Practicality of using capacitors as a power source

ble on

You should look at the energy density. Joules/ kilogram or Joules/ liter.

Can you do your own wiki search?

(Besides the obvious answer that as you discharge a cap the voltage drops a lot. Q=3DCV and all that.)

George H.

The energy storage ratio is extreme, four or six orders of magnitude.

John

ble on

Here are a couple of web sites on the subject of using capacitors as a power source.

Alexander Bell's site seems to me a bit biased and doesn't give a fair representation of the trade offs. Remember the words of the great Oliver Heaviside. "Some think electricity is energy. Others think electricity is power. Some manage to think both these things at the same time."

Capacitive power storage and battery power storage are two different animals with different applications. Capacitors have high power density. Batteries have high energy density. So if you need high power for a very short time, you use a capacitor. Think flash bulb. If you need low power for a very long time, you use a battery. Sometimes you need a combination of both. You use the battery to store the energy and charge the capacitor. Bursts of power are then taken from the capacitor.

Thanks for the links. Although I had already read wikipedia's article on supercapacitors (it was one of the reasons I subscribed to this group and subsequently started this discussion), I hadn't read the link from alexanderbell.us, which was interesting.

Regarding capacitor's rate of discharge, is it possible to release energy from a capacitor in a gradual manner without much waste instead of releasing it in intense burts?

Thanks for the help, Rui Maciel

Thanks for the link, Charles. The link on capacitors and supercapacitors[1] was particularly informative.

Rui Maciel

Yes, it appears that those values are coherent with the information which has been presented in this thread. Yet, in some applications energy density may not be the decisive property of a energy storage device. For example, in applications such as small LED flashlights energy density may be less important than recharging speed and even the ability to recharge them through simpler means, such as mechanical generators. Is there any reason that makes this sort of applications impractical?

Rui Maciel

Yes, energy density. Capacitors just don't hold enough charge to make this sort of use practical. Batteries aren't all that scary.

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They're working on better capacitors constantly. they have certain advantages over rechargeable batteries.

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Lets just do it.

Here's a supercap from digikey 1 Farad 6.3V around 4 dollars.

-ND

Lets say we can charge it to 6 volts and discharge it to 2 volts and we build a constant current circuit to drive 10mA into a 2 volt Led.

Energy in a cap is (1/2)*C*V^2 so at 6 volts the cap has 18 joules and at 2 volts it has 2 joules so we can supply 16 joules.

The LED at 10mA and 2 volts uses 0.02W.

A joule is a Watt*second so 2/0.02 =3D 100 seconds =3D 1.6 minutes

Now a AAA alkaline has about 1000mAh at 1.5v. or 5400 joules. Let say we boost that to 2v and we're 50% efficient. So, we supply 2700 joules to the LED. 2700/0.02 =3D 135000 seconds =3D 37.5 hours.

So you will need a supercap with lots of Farads of capacitance and a high voltage rating to compete with the battery.

ble on

Capacitors use surface area of plates. Batteries use molecules. That explains why capacitors hold less charge. Tantalum capacitors use plates in convoluted shapes, so the surface area is bigger than parallel plates. Batteries use smaller structures called atoms and molecules to hold charges, so they have many crannies of smaller sizes than the surfaces of plates. It is about nano versus micro shapes.

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eason

Opps, shouldn't that be 16 Joules/0.02 Watts =3D 800 seconds. (but still much to short a time.)

Oh, you must use a AA battery or Miso will have another rant. :^)

George H.

For some applications capacitors are eminently suitable as a power source today. As for replacing high power storage batteries - they still have a way to go.

They are already experimenting with using super caps in conjunction with car batteries to save on battery costs or to allow the battery be located further from the engine (where the heat won't kill it as quickly).

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Oops. Yes you're right. 800 secs =3D13.3 minutes

Yes, but you don't get more energy. A capacitor is a storage container. It can hold only so much, and it cannot release more than it can hold. Think of a liter bottle of soda. You can't get more than a liter of soda into the bottle, and you can't get more out of it, regardless of how slowly you sip it. A capacitor is like that.

Ed

ND

I see what you mean. Yet, the capacitor you've shown is a somewhat smaller than a AA battery and it appears to be possible to stack around 5 of those to reach the same height. If I'm not mistaken, with this arrangement it would be possible to store enough energy to power that LED for around an hour. Although it still lags behind what can be had with batteries, it is already a decent amount of time. If it was possible to recharge the capacitors through some means, such as a mechanical generator, then it wouldn't be a bad thing to have.

Rui Maciel

An easier way to look at this is to consider that an Ampere is a Coulomb per second (C/s) (how much of a charge of electrons flow past a point per unit time) whereas a Farad is a Coulomb per Volt (how much charge is crammed into a capacitor for a given potential difference).

At 6V, our 1F cap stores 6 coulombs, and discharges to 2 coulombs, so we lose 4 coulombs. These 4 coulombs are being doled out using an active current limiter at a steady 10 mA trickle (in other words 0.01 C/s), so we get a linear discharge, rather than the typical RC discharge.

We thus easily calculate the discharge time:

Thus 4C / 0.01 C/s) = 400 s.

We get 6 minutes and forty seconds. (Of course, a real current limiter will have a voltage drop, such as the VCE(sat) of its transistor or whatever: it will stop working somewhere above 2V).

From this we can also see how much energy is wasted. We have 16J, right? But we got to run our 0.02W LED for 400 seconds, which gave us 8J of energy. Watts is Joules per second, so 0.02 (J/s) * 400s = 8J.

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A cap is perfect for getting you started going through the intersection. That way the $15,000 battery isn't damaged by 100 kW loads.

An EV battery only needs to put out 10 kW on the freeway.

Bret Cahill

Isn't it possible to at least limit the voltage output of a capacitor?

Rui Maciel

Hey, Bob.

How about a 2600F 2.5V cap for $10?

Maxwell 2600 Farad 2.5VDC Boostcap

A couple of these hould be enough to power a white LED for some time, yes?

Enjoy...

Frank McKenney