I disagree partly with Byron. The load does indeed have a significant
effect on the terminal voltage, but you can fairly well characterize this
effect if you know the load.

As Russell says, the best way is to measure current in and out, measure
temperature, measure terminal voltage, and put all these together with a
mathematical model of the battery. For one particular type of battery, with
decent experiments to find the model parameters, that should be able to get
you to better than 5% accuracy.

Another important caveat is that terminal voltage versus SOC depends not
only on load current but also it has hysteresis. The curve for positive
current and negative current are different and there is hysteresis between
them.



On Tue, Nov 11, 2014 at 2:09 PM, Byron Jeff <byronjeff@mail.clayton.edu>
wrote:

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