Alt-BEAM Archive
Message #02076
To: "Paul B. Webster VK2BZC" pwebster@clinipath.com.au
From: Steven Bolt sbolt@xs4all.nl
Date: Wed, 7 Apr 1999 09:43:42 +0200 (CEST)
Subject: [alt-beam] Re: Microcore memory for 100Farad caps.
On Wed, 7 Apr 1999, Paul B. Webster VK2BZC wrote:
> > Let's say 3V: 100 * 3 = 300 ampere * second
> > Translate that to wattsecond by again multiplying by volt: 900ws
> > Convert that to milliwatt-hour: 900 * 1000 / 3600 = 250 mAh
> > Which equates to two AAA nicads.
>
> Eh? Let me try that. 100 Farad is it? It=CV
>
> Let's say that we can afford a drop from 3V to 1V, so C*V = 100 x 2
> which is 200 Ampere-seconds. Divide by 3600 to get Ampere-hours and
> scale up by 1000 to get milliampere-hours, gives 83 mAh.
Thank you Paul, I stand corrected. mWh obviously isn't equal to
mAh. I wanted to get to energy and then lost it completely (other
distractions, early morning, getting old?). Sorry about that.
But I see that you can also miscalculate: 200 ampere-seconds
divided by 3.6 is actually 56 mAh :)
I prefer mWh because a capacitor doesn't pay in mA. Almost the
full discharge of a nicad happens at 1.2V; a capacitor gradually
drops to zero. When Vcc drops from 3V to 1V, the current
consumption of an innocent 74HC pulse generator drops from 40uA to
1uA, as we can see in the table recently posted by Wilf Rigter.
A fortyfold difference.
The little geared motors I like to use go from about 28mA to 14mA,
a less dramatic percentage but plenty in absolute value.
Note that as the voltage drops from 3V to 1V, almost 90% of the
available energy is used: 1/2*100*9/3.6 = 125mWh, and at 1V we
leave behind 1/2*100/3.6 = 14mWh
So imho caps and nicads are best compared by looking at mWh,
keeping in mind that most applications will do better with the
constant voltage discharge of a nicad.
An exception might be the small model aircraft. The high power
first part of a capacitor discharge allows a nice climb, followed
by a few circles at more or less constant altitude and a gradual
descent.
I've seen a model fly on a mere 3.3F for about half a minute. The
cap was charged to about 4V, making 1/2*3.3*16/3.6 = 7.3 mWh
available. About 75% of the energy contributed to the flight (while
the voltage dropped from 4V to 2V).
> The big question is indeed internal resistance.
That seems to depend mostly on the shape of the plates. In the
high, narrow cilinder type (as used in that model aircraft), the
ratio of surface area to circumference is high, making room for a
fair amount of current. In the flat pillbox type, the plates are
very long and narrow, creating a high internal resistance.
Btw, I can't help but wonder what happens if a 100F cap is
overcharged. Nicads are supposed to vent the overpressure and die.
If they don't vent, they explode like grenades, but that is
fortunately a rare occurrence. Caps don't have a vent and are
presumably more likely to explode. I saw it happen once and was
quite startled by the bang of 100uF at 20V, a mere 0.006 mWh. That
cap was intended for max. 16V.
100F at some overload, perhaps 4V would contain 220 mWh. I guess
(and certainly hope) that the natural current limit would prevent a
too sudden release.
Best,
Steve
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# sbolt@xs4all.nl # Steven Bolt # popular science monthly KIJK #
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