What are the limitations of a capacitor versus a battery?

Question

Would it be possible to power a small electronic device with a capacitor, like a laptop? What are the pros and cons of a capacitor versus a chemical battery? Any info would be appreciated.

Answer 1

The capacitor would have very limited storage capacity and the voltage starts dropping as the current starts flowing. Even a small battery is much more efficient in this regard compared to a capacitor. Except for the disposal problems of some chemical batteries (which contain mercury or lead for example), a battery is a better option.

Answer 2

The answer is simple once you understand a few things. First Capacitors have many general uses. Power factor correction and power storage are 2 of them. For power storage which I believe you are more concerned with given your reference to batteries, they serve quite well. However they are MUCH more dangerous than batteries. Reasons why: 1. As opposed to a battery, a capacitor can discharge its ENTIRE charge in a very small fraction of a second. Batteries can not. This makes the total energy involved in a short circuit much higher with a capacitor than your typical battery. 2. Capacitors can explode. Yes, explode not unlike a grenade. This can happen when you have a direct short on a capacitor or some other conditions like wiring a charge capacitor backwards and applying voltage. 3. Given #1 and 2 above do you need more? I hope this answers you questions.

Answer 3

The previous answers are correct, but to give you an example you can visualize let me give you some definitions.
A capacitor's ability to store charge is measured in Farads, where 1 Farad can store one Coulomb of charge when one volt is applied to it.
You are hoping to use it as a power source so you are more concerned with the discharge of a capacitor. For example a one farad capacitor is sometimes installed into a car to help with a high power audio system. It is likely to be about the size of a 1 liter bottle of soda. Note that the applied voltage is usually about 14.4v, so the charge stored would be 14.4 Coulombs for the 1 farad capacitor (C=FV).

Compare to a battery. A quick search for 'laptop battery' found some rated at 4500 mAh at 10.8v. So we can roughly describe it as a capacitor that could discharge 4.5 amps for 3600 seconds and maintain 10.8v output.

To maintain the voltage there has to be enough charge in the capacitors to provide that voltage.
For example you could take a single 1 farad capacitor and charge it by 10.8v and it would store F*V or 10.8 coulombs. This would be the empty state. if you pull off any more charge the voltage would drop and the power supply would be useless.

now here is the real core of the problem. in order to match the lattop battery you have to put into the capacitor another 16,200 coulombs (4.5amps for 3600 seconds). To do it in a one farad capacitor you would have to charge it with 16,210.8 /1 (C/F) = 16,210.8 volts. 16,000 volts is unmanagable. the capacitors would explode, people might die.

If you were to use 100 farads, then you would need for the minimum charge.
C=FV =100*10.8 =1080 coulombs.
On top of that you want 16,200 for operation, for a total charge of 17280.8 C.
And the voltage for that charge is V=C/F = 17280.8/100 = 172.80 volts. Large, but under the 200v value that is often seen. But that is stilll 100 liter bottle sized 1 farad capacitors.

Allow yourself 1000 farads then the 'empty' charge is C=FV =1000*10.8 10,800 C.
And the operating voltage is V=C/F =(16,200+10,800)/1000 = 27volts. That's a great voltage but way too many farads to handle.

And that's why they don't do it.
Hope this helps.

Answer 4

A Capacitor is similar to a battery in the way it temporarily stores unused energy - However, it's drain rate is much higher. Unless you plan on setting up a "huge" series of capacitors, you should not try to use them for the regular powering of your electronics - Unless it's for an experiment!

Answer 5

Capacitors are only used in high surge discharge appliances in conjuction with a battery (ie a taser, soleniod controller, etc).
A laptop is not a small device most have fairly high power draws and would require a physically massive capacitor bank. Remember tha capacitor discharge/charge according to their time constant.

Answer 6

Yes, it is possible,but there are practical and economical considerations, primarily related to energy density considerations. Batteries, as energy storage solutions (which they are designed to be) store far more energy per dollar and per pound than even the most advanced "supercaps".

The main problem with caps that limits their storage capacity is that the energy is stored as an electrostatic field, which doesn't compare very well with the chemical method of batteries. For example, look at the battery in my Makita drill. It weighs about 2 pounds, is about 1" x 2" x 7" in size (14 cubic inches), and costs about $20. It says it's rated for 1000mA-hr and is 9 volts. Multiply the 2 and you get 9 watt-hours, which times 3600 seconds/hr = 32,400 watt-seconds, aka 32,400 joules of energy.

Now I'm looking at a nice electrolytic cap here, which is rated at 1500 microfarads at 200 volts. The energy in a cap is

E = CV^2/2, in joules,

so if I charge this cap all the way up to it's limit, I have

(1500 x 10^-6)(200^2)/2 = 30 joules

Two of these caps are about the same weight, volume, and price as my battery. Even then, 60 joules from these 2 caps compares very poorly with the 32,400 joules of the battery; it would take about 1000 of these caps fully charged to hold as much energy as this one battery.

[REPOST] I should add that the one big advantage caps have over batteries is that they can be charged and discharged at much higher rates, certain types into the billions of times per second.

Answer 7

Capacitors are generally used where a high power burst is needed. They are better at taking a charge and letting it all go very quickly where a battery is better at letting out a little power over a longer period of time.

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Answer 8

A capacitor is made to hold charge for a limited time. If it were feasable, it would be done.

Answer 9

Both voltage and amperage.
Capacitors or only for a brief spike of energy.