most fundamental level, these two things are more-or-less the same. But, that's not the case; they are only superficially the same once you look at their functionality.

You wouldn't say that an automobile and an airplane are "basically the same," would you? After all, they are both machines, and they both move people.

Or is a Steak and a cupcake "basically the same?" After all, they both provide nutrition.

The OP asked if a Battery is a capacitor.
It isn't.
No matter how many people want to try to make it into one.
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