Got used Clark, how to tell when battery is at 20% capacity?

Got a used Clark NP-300-30B with 12-125-13 battery, 750 ah @6 hrs. We think the Clark was taken out of service somewhere maybe 2 mos. ago. No idea what charge was on battery when we got it a week ago, but it showed 24 volts on built-in voltmeter when ignition is on. We have run it for about an hour and meter still shows 24 v. When idle, but that drops down when motors running.

How do I tell when it is down to 20 % charge so I will park and charge it

You can check voltage and specific gravity.

Lead acid batteries typically are dead at 1.75 volts per cell and fully charged at 2.25 volts per cell as a generic statement.

You can contact the battery manufacturer are get a data sheet specific to YOUR battery that will have all of this information.

You can make a spreadsheet to enter the VPC numbers from the data sheet for full charge and discharged then multiply those numbers by the cell count to get actual voltages.

You can take the difference between the 2 voltages and divide that be 10 then create a column starting with the lowest number then next row is formula =(above row) +(whatever your divide by 10 number is) then cop that down 9 rows.

This will give you a general close enough voltage yo estimate the state of charge based only on voltage.

Best is just charge at end of shift/day/when done using and if parked for extended time then charge at interval manufacturer recommended maintenance.

Batteries not cheap and can be ruined by in proper care.

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