Question: capacitors as special use batteries?

The fact that something can keep a charge for a significant amount of time is irrelevant. The fact that those capacitors are isolated from the circuit so there is virtually no load is the giveaway. If you keep a load on the capacitor, it will discharge. A really high impedance load will not affect the voltage on the capacitor for a long time, but the moment you try to draw any real current and the voltage on the capacitor goes away immediately.

Capacitors don't have the capacity to do what you want. They are good at storing voltage, they are lousy at storing power, and you need power (ie voltage times current) if you want to do anything with this.

You will very easily learn that you would need massive capacitors (in terms of capacitance and thus in terms of physical size) before you could even come close to doing anything useful this way.

Spend your efforts on lowering current draw, which then requires less power. I have one of those "atomic clocks" that is at least four years old and still running on the two AA batteries that I put in when I bought it. That's low current, considering there is radio circuitry that has to come alive each night to get the update from the time station. No need to fuss with large capacitors, or rechargeable batteries, it just does its job well with very little current draw.

In a lot of cases, you can't do that, since the device is required to supply power of some sort (like that stereo amplifier over there). Radios are another place where low current isn't necessarily the best route; I just got one of those crank radios and the crank function is great, the radio is horrible, likely the result of low current being seen as the primary design factor, rather than good reception.

Michael

I agree with Michael Black, you are asking capacitors to do the job of power cells. Capacitors can supply large amounts of energy for short periods of time or small amounts of energy for longer times. Look at the discharge curves of a charged capacitor as it discharges through your load. You can find this info if you search. Regards, Tom

Modern capacitors are starting to have enough capacitance to be used in this sort of application.

Here is a research starting point:

Alan Nishioka snipped-for-privacy@nishioka.com

Basically no. The energy stoage capacity per volume / weight is absurdly low when compared with any other method.

So-called 'super caps' have niche applications but will never be used like that.

Graham

The capacities aren't yet at that level, but one company is now marketing a capacitor powered cordless drill with a recharge time of less than 2 minutes.

.

And I bet the drilling time is a lot less than that.

John

Tecate sells the Maxwell Boostcaps that are the size of a D-cell,

2.7V, 350 Farads. Its takes several minutes to charge one up on the lab supply supplying 2.5V 3A. Several 1000 watt-sec.

You're not going to get more power out, on average, than you put in. Drills use a lot of power, so if a 2-minute charge yields 2 minutes of drilling, it's going to be a *big* charger.

Batteries aren't as efficient as caps (joules in:out) but batteries tend to run constant-voltage during use. Caps lose voltage linearly, so will need downstream electronic switching regulators to keep up the load as they discharge.

John

On Jul 11, 6:03=A0pm, John Larkin

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D I bet you could build form-fit-and-function battery packs for a drill using boostcaps and a wide input range switcher like you said. You drill for a couple minutes then change the pack out. The charger could hump 50 or 100 amps into the pack being charged and charge the thing up 10x faster than it discharged under use. Better than the charge for several hours arrangement they have now.

You're absolutely right, John. A couple of minor points, though...

Caps lose voltage exponentially, not linearly. That's one of the downsides of using them as battery replacements.

We've been using supercaps as intermittent storage devices and they have their good sides and bad sides. No matter what the size of the cap, if the load is fairly low, the dissipation of energy through the load happens very quickly. Since E = 1/2 C * V^2 (energy = 1/2 * capacitance * voltage squared), if a 4F cap is charged to 5VDC, it has an initial energy value of

50J. When connected to a load R, in one RC time constant of time, the energy has dropped to 6.8J or roughly 86% of the total initial energy is dissipated in the first RC time constant.

Another downside of using supercaps... their intrinsic voltage rating is usually very low, typically around 2.5VDC to 2.7VDC. To get any sizeable voltage rating (for instance 100VDC) you need a LOT of caps in series, which significantly drops the capacitance value. Even the caps rated at 5.0VDC or

5.5VDC are typically made up of a set of the lower voltage-rated caps.

What we really need is a 200F, 100VDC-rated cap the size of an 0805 surface-mount device :)

Dave

Someting like that would be dynamite!

Infact, better than dynamite. Dynamite only manages an energy density of 4300J/g at about 0.002g each, mass for mass your (dream) caps would pack 1000 times the punch when fully charged.

:)

Bye. Jasen

Don't you watch Star Trek? :) ...I'm only asking for the power source in a phaser!

Dave

:I recall from my hobbyist days that high-voltage capacitors can keep :a :charge for a significant amount of time, especially if they are cut- :off from the circuit subsequent to charging. : :I'd like to know whether it might be feasible, from a technical and :cost-effectiveness standpoint, to use a large array of specially :designed capacitors to hold excess electricity from solar plants for, :say, periods of a week or more. This would be helpful to bridge gaps :caused by overcast weather, and for other purposes. : :I know that there are potentially problems with dielectric breakdown :and leakage. On the plus side, such "batteries" could be charged and :discharged very quickly, as needed, and without the complications :involved in conventional batteries using chemical electrolytes.

At this stage supercapacitors are only useful and cost effective for supplying short term, high current delivery.

In 2000,CSIRO and other Australian auto companies developed a hybrid electric demonstration vehicle called aXcessaustralia LEV

CSIRO developed a lead-acid battery with combined supercapacitors to produce a battery pack which was about 20% the weight of battery packs normally used in electric vehicles. The Supercaps were charged by regenerarive braking and supplied the short term boost current for rapid acceleration. When running on electric only, the vehicle was capable of around 20 minutes use in urban environment.

Linearly with constant-current load. Drooping exponential with constant-resistance load. If you hang a switching regulator on the output and drive a constant-power load, current will increase as voltage drops, which is a growing exponential upside-down, because the load is a negative resistance.

How many watts does a drill need? A lot, I think, if it's doing any serious drilling.

A AA alkaline battery can deliver a couple of kilojoules. So spending multiple dollars for 50 joules isn't very appealing

John