Battery Monitoring

It should work reasonably well for gel cells, particularly if you know
what the
current drain is at the time of measurement.  If you don't know the
current
drain (especially if the instantaneous drain might be high), you may
not get
there.  It's a pretty straightforward application of Ohm's law - the
terminal
voltages drop by the IR-losses within the batteries.

Hope that helps!
    -f

On Mon, 9 Mar 2009 10:50:45 -0400, "John O"


You probably need to look at the discharge curves for the battery
you are using. If you don't have a curve, then generate one by
putting the battery under load and measure its output voltage
over time as it discharges.

Yes, I have that curve.

There are commercial testers which work by applying a (modest) known load
and measuring the before and after voltages. I assume they work by looking
at the change in battery voltage after a defined quantity of charge has been
removed ...

Dave

John O wrote:

Yes, that's almost exactly what was suggested to me this morning. That type
of system may be more than I need, and it's certainly more expensive and
complex.

The system is self-powered, so the batteries are under a nominal load before
the sensors can work at all. I'm thinking that the voltage under such a load
is a "reasonable enough" predictor of charge status.

-John O

The reasons for applying a load to a PbA battery are to eliminate
surface charge. If you don't, voltage will not correlate at all well
to capacity. Here's a good explanation of surface charge:
http://batteryuniversity.com/parttwo-42B.htm  (See under
stratification. That's what they call it.) When you have a battery
with one or more "weak" cell(s), it will show 12-13V under no/light
load, but will drop sharply under a moderate or high load. A "weak"
cell is really a dead cell (electrodes are shot) that looks ok under
light loads only because it has some surface charge. Soon as you break
that down, voltage for the whole battery plunges. The reason the load
helps is that current flow causes mixing. From this you can also see
why the older the battery, the less reliable no/light-load charge
levels are as a capacity measure.

Even under a load, unless you're actually monitoring AHs in and out,
you'll never get more than a rough estimate. Actually, even with AH
measures, precision is just not very high. I did a fair bit of battery
testing back in the early 80s, while working on the electric-vehicle
program at jpl, and my experience was that battery measurements are
always very noisy because capacity depends on so many factors. (I got
to test drive an early prototype of the EV-1. I had no idea at the
time that it would someday be famous :)

Robin

On Mon, 9 Mar 2009 10:50:45 -0400, "John O"


The discharge curves for a typical lead acid battery have pretty
noticeable slopes for a given discharge rate, so it's not infeasible to
estimate the remaining capacity if you measure the voltage and discharge
current *and* the battery temperature. You might be able to infer the
current based on a known draw in a given state of the robot and assume
"room temperature" but measuring them both, in addition to voltage,
would be better.

The manufacturer may publish the necessary curves or you might end up
having to create them yourself. Good project...  ;-)

--
Rich Webb     Norfolk, VA

The problem of course is the variations in voltage that occur as the
battery
is loaded. My forklift for example will be reading 50% but lift a load
and move
the forklift and you have 10-25% on the guage.
So to have an sort of accurate estimation of battery life you need to
take a reading when applying a set load. You can then do a few
experiments
to see the actual time remaining of battery life.

You measure the voltage with no load. There are charts that describe total charge
left. For a given manufacturer and temperature, the voltage is fairly
predictable.
they sell those charge indicators for boats. they also include it on many
trolling motors. For a given load you can also predict voltage vs charge.
Measure the voltage directly on the battery terminals ONLY.
If the battery is old or defective, its likely the measurments will be off.

greg