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  1. #1
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    Charging parallel batteries & Connecting trolling motor to parallel batteries

    I've been searching and finding posts that say it's ok either way but if I have 2 12v deep cycle batteries in parallel can I connect the battery charger to one battery and charger both until they are full? Or will it only fully charge the battery it's connected to and leave the second battery less than full?


    Similar question, does it make a difference if I connect my trolling motor to one of the batteries? Or to the positive of one and negative of the other?

    I've seen some people post saying it makes one large 12v bank so it doesn't matter how you connect it and I've found illustrations, websites, and videos saying you have to connect trolling motors and chargers to both batteries. I'm not concerned with the extended charge time just want to fully charge the batteries and be able to use both batteries.

    No point in carrying the extra weight if I'm not connecting them correctly.
    Last edited by chrisg_sdcf; 08-14-2019 at 06:30 AM.

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    #2
    If they are in parallel, with both positives connected together, and both negatives hooked together, it's just like one (big) battery. It will charge and discharge just like one. There can be some equalizing at rest, or when not being charged or discharged, but if they are both healthy, and of similar cycles, I see no issues.

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    #3
    The key here is the capacity needs to be closely matched between the 2 to make any charging/discharging hookup work well(2 batteries of equal age and listed capacity). I would go by what places like Battery University or other sources of that level of expertise say. I have my opinion but everybody has one. Use a few expert sources from your searches and go with it. Bob

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    #4
    Do not know about the trolling motor hookup. I had two gp31s in parallel for a few years for start and electronics. I hooked the charger to + on one and - on the other. Never had a problem, always charged
    Ed R.


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    #5
    They are both the same size, same brand and same age

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    #6
    To charge two healthy lead acid batteries that have been discharged in parallel, and using only one charging source, better to connect charger positive to one battery (+) and the negative to the other battery (-) .

    Similarly split the connections of the load (through trolling motor's OCP device) separating the connections to different batteries....just as large parallel battery power backup facilities are designed.

    Until end of life, always being kept in parallel, both deep cycles will continuously be at same state of charge regardless of their individual capacities, each contributing their appropriate share to power the load. Always kept in parallel, it is impossible to discharge one more than the other with a trolling motor.
    Last edited by Lou r Pitcher; 08-15-2019 at 04:43 PM.

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    #7
    All good advice, and it can be done.

    But in the long term, normally one battery will fail before the other and cause issues. Also I don't recommend it with todays smart chargers, not so much that they would not charge but some of the other advanced features. It really depends on the charger if it matters or not.
    And in rare instances and depending on how much they are drained some chargers may have a problem charging two 12volts in parallel due to the capacity.

    This is what I did when I ran two batteries in parallel for a trolling motor.

    I installed a blueseas battery selector switch. This switch gives the ability too select battery 1, 2, Both or Off.

    My charger was hooked the terminal output to the trolling motor. In this way I could charge each battery individually or both at the same time.

    The main reason I did this was for kind of a reserve gas tank on a vehicle. While fishing if trolling motor started to get weak I would switch over. The second reason was buy charging individually I would get an indication on my smart charger of a failing battery, it would not tell me that when charging in both at same time.

    I do not recommend this way with a digital trolling motor with spotlock (not sure if a 12volt motor with spotlock is even made), due to the charger is hooked to the trolling motor.
    Last edited by moetorola; 08-15-2019 at 02:48 PM.

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    #8
    Right now I have the trolling motor connected to battery 1 and the charger connected to battery 2 and both batteries are connected positive to positive and negative to negative with 4ga tinned copper wire. I'll have to lengthen either the positive or negative cable for the charger to extend to battery one and then buy another 4ga wire to connect the the trolling motor either positive or negative to battery two. Not too difficult, just have the batteries on opposite sides of the boat to balance the weight.

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    #9
    It can be frustrating but projects have a way of snowballing even when we don't screw up! Bob

  10. Member wmitch2's Avatar
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    #10
    If you're running a 12v trolling motor you can charge two 12v batteries connected in parallel by hooking the pos of one and to the neg of the other. You can NOT charge them like that if you're TM is a 24v as they should be connected in series ! You need a pos and a neg from the charger to each battery.
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    #11
    Right, it's a 12v motor. Thank you for the information though.

