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From: jmuchow@SPAMMENOTcamlight.com (John Muchow)
Subject: Re: Reducing contact resistance for low volt use?
Date: Fri, 13 Dec 2002 07:34:10 GMT
Organization: MindSpring Enterprises
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X-Server-Date: 13 Dec 2002 07:34:32 GMT
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>>>I have been looking at the discharge curves for battery packs thinking
>>>about what a translation routine would look like.
>>>The most important value, the total capacity of the battery, is
>>>computed by multiplying Amps with time in the constant current test.
>>>In the constant load test it is a little more complicated but no
>>>We take the value of voltage and resistance for each ten second
>>>measurement and compute each such period separately and then add all
>>>these periods. The total capacity and the time when the voltage has
>>>fallen to a certain value can be found with this method. A special
>>>routine is used for the first 10 seconds where the curve has another
>>>shape, but a very easy shape to compute as it is a straight line.
>>>The shape of the curve is computed with one routine for the first 10
>>>seconds and another routine for the rest of the curve.
>>>Use the values for 10 sec point and the end point where the voltage
>>>has fallen to .9Volt, to choose a typical constant current curve which
>>>Check manually (visually) that the discharge curve from the constant
>>>load test has the typical shape to know that the translation can be
>>>There are some more details to work out but it is no problem to write
>>>such a translation routine.
LOL, of course!
It's been a while, but that sounds like calculus. Working out the
area under a curve by dividing it up and adding up the area of a bunch
of (essentially) rectangles....if you chop the curve up into enough
I think we'd only have to compute the area underneath each segment of
the curve and not have to use different calculations for different
portions of the curve. If we divide it into enough pieces that is.
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.
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
discharge 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.
It may not be enough of a difference to matter, I don't know. I think
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.
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