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Everyone who owns a mobile phone knows something about rechargeable batteries, but not that many non-engineers really know about capacitors. Even so, big capacitors - aka supercapacitors or ultracapacitors - have just gone mainstream.

It was announced last week that the Honda FCX will be sold to ordinary drivers from 2008. Like most other fuel-cell vehicles, the FCX needs buffer storage for electric power between the fuel cell and the motor: and the FCX uses an ultracapacitor rather than a conventional chemical battery.

Electrical storage is needed because fuel cells can't put out a lot of peak power for a given weight. Over time, Honda is confident its hydrogen cells can supply enough energy to meet the demands of an ordinary motorist; but those demands aren't evenly distributed. Going uphill and overtaking, a driver needs to take juice fast; when idling or freewheeling, he or she hardly needs any; when braking, energy should ideally go back into the system.

So the FCX needs a way to store electric power during periods of low drain or braking, so it will be there when required. In effect, the FCX is a hybrid design not unlike the Toyota Prius; it just replaces the hybrid's petrol engine with a fuel cell.

But chemical batteries aren't really ideal for this kind of use. They hold a lot of energy as electrical storage devices go, but they don't like being made to give it up fast. Worse, repeated charging and discharging wears them out quickly.

Enter the ultracapacitor, which doesn't hold anything like as much juice but laughs at charge cycles and high drain. MIT electrical and electronics boffin Joel Schindall writes about ultracapacitors in this month's IEEE Spectrum, explaining his own lab's efforts to make capacitors hold a lot more energy than they formerly could.

"It's almost engineering heresy to suggest that a capacitor could power a car," says Schindall. "The common capacitor stores a puny amount of energy... a chemical battery can store at least a million times as much."

He goes on to say that if you used ordinary capacitors in a torch it would go out after a second, and that even the best modern ultracapacitor tech would only give two minutes of light.

That doesn't sound too good for FCX drivers; any hill that takes more than a couple of minutes to climb could see the Honda's reserve power exhausted and the car slowing to a crawl.

But Schindall reckons his team has a solution. "We believe that within a few months we'll be able to demonstrate results that outperform today's designs by a wide margin," he says.

The lab boys and girls reckon they can do this by covering the surfaces of their ultracapacitor electrodes with a forest of - you guessed it - carbon nanotubes, creating a seventies-carpet style structure dubbed "electric shag".

This greatly increases the electrode surface area for a given bulk and weight. As capacitors store all their energy on their electrode surfaces, this is a good thing.

"We think... we can create a device that can hold up to 50 per cent as much electrical energy as a comparably sized battery," says Schindall.

"This feat would allow ultracapacitors to supplant batteries in a number of mainstream applications."

He goes on to mention other approaches taken by competing ultracapacitor teams, but curiously makes no mention of Honda's FCX kit, developed inhouse by the Japanese motoring giant.

It seems comparatively unlikely that even Schindall's ultra-ultracapacitors will serve to power cars on their own. Fully-electric vehicles will presumably continue to need the greatest total storage they can get, and that will mean batteries. Even fully-electrics, however, might use ultracapacitors to smooth out power demands and eliminate unnecessary charge cycles.

As for hybrids, by Schindall's analysis one might suppose that vehicles such as Toyota's Prius hybrid should really have been using ultracapacitors instead of batteries for some time. But Toyota's vision is, rather, to move toward the so-called "plug-in hybrid". Here the vehicle has enough storage capacity that it becomes worthwhile to charge up from the grid overnight, with the engine perhaps only coming into play occasionally during long journeys.

Critics of that method - including Honda, funnily enough - say it makes little sense to haul a seldom-used engine and tank of petrol to and fro on the daily commute.

Google, however, endorses the plug-in concept, saying cars parked at the office could pour stored juice back into the grid during the day and minimise the use of polluting gas-turbine power stations.

Interesting times ahead for tech lovers, anyway, as the competing philosophies slug it out. ®

*The term high-capacity seems to have been avoided, for obvious reasons

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