Quote Originally Posted by MDCrappie View Post
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Need more information before making that assumption. First off are the batteries flooded? Gel? or AGM? If 12.5 is the voltage immediately after removing the charger (the floating voltage) then sure it's gone. But if this is a flooded lead acid and that is the standing voltage that battery is still in the 85-90% capacity range. If they are Gel or AGM they are at about 1/2 life.

To the OP - If this is not how you are testing..... Charge the batteries. Remove charger and run your trolling motor for about 10 seconds. Wait a minute and then check the voltage. Turning the trolling motor on for a few seconds will get rid of that floating voltage and you will get a much more accurate measure of the batteries capacity.
Doesn’t matter what kind of battery or how long it’s been off the charger. If it won’t charge to 12.65 or more it is junk. Low full charge voltage indicates a significant loss of sulfur in the electrolyte. Likely from sulfation, but maybe loss of electrolyte.

You can’t estimate capacity by charge voltage. Charged voltage is a function of electrolyte specific gravity and internal resistance. A battery can be at 25% of its original capacity and still charge to the same voltage as when it was new. A reduction in voltage indicates a battery with damage of some type.

You are confusing state of charge with capacity. A conventional flooded cell battery at 12.5V would be at 80% state of charge, but that does not mean it’s at 80% capacity. It would track pretty close with a new battery, but as it ages there is no relationship.