Hi William, Thinking out loud here, and I am guessing that there are reasons why not to do this. But what about putting a voltage tester across each battery and running a quick test? I mention this because just a couple days ago, I went to check on a grid tied battery backup system where capacity was much less than expected. It is two strings of the Outback Northstar Blue+ batteries with a Radian inverter. There should be about 9kwh of usable energy at 50% DOD. I was able to get the data logs and found that on a recent grid outage the inverter put out about 3kWh of energy before it shut off loads when the batteries got to 48V. These batteries are only about 3.5 years old. Their resting voltages were all pretty close (although, they were only resting - not floating - for about 10 minutes)
But then I put a 100A battery tester across each battery. While most of them dropped from about 13.8V to about 12.8V, I found one that dropped to 11V. There was also another that dropped to about 12.3. So, those two batteries seem suspect to me. This is not a perfect method, but seemed to quickly identify a couple batteries with issues. Cheers, Dave On Sat, Mar 9, 2024 at 3:01 PM William Miller via RE-wrenches < re-wrenches@lists.re-wrenches.org> wrote: > 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 > > > > > _______________________________________________ > List sponsored by Redwood Alliance > > Pay optional member dues here: http://re-wrenches.org > > List Address: RE-wrenches@lists.re-wrenches.org > > Change listserver email address & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > There are two list archives for searching. When one doesn't work, try the > other: > https://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/ > http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org > > List rules & etiquette: > http://www.re-wrenches.org/etiquette.htm > > Check out or update participant bios: > http://www.members.re-wrenches.org > > -- [image: Logo] <https://www.sungineersolar.com/> Dave Tedeyan, P.E. Owner | Sungineer Solar p: he | him | his a: 1653 Slaterville Rd. | Ithaca, NY 14850 w: www.sungineersolar.com <http://www.sungineersolar.com/> c: (607) 270-0370
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