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Old 05-18-2021, 12:14 PM   #1
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Battery State of charge

The date on my 6v Interstate batteries is 2014, started to be used late 2015, now 6+ years old. They are maintained and connected to solar.
They have been disconnected for a few days while doing other work, so I measured them using a voltmeter they are 12.65 volts. Some charts say this is close to 100%, Trojan chart shows about 90% and others show 60%.
I know they are getting tired, due to replace, not sure yet. Does anybody have advice on what 12.65 really means?
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Old 05-18-2021, 12:47 PM   #2
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It depends

Complex topic. Open Circuit Voltage (OCV) gives a crude state of charge (SOC) value. Lead acid batteries are often tested when under load for truer determination of overall capability.


12.65 OCV is pretty good. Different values are given by different manufacturers for % SOC based on OCV. 6 year old batteries are getting somewhere towards end of useful life, but if they've been kept charged with solar all the time, that helps. Deep discharge of lead acid batteries shortens their life.


Best if you read up on the topic, and there's lots of venues available.
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Old 05-18-2021, 01:14 PM   #3
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Quote:
Originally Posted by Chris R View Post
The date on my 6v Interstate batteries is 2014, started to be used late 2015, now 6+ years old. They are maintained and connected to solar.
They have been disconnected for a few days while doing other work, so I measured them using a voltmeter they are 12.65 volts. Some charts say this is close to 100%, Trojan chart shows about 90% and others show 60%.
I know they are getting tired, due to replace, not sure yet. Does anybody have advice on what 12.65 really means?
My bet is you have some more life in them. I'd be interested in what a hydrometer tells you. The specific gravity of the electrolyte in each cell will tell you more than just a voltmeter and you can see if there are any significant differences between the cells. Also I never thought about this until now but you should be able to put the batteries under a load test like the auto shops do to a suspect car battery. Harbor Freight makes a unit that also tests 6V batteries.
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Old 05-18-2021, 01:23 PM   #4
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You could reconnect them at night and turn on a few things- fan, lights, etc- and see how fast that voltage drops. They should be able to sustain a load. Mine kept measuring fine but then would run down really fast- I finally figured that out when I turned the fan on while watching the plug-in voltmeter.
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Old 05-18-2021, 06:08 PM   #5
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Originally Posted by rubicon327 View Post
My bet is you have some more life in them. I'd be interested in what a hydrometer tells you. The specific gravity of the electrolyte in each cell will tell you more than just a voltmeter and you can see if there are any significant differences between the cells. Also I never thought about this until now but you should be able to put the batteries under a load test like the auto shops do to a suspect car battery. Harbor Freight makes a unit that also tests 6V batteries.

Thank you, I forgot I had a hydrometer, I wonder what other things I have forgotten. Just tested each cell, they are all midline in the green, all the same.
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