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Old 10-28-2018, 08:57 PM   #1
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optima battery how to tell when fully charged

I have optima batteries, and would like to know how to tell when they are fully charged. I have heard that voltage is not an accurate way to tell, and because it is an AGM battery it is not possible to use a hydrometer. My understanding of a battery monitor is it counts what goes in and what goes out, well enough if the battery is starting at a full state of charge, of course that gets me back to what is a full state of charge? So what am I missing here?
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Old 10-28-2018, 09:21 PM   #2
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good point.Very hard to tell. For lead acid they "invented" float charge. That is a voltage of 13.5 to 13.8 volt at 25 degree Celsius. At that voltage it keeps charging a small amount all the time. The excess electricity is wasted in heat and off gassing. For AGM and zero maintenance the gasses are recombined in the battery so that no electrolyte gets lost.
So after your charger has bulk charged , topping charge and float chargedyou can consider it full.


If you have a true battery monitor that counts ah in and ah out... it needs to have a shunt in series with all loads and all chargers...then you can check the real capacity of your battery by discharging to 11.6 volt with a relative low current . only once in a while because discharging 100% is bad for your lead acid battery.
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Old 10-28-2018, 09:27 PM   #3
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You measure declining amps while holding Absorb voltage.

When it gets to .005C they are Full 100% and you drop to Float.

That is half an amp per 100AH.

Old worn batts may not get there, so can use

amps drop less than .1A in an hour

instead.
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Old 10-29-2018, 08:00 AM   #4
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Originally Posted by john61ct View Post
You measure declining amps while holding Absorb voltage.

When it gets to .005C they are Full 100% and you drop to Float.

That is half an amp per 100AH.

Old worn batts may not get there, so can use

amps drop less than .1A in an hour

instead.
Ok this makes sense to me.

Does the battery offer more resistance to charging as it gets full? and that is why the charge rate drops?
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Old 11-08-2018, 11:01 AM   #5
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I came across the following link that may be helpful - very long and detailed

https://marinehowto.com/programming-a-battery-monitor/

Toward the end it talks about how to know your battery is at 100% so that you can reset your Ah meter.
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Old 11-08-2018, 01:05 PM   #6
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Originally Posted by Ronnie View Post
Ok this makes sense to me.

Does the battery offer more resistance to charging as it gets full? and that is why the charge rate drops?
Exactly. And MaineSail's site above is well worth close study
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Old 11-08-2018, 01:43 PM   #7
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Thanks, am looking at the link now. It certainly is detailed, and so far seems to be what I am looking for.
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