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Thread: Batteries parallel, charging different size group(27 & 24) is it ok

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    Default Batteries parallel, charging different size group(27 & 24) is it ok


    Just spent 3-4 hours researching online and read allot of opinions, thought I would ask fishermen they'll know.

    14' gruman(x-wide/deep & pull start 20hp), last year had a battery at both ends and got tired of the one in the back plus most of the weight is in back(me/Bat/motor). Going to put both 12v batteries upfront in Parallel to even out the weight.

    From research some say you shouldn't charge 2 different size bats in parallel while others say it won't hurt a thing. I'll have a 6 mth old group 27 and a new group 24 running the TM, finder, and livewell.

    Does anyone see a problem charging them both in parallel since the AH will be different.

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    Default Batteries parallel, charging different size group(27 & 24) is it ok

    I would think the larger AH battery would charge quicker than the smaller. From the way I understand charging 2 different size batteries it’s like filling up a swimming pool. All the water goes to the deep end 1st. But, I don’t think it would hurt anything. If your group 24 is new and it damages it then swap it out before the 12 months is up.


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    Never thought to put this in the 1st post. But has anyone who paralleled 2 batteries of different groups or AH had a bad problem.

    Read one report that sometimes "salesmanship" can influence advice, they will say when replacing one replace all to make sales. That makes sense cause a salesman wants to sell batteries. The OPTIMA site said that, if replaceing one replace all.

    Another thing I read that all agree on is not to mix different types like lead acid, gel, or AGM, that makes sense. I'll be using 2 wet 12v.
    Last edited by Charly; 04-05-2019 at 03:28 PM.

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    Quote Originally Posted by shmelton View Post
    I would think the larger AH battery would charge quicker than the smaller. From the way I understand charging 2 different size batteries it’s like filling up a swimming pool. All the water goes to the deep end 1st. But, I don’t think it would hurt anything. If your group 24 is new and it damages it then swap it out before the 12 months is up.
    Sent from my iPhone using Tapatalk
    I like your layman explanation, easy to understand. You are saying exactly what I read at this site but simpler

    Electrical Myths, Part 3: Mixing Batteries of Different Ages/Capacities - RV Nerds

    Let’s suppose you have 3 different 12V batteries, wired in parallel to supply 12V power to your RV. They can have different capacities on account of size or age, but the same chemistry (e.g. all flooded lead acid or all AGM). Before you start charging, the voltage across each of them is the same–even if one is fully charged and the others aren’t. Charge will flow from one battery to the other two until they’re balanced. With a lead acid battery bank, the internal resistances are limiting to a point that you don’t have to worry about arcing or your battery cables overheating when you connect them (not the case with lithium-ion banks…).

    So when we start charging, all of the battery banks are very close to the same point as far as state of charge. Each battery acts like a resistive load, and current will flow to the battery with the lowest resistance, or highest capacity, more than the rest. All of them will reach the end of bulk charging together, when they’ve all reached the same voltage


    Last edited by Charly; 04-05-2019 at 03:59 PM.

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    skeetbum is offline Crappie.com Legend - Moderator Jig Tying Forum
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    Cat fan is the man to ask. Batteries are his living and he’s an electrical engineer.l
    Creativity is just intelligence fooling around
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