On Tue, Nov 11, 2014 at 10:15:08AM -0700, Dwayne Reid wrote:
> Good day to all.
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> I'm just starting a project which is powered by a 12V 18AH AGM Lead=20
> Acid battery.  The customer greatly desires a display that shows the=20
> amount of charge remaining in the battery.
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> I remember looking at this long ago and concluding that it was=20
> difficult to do when the battery temperature varies greatly above and=20
> below room temperature.  I'm hoping that someone 'out there' knows of=20
> some technology that does this.
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> Can anyone suggest either a simple method for me to do this myself=20
> or, better yet, point me towards an already existing off-the-shelf=20
> solution that I can purchase?  Quantity is low - only a few dozen units.
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> Many thanks!
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> dwayne

Dwayne,

The amount of usable charge is not only dependent on temperature but also
on current draw from the battery. That 18AH capacity is specifically at a
C/20 rate, which is 900 mA. Draw more and the capacity is lessened, whereas
if you draw less, there is more usable capacity.

The other major issue is that you have a sealed battery. So the only real
method of determining state of charge is to measure the open circuit
voltage (OCV). If any load, even parasitic ones, are applied during the
measurement, then any hope of getting an accuate state of charge is gone.
Of course this is a challenge because the likely purpose of the battery is
to in fact power the project.

The bottom line is that the best you can do is use the OCV. This document
shows a chart between OCV and SOC:

www.jaycar.com/images_uploaded/slabatts.pdf

Hope this helps,

BAJ

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> --=20
> Dwayne Reid   <dwayner@planet.eon.net>
> Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
> (780) 489-3199 voice          (780) 487-6397 fax
> www.trinity-electronics.com
> Custom Electronics Design and Manufacturing
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Byron A. Jeff
Chair: Department of Computer Science and Information Technology
College of Information and Mathematical Sciences
Clayton State University
http://faculty.clayton.edu/bjeff
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