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
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