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From: Roger Johansson
Subject: Re: Reducing contact resistance for low volt use?
Date: Fri, 13 Dec 2002 11:16:39 +0100
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jmuchow@SPAMMENOTcamlight.com (John Muchow) wrote:
>Hmm...with the data logger going at 1 voltage measurement per second
>(and knowing the fixed resistance), I could do the math for each 1 sec
>segment of the curve....adding up a bunch of watt-second readings.
It should be enough to have something like 4-10 seconds slices, take
the average of the start and end voltage of each section and use that
voltage value for that section.
>Oops, wait a sec. I don't know if a cell is linear in it's response
>for changing currents, especially where it starts to fall off near the
>end of the curve. The discharge time will be shorter with a CC
We can choose a constant load which gives the same discharge time in
total. We choose the load to give a curve which is a little higher in
the beginning and a little lower at the end, the total measurement
should not give much different results.
>and the cells may not respond the same way as a longer CL
>discharge. The higher cell temperatures for the faster CC discharge
>may influence the resistance of the cell.
There is no reason for CC to be faster in total, we can adjust the
load to give the same total time as a comparable CC test.
The method might need a bit of tweaking, ya know :-)
You might even want to consider changing the constant load in the
middle of the measurement, after 3 minutes, to compensate for
differencies at the end and the beginning.
Use the parallell port of a modern PC and you can have both inputs and
outputs so you can switch in a higher resistance in parallell with the
main load, to adjust it a little.
When the main load is connected all the time you need much simpler
circuits to adjust it during the measurement.
Hey, this could be made into a constant current method again!
If we use a "constant load" copper bar which gives the right current
at one end of the curve, we can switch in other loads in parallell to
make the curve essentially a constant current test.
Use 4 mosfets to add 4 other resistors during the test, that will
adjust the curve enough to be a "constant current" curve.
>It may not be enough of a difference to matter, I don't know.
That is what I think too.
>I would need to work up a couple of easy low-current CC and CL
>circuits and check out the results. Gotta get a data logger going
>first though...or do it manually.
A simple A/D converter for the parallell port for the computer and a
small program in your preferred language.
I still use Borland Turbo Basic so I might not be the authority on
programming around here :-)
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