Alt-BEAM Archive
Message #05178
To: beam@sgiblab.sgi.com
From: Steven Bolt sbolt@xs4all.nl
Date: Fri, 9 Jul 1999 11:46:12 +0200 (CEST)
Subject: [alt-beam] Re: Supercaps (a strange tale)
On Thu, 8 Jul 1999, Bob Shannon wrote:
> > In coin type caps with a large diameter, the strips are very long
> > and narrow --> high internal resistance, not good for the usual
> > BEAM applications.
>
> Actually the large supercaps use bulk porus (sintered) material
> so that the energy storage happens throughout the bulk material
> rather than on the surface of the foil (technically the space
> between the foil).
Now that you mention it, that micro-porous surfuce does ring a bell.
The relationship between internal resistance and shape is however a
fact, discovered - to our annoyance - by Dave H, myself and others.
So my explanation (correct for oldfashioned caps) doesn't hold much
water here. But the same volume of porous stuff should have about
the same properties, no matter what shape - within reason. So what
gives? Doing something I should have done earlier...
...I just cracked open the case of a flat (7mm high), large
diameter (20mm) 1 farad supercap, known to have rather excessive
internal resistance. What do I find?
Inside the case is another case, looking much like a button cell
battery. Cutting away the shrink-foil...
...And out pop two very flat button cells! Each 2mm high, 18mm
diameter. Both behave like large capacitors. So what I have here
are two caps of 2 farad each, put in series to jack up the voltage
rating. The thing was rated at 5.5V, so presumably the coins can
take 3V each. And the internal resistance is just about low enough
for a very light photovore using a monostable-based SE. Hmmm.
Live and learn. Thanks for making me take that cap apart, Bob!
However, an explanation for the difference in internal resistance
between tall and flat supercaps is now harder to dream up. Perhaps
the tall ones are monolithic, allowing more active substance per
unit of case volume. There is very little of it in this one; less
than 0.6 cm2 (button diameter less squeezed rim just 14mm) inside a
case volume of 2.2 cm2.
Best,
Steve
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# sbolt@xs4all.nl # Steven Bolt # popular science monthly KIJK #
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