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Old 07-17-2019, 04:39 PM   #1
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Drawing down solar bank

We are using FLA batteries for our solar battery bank (12v, 4x230ah). I understand that drawing these down under 50% of their capacity is not good for their longevity.

The inverter and solar controller both have a remote display that tells us how many volts are hidden away in the batteries, their temp and stuff like that. My question is, how do we know when they are getting close to 50% drawn down?

I did read something about checking the specific gravity of each cell (24 in all) and adjusting for their temp to get the actual number. Is there some other number we can look for on the displays that gives us a clue without going through checking each cell?
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Old 07-17-2019, 05:02 PM   #2
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Grab yourself a battery monitoring system off of Amazon. I got a Victron, but there are many others out there....some cheaper, some more expensive. It gives us a pretty reasonable idea of the state of charge just with a quick glance. I've also got a hydrometer to check the SG of the batteries, but I only use that occasionally.
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Old 07-17-2019, 06:12 PM   #3
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A correspondence table between voltage and SoC would require at least a few hours rest, as voltage is varyingly affected by loads and sources' current rates. Also

good for that battery only

change as it ages

only approximate best of times.

For a lead battery the most accurate SoC meter is Merlin SmartGauge, but it does not show amp rates or Ah in and out.

Coulomb counting SoC meters require a fair bit of attention

Go here https://marinehowto.com And CTRL-F for battery monitor and read closely those three articles.
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Old 07-17-2019, 09:01 PM   #4
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Basically, the deeper the discharge the shorter the life.

About 12.2 volts is considered 50% charge in a lead-acid battery (12.5v in AGM). Deep cycle batts have a finite number of deep cycles available (from 100% to 20% and back to 100%). You could get, say, 150 deep cycles from your bank and maybe 5 times that at a 50% discharge.
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Old 07-18-2019, 01:40 AM   #5
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It's not black and white but

99% of even top-notch quality deep cycling batt models should only be drawn down much below 50% in rare extenuating circumstances.

30% is better than 40% average, 70% is worse than 60% etc
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