[LINK] ultracapacitors

[Karl Auer]

On Thu, 2007-09-06 at 12:53 +0930, Glen Turner wrote:
> On Wed, 2007-09-05 at 12:25 +1000, Karl Auer wrote:
> 
> > Well, it's certainly useful, and a big electrical zap is no more
> > dangerous than a petrol explosion - in fact, probably less so. So I'd
> > say it wasn't too far off the mark. We downgrade the risks we are
> > familiar with.
> 
> Hi Karl,
> 
> Not too sure about that.  Petrol has the nice property of
> needing ignition. A capacitor has huge voltage just sitting
> there waiting for a path to earth.  Touching such a car
> involved in a traffic accident might not be wise.

I'm not saying the electricity is NOT dangerous, just that it's a bit
hard to compare the two risks. Because we tend to downgrade risks we
have lived with for a while, people will tend to see petrol as safe and
batteries as dangerous. Some things to think about:

A liquid can move about without benefit of conductors; it can spread and
pool, endangering a relatively large area. Petrol has fumes, which can
be extraordinarily easy to ignite, and far easier than the liquid form.
Petrol pollutes. Petrol is not only flammable and explosive, but also a
poison. Petrol is a solvent.

The electrical bolt will tend to be all gone at once. If it discharges
through someone, it will tend only to damage that one person involved,
it won't e.g. take out the service station, and it is unlikely to affect
any other batteries, even quite close ones. The discharging battery is
not itself explosive - the danger is solely from the discharge, not from
heat, shrapnel, fumes, smoke, or impact (OK, heat/fumes/smoke are
maybes, a slow discharge might develop sufficient heat to start a fire).
The danger is highly localised, and conductors are relatively obvious.
The danger can be neutralised relatively easily by grounding the
wreckage. The danger can be relatively easily avoided by avoiding the
wreckage, and even a very small distance is safe, whereas to avoid a
dangerous petrol explosion you have to move *well* away. Capacitance
relies on very precise arrangements within the battery; the chances are
that the capacitive ability of the battery would not survive a serious
crash. Simple emergency grounding devices could dump the battery to
earth in case of impact, whereas there is no way to make petrol go
non-flammable on impact.

So the dangers are in both cases serious, but I would not say that they
dangers of the high-capacity battery are worse than those of the
ruptured fuel tank, and in some ways the batteries are are clearly less
dangerous.

Regards, K.

-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Karl Auer (kauer at biplane.com.au)                   +61-2-64957160 (h)
http://www.biplane.com.au/~kauer/                  +61-428-957160 (mob)