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    #12
    Either way will charge the battery, with neither being incorrect. I've always put the positive on one, negative on the other -- but that's because it just looks neater to me. In working with starters and alternators, you'll find 18wheelers which require a lot of amperage to turn the starter connected to the same battery, the two more in parallel with that battery (positive to positive to positive and negative to negative to negative). The alternator charges is great and the starter all works great. The same principle applies here. However, with that in mind, be aware that a bad battery in the system becomes a load and will draw power trying to equalize from the other batteries in the parallel system. A more efficient way in a two battery system is use a battery selector switch. This isolates the batteries, when one runs low, simply turn to the other battery and you have a full battery again to work with. When charging, change to both and the charger will charge each battery in parallel.
    John
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    #13
    Quote Originally Posted by Jonestrollingmotor View Post
    Either way will charge the battery, with neither being incorrect. I've always put the positive on one, negative on the other -- but that's because it just looks neater to me. In working with starters and alternators, you'll find 18wheelers which require a lot of amperage to turn the starter connected to the same battery, the two more in parallel with that battery (positive to positive to positive and negative to negative to negative). The alternator charges is great and the starter all works great. The same principle applies here. However, with that in mind, be aware that a bad battery in the system becomes a load and will draw power trying to equalize from the other batteries in the parallel system. A more efficient way in a two battery system is use a battery selector switch. This isolates the batteries, when one runs low, simply turn to the other battery and you have a full battery again to work with. When charging, change to both and the charger will charge each battery in parallel.
    John
    So charging them this way works, how about running the trolling motor hooked up to one battery (both batteries connected but trolling motor hooked up to one battery) I'm new to this but would both batteries, the trolling motor, and the charger all connect to this switch ? Can you send me a link to the switch you are referring too?

    Thanks for the help

  14. Member wmitch2's Avatar
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    #14
    Quote Originally Posted by chrisg_sdcf View Post
    So charging them this way works, how about running the trolling motor hooked up to one battery (both batteries connected but trolling motor hooked up to one battery) I'm new to this but would both batteries, the trolling motor, and the charger all connect to this switch ? Can you send me a link to the switch you are referring too?

    Thanks for the help

    It used to be when in parallel, you used the Pos of one Battery and the Neg of the other. I still do it that way, but now it's said you can do it either way.
    Ranger Boats / Mercury Motors
    G Loomis Rods / Shimano Reels
    Raymarine / MinnKota Ultrex
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    Pepper Jigs / Robo Worms
    Troll Bridge / V-T2 Vents


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    #15
    I am running my trolling AND sump pump batteries in parallel.

    I connect one charger clamp to each battery, and I alternate.

    I use a hydrometer and a SOLAR BA9 to determine battery health.

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    #16
    You can charge in parallel and do so with different size batteries. In parallel, they will both charge and discharge proportionally. But both batteries need to be the same type (both AGM, both wet cell, etc.)

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    #17
    Quote Originally Posted by skid00skid00 View Post
    I am running my trolling AND sump pump batteries in parallel.

    I connect one charger clamp to each battery, and I alternate.

    I use a hydrometer and a SOLAR BA9 to determine battery health.
    Look at it as running the trolling motor and the sump pump off one big battery, chargeable by placing the charger on a positive post and a negative post.
    John
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    #18
    Quote Originally Posted by Jonestrollingmotor View Post
    Look at it as running the trolling motor and the sump pump off one big battery, chargeable by placing the charger on a positive post and a negative post.
    John
    And it doesn't matter which positive or negative since the two batteries are now one? (Besides the fact one battery could die sooner)

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    #19
    Quote Originally Posted by chrisg_sdcf View Post
    And it doesn't matter which positive or negative since the two batteries are now one? (Besides the fact one battery could die sooner)
    Correct
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    #20
    Not trying to hijack this thread, but if I have 12 v terrova and parallel series setup do I need a off/on breaker to charge with the selector set on both for charging so the charge won't get to the terrova?
    thanks

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