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Thread: Battery Question

  1. #1
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    Default Battery Question


    One of the two interstate 27 marine batteries in my boat has gone bad. They are several years old, but the other battery still charges and shows 12.5 volts. I was looking at replacing the bad battery with an Everstart 27 marine battery. Do I need to replace them both or can I replace just the bad one with a different brand of battery. Thanks for help in advance

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    It depends on how you are using them. What is connected to which battery..
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    These are both connected to my Ultrex trolling motor

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    A battery that only gets to 12.5V is shot.

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    Quote Originally Posted by CatFan View Post
    A battery that only gets to 12.5V is shot.
    ??

    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.

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    I would replace both. Less problems later on. And with batteries in parallel, the weaker one will most likely pull the stronger ones down, thereby reducing your run time. Good luck.
    Likes Kevin22, SpeckledSlab LIKED above post

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    Quote Originally Posted by CatFan View Post
    A battery that only gets to 12.5V is shot.
    Thats true as 12.5 volts isn't fully charged.
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    Quote Originally Posted by MDCrappie View Post
    ??

    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.

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    , I too would replace both. For me it would be the knowledge that both were same age and the extra assuredness they were as strong as possible on the water.
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    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.

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