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I am using a flooded deep cycle lead acid 24v 450ah battery bank. I don't want to discharge them beyond safe levels so they can last longer. I don't know how to check the state of charge. I have a 3000 watt 24 volt 120v reliable electric pure sine wave inverter.

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Get a hydrometer and learn how to use it for detailed analysis.

For less detailed analysis, if a lead acid bank is below 2.1 volts per cell (in use) the smart money is on cutting it off if you care about long life. So if your 12 or 24V bank actually reads 12 or 24 volts, you are probably dipping below a healthy discharge level.

Better quality battery operated equipment will disconnect itself, rather than depend on you checking the voltage and taking it off-line. But we have no idea what your inverter is since you have not told us, and if it's a lower-end model it may not have a very good behavior for when it "alarms." (Even with the model information it may be hard to know unless it happens to be a well-known quality model with good documentation. Cheap junk tends not to document the things that reveal it to be cheap junk.)

Much better quality battery operated equipment will record the number of amp*hours put in and taken out (and know how big the battery bank is) rather than using the voltage as a rough yardstick.

This website provides some more food for thought, but practical and perfect don't tend to get along well in real life battery bank use scenarios. i.e. you are unlikely to disconect it for 4 hours before checking the voltage, at least in normal use. Occasionally for better numbers when doing maintenance, perhaps.

...And voltage on the whole bank does not give you the detailed info that checking each cell will - if you don't have 12 individual 2 V batteries making up your bank, the only way to get that into is with a hydrometer.

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  • Plus 1, but a warning about acid may be a good idea... – Solar Mike Mar 27 '20 at 10:02
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    Man, are people still selling the leftover brown LSD from the original Woodstock??? :-) I tend to think that the battery manufacturer has already plastered a bunch of warnings about the sulfuric acid in the batteries, on the batteries, and if you don't read those, not much that I write will likely make a dent. Sure, fine, wear gloves and goggles and don't smoke or otherwise ignite the hydrogen gas, and all that other stuff it says on the batteries. If feeling particularly wary have some baking soda handy to neutralize, but keep it OUT of the batteries unless you like expensive doorstops. – Ecnerwal Mar 27 '20 at 14:24
  • Thank you, but I want to be able to check the state of charge while everything is running. – Superhuman Mar 27 '20 at 16:11
  • If (or since, I guess) your inverter/charge controller does not have one, you can add a separate "Coulomb Counter" meter which tracks charge in and out of the battery - they work well with Lithium batteries, not very well with lead acid, since lead acid has a lot of sysem losses (charge out will always be considerably less than charge in.) – Ecnerwal Mar 27 '20 at 22:44
  • What inverter model? the low voltage cutoff may well be adjustable. – Polypipe Wrangler Mar 29 '20 at 1:35

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