[RE-wrenches] Evaluating battery health

[William Miller]

Friends:



Evaluating the state of health of a battery array is one concept I struggle
with.



It pretty easy with flooded batteries—charge them up and check SG.  Sealed
batteries are different.



I do track “at-rest” voltages of discrete 2 volt VRLA cells.  This
generally can’t be done for monoblock batteries.  I see trending towards
greater deviation over time.  For example a UnigyII array I am currently
working with is about 8 years old and has been worked hard.  Recent at-rest
voltage readings show a deviation of 8%.  Four years ago it was 2%.  How
much deviation is too much?



The O&M manual for UnigyII batteries suggests making ohmic measurements.
My research indicates this requires a rather expensive meter, such as a
Hioki 3554.  I suppose it may be cheap of me to not invest in one of these
meters and I am still considering doing so.  Have any of you worked with
one of these meters?



Here is another idea:  As we have discussed here, I am not a fan of battery
monitor modules in a battery inverter BOS package.  I am an Outback
integrator so the option is the FNDC.  The FNDC will generate SOC values
which are often inaccurate and confuse my clients.  These inaccuracies
occur when the programmed efficiency does not reflect an accurate value.
The eff will change over time, with different ambient temperature and at
different states of charge.  (Another problem with the FNDC is it has only
3 shunt inputs.  This can be difficult to implement if you have a large
battery array and many inverters and charge controllers.)



My idea is to turn this concept around.  Instead of calculating SOC one
could use the ratio of AH in to AH out over time to determine the actual
efficiency of a battery array.  Battery efficiency seems like as good an
indicator of battery health as any other value.  It is easy to obtain data
files for the FNDC in CSV format.  (I suspect this is kind of like what is
done internally for the Sunny Island battery health reading.)



Have any of you pondered these questions?  I am interested in your thoughts.



TIA.



William



Miller Solar

17395 Oak Road, Atascadero, CA 93422

805-438-5600

www.millersolar.com

CA Lic. 773985
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20240309/a7616adb/attachment.htm>