Quote Originally Posted by CatFan View Post
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.
But you see what you are ASSUMING is that the 12.5 is what the battery "is charging to". He did not say that. He said " They are several years old, but the other battery still charges and shows 12.5 volts". Now if that 12.5 is immediately after removing the charger (which is not the correct way to check a batteries voltage) then you would be correct. If that 12.5 is after he has removed the float voltage then that battery is NOT shot and likely still has plenty of life left. Hence my comment of "Need more information before making assumption".
BTW - I'm not confusing anything, I'm just not making assumptions with too little information.