This technology seems to be moving along pretty well, and it's one of the revolutions that will constitute a major solution to our energy problems.

This approach allowed the engineers at Standard Oil to build a multifarad device. At the time, even large capacitors had nowhere near a farad of capacitance. Today, ultracapacitors can store 5 percent as much energy as a modern lithium-ion battery. Ultracapacitors with a capacitance of up to 5000 farads measure about 5 centimeters by 5 cm by 15 cm, which is an amazingly high capacitance relative to its volume. The D-cell battery is also significantly heavier than the equivalently sized capacitor, which weighs about 60 grams.

I've probably told this story before about innumeracy, even of physics students, and the inability of some to think through a problem. When I was teaching an E&M lab in college, we were doing experiments with capacitors, and someone came up and said, "The lab instructions say to use a microfarad capacitor, but this one says MFARAD instead of (greek letter mu--the symbol for micro)FARAD. I assume it means megafarad. Don't we have smaller ones?

I explained to him that the largest capacitor I'd ever seen (this was in the late seventies) was a quarter farad, and it took a truck to deliver it. Did he really think that he was holding something in his hand with four million times that capacity?

The article is also a good tutorial on capacitors in general, for those unfamiliar with how they work. The way that I like to think about this is that the positive charge accumulates on one plate, and the negative on the opposite one. They are held in place by their mutual attraction (being opposite charges), but cannot combine because of the insulation gap represented by the dielectric. The more accumulated charge, the higher the attraction (and field force) and accompanying voltage and potential energy. When the plates are allowed to connect to each other through an external circuit, the charges flow toward each other and create current (and power). The breakdown voltage is the voltage at which the gap can no longer restrain the attraction between the two groups of charge, and they jump across it to meet their destiny. This is to be avoided.

[Update a few minutes later]

Sorry, link was slightly mangled (though usable) before. Fixed now.

The provided link is to "Page 2 of 4".

The start is the URL below, and there's an interesting picture there.
http://www.spectrum.ieee.org/nov07/5636

Thanks for the excellent reading material.

So any idea why we're calling these both "ultracapacitors" and "supercapacitors"? Is it a branding thing or jargon conflict between different research groups?

The Riverworld series by Philip Jose Farmer (from back in the 70's, I think) called them "batacitors".

What surprises me about these supercapacitors is that they won't discharge instantly if you hook them up to a circuit. I suppose the time it takes to discharge must depend on the resistance just like a battery?

I've always thought of the "Shipstones" often mentioned in Heinlein's later work as a form of capacitor.

I've already seen an ad for a cordless screwdriver that uses capacitors rather than a battery. It had a fairly low "battery life", for lack of a better term, but could be recharged in 90 seconds.

The article spent a lot of time talking about electric cars. I think the cordless powertool market will be much more lucrative, at least in the short term.

I'm just glad that we're hearing claims from more than just EESTOR, about whom much muck has been raked.

I think the micro R/C cars use capacitors. They take about 5 minutes to charge up with a 9v and run for about that same amount of time.

I suppose the time it takes to discharge must depend on the resistance just like a battery?

The capacitor discharges at a rate proportion both to the charge left and to the resistance. In other words, the charge (and the discharge rate) exponentially decays to zero with the rate of decay proportional to the resistance of the circuit.

How quickly they discharge is dependent on how much current you pull out of them, Mark, as Karl noted. The charge in a capacitor is proportional to both the capacitance and the voltage present in the capacitor:

Charge (in coulombs) = capacitance (in farads) x voltage (in volts)

or

Q = C x V

Current is nothing more than time-rate-of-change of charge, so

I (current in amperes) = Q/t = (C x V)/t

or, after rearrangement, as the capacitance is constant

V/t (volts per second) = I/C

Example:

If a 10-farad capacitor is feeding 0.005 ampere to a load, the voltage falls off at a rate of

V/t = I/C = (0.005 A)/(10 F) = 0.0005 volts/sec, or 500 microvolts per second.

Put another way, it would take 2000 seconds, or about 33 minutes, for the voltage to fall one volt.

I'd give a more general answer based on the idea of RC time contstants, but I've blathered enough here already, and there's always Wikipedia.

The fact that I've written this at all tells you that I really should not have had a Coke an hour or two ago, and it's already 3 AM. Urrgh....

:)

Just remember, boys and girls, when working with supercapacitors charged up to even just a few tens of volts, Mr. Screwdriver is NOT your friend.

I remember in electronics lab when we used to deliberately reverse electrolitic caps on a breadboard.......those were fun times!

I don't see the revolution until the capacitors are a) cheaper than batteries per energy, b) lighter than batteries per energy, or c) more easily conformable than batteries to use as device structure or insulation or other dual use. That is, higher price or mass efficiency of some larger system that has multiple functions of which the capacitors are a part. Does anyone see more widespread adoption of capacitors than batteries prior to crossing one of those milestones?

Does anyone see more widespread adoption of capacitors than batteries prior to crossing one of those milestones?

Capacitors have other attributes in which they may outperform batteries, including (1) much longer cycle life, and (2) possibly higher discharge rate. We may see batteries and ultracapacitors combined, for example to soak up power spikes from regenerative braking and prevent excessive cycling of the battery.

What scares me about capacitors is that-- in theory, anyway-- there is NO upper limit on their discharge rate.

Like I said before, above a certain energy level, Mr. Screwdriver is not your friend.

And that may be a showstopper for some applications unless the discharge rate can somehow be tamed from the explosive (Mr. Screwdriver vaporizing) to the merely incendiary (laptops catching fire).

Hale Adams:

How dare you give away the solutions to my ECE 376 exam on Thursday!