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  1. #1
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    (2) 24 volt batteries in parallel?

    Ok electrical engineers chime in. Wanting to run (2) 24 volt batteries and have the capability to run one at a time or both to increase AH. Would use (2) 3 pole switches with a battery 1, battery 2, or both batteries capability. Will this work?

    57F9D4D9-539E-45CB-A30B-07358730CBD3.jpeg CCE209DF-596A-4C9A-9018-ACE9009951D6.jpeg

  2. Electrical/Wiring/Trolling Motors Moderator CatFan's Avatar
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    #2
    No need for two switches. Just connect the negatives together and to the load, then switch the positives as needed.
    If you have integrity, nothing else matters. If you don't have integrity,
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  3. BBC SPONSOR/ Shallow Water Anchors Moderator
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    #3
    Why would you want to run 1 at a time. Trying think if a good scenario why someone would want to.

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    #4
    Quote Originally Posted by KandKKustomz View Post
    Why would you want to run 1 at a time. Trying think if a good scenario why someone would want to.
    Maybe if he has a failure on one battery he could just turn the switch to run only one battery. It would only be half the AH but super easy to get straight back to fishing.

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    #5
    I have customers doing this with electric only boats with 2 24v trolling motors, and need to be able to send power from both batteries to the main motor to get back to the ramp
    Drewcraft Specializes in Lithium Batteries and Chargers, We carry all Brands of Trolling Motors and Graphs, Minnkota, Lowrance, Garmin, Motorguide
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  6. Member
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    #6
    It's cool, but with the abuse a lot of trolling motor batteries get I would only leave them in parallel while in use.

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    #7
    Just run them in parallel and be done. No need to isolate

  8. BBC SPONSOR/ Shallow Water Anchors Moderator
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    #8
    Quote Originally Posted by Mullins057 View Post
    Maybe if he has a failure on one battery he could just turn the switch to run only one battery. It would only be half the AH but super easy to get straight back to fishing.
    if you ran 2 in parallel if there was a failure on 1 he would still be running. Motor still getting 24 volts and nothing to switch. Just down half the ah. (That's double redundency) Only reason could see is what Andy described.

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    #9
    Quote Originally Posted by KandKKustomz View Post
    if you ran 2 in parallel if there was a failure on 1 he would still be running. Motor still getting 24 volts and nothing to switch. Just down half the ah. (That's double redundency) Only reason could see is what Andy described.
    A failure of one battery in parallel will run the other battery down even if they are not in use. Current goes from a high potential (voltage) source to a low potential source when connected. The "good" battery would try to charge the "bad" battery.

  10. BBC SPONSOR/ Shallow Water Anchors Moderator
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    #10
    Quote Originally Posted by billnorman1 View Post
    A failure of one battery in parallel will run the other battery down even if they are not in use. Current goes from a high potential (voltage) source to a low potential source when connected. The "good" battery would try to charge the "bad" battery.
    no because the bad batterys bms clicked off, rendering it as not even there. These are lithiums not agm or lead acid so that concept of thinking doesnt apply

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    #11
    Quote Originally Posted by KandKKustomz View Post
    no because the bad batterys bms clicked off, rendering it as not even there. These are lithiums not agm or lead acid so that concept of thinking doesnt apply
    The post author did not say Lithium, but I realize that is your area of expertise.

  12. BBC SPONSOR/ Shallow Water Anchors Moderator
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    #12
    Quote Originally Posted by billnorman1 View Post
    The post author did not say Lithium, but I realize that is your area of expertise.
    don't know of anyone running 2 non lithium 24 volt volt batteries in there boat.

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    #13
    I think this came from John Jones years ago. Not my original.

    Batteries don't blow, devices suck.

    I hope understandable.

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    #14
    Quote Originally Posted by MCIPinkie View Post
    I think this came from John Jones years ago. Not my original.

    Batteries don't blow, devices suck.

    I hope understandable.

    But John Jones was wrong!

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    #15
    If in a non critical application running two (or more) batteries of any type (Pb or Li) in parallel should be OK.

    If in a critical application, then I might want to keep each battery independent of the other battery(s). So that one of the two (or more) batteries can not adversely affect the other battery(s).

    Salt water boaters generally have two Pb batteries when boating far offshore. So that if one craps out the other can start the motor. There is talk of paralleling the two batteries to start the motor. But if I had one known good battery and one known bad battery, I will be damned if I am going to parallel the two.

  16. Electrical/Wiring/Trolling Motors Moderator CatFan's Avatar
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    #16
    Quote Originally Posted by yam911 View Post
    But John Jones was wrong!
    No. What it means is that the load determines the current flow, not a fixed voltage power source.
    If you have integrity, nothing else matters. If you don't have integrity,
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    #17
    Quote Originally Posted by CatFan View Post
    No. What it means is that the load determines the current flow, not a fixed voltage power source.
    It is the electrical pressure within the battery that is the impetus for the current flow through a load. The electrical pressure (voltage) within the battery is going to determine the current flow, based on the electrical load. Resistance of the load actually.

    The load can stay the same. As the voltage increases, so does the current flow through the load. As the voltage decreases, so does the current flow through the load.

    To use the sucking terminology, a piston does not suck air in. A piston going down reduces the air pressure within the cylinder. The ambient air outside the piston pushes air into the cylinder. It is the higher pressure air (just like the higher voltage battery or power source) that is doing the work.

    High to low.

  18. Electrical/Wiring/Trolling Motors Moderator CatFan's Avatar
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    #18
    Quote Originally Posted by yam911 View Post
    It is the electrical pressure within the battery that is the impetus for the current flow through a load. The electrical pressure (voltage) within the battery is going to determine the current flow, based on the electrical load. Resistance of the load actually.

    The load can stay the same. As the voltage increases, so does the current flow through the load. As the voltage decreases, so does the current flow through the load.

    To use the sucking terminology, a piston does not suck air in. A piston going down reduces the air pressure within the cylinder. The ambient air outside the piston pushes air into the cylinder. It is the higher pressure air (just like the higher voltage battery or power source) that is doing the work.

    High to low.
    Battery is fixed voltage, so the load determines current. Pretty simple.
    If you have integrity, nothing else matters. If you don't have integrity,
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    #19
    Let's burn this some more.

    Assume we have a 100AH battery and a 50AH battery wired parallel. At the end of the wire, we have 150AH available something to suck on. The consumer knows nothing other than we'll say 12 volts is available to him.

    Now that is 10A consumer. In theory, we have 15 hours of use available to him. When he sucks, does he suck more out of the big battery, more out of the small one, or does he suck 5A out of each? He don't know. He's just sucking 10A. IMHO (I'm a structural, not a EE), he's sucking 5 out each and the little battery will die after 10 hours and the big battery will die after 20.

    Really got no skin in the game. I'm just bored. I did notice that one of the big lithium suppliers has proposed a 50AH and a 125AH cranking system for those that aren't comfortable with 1 - 125AH

  20. Electrical/Wiring/Trolling Motors Moderator CatFan's Avatar
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    #20
    Quote Originally Posted by MCIPinkie View Post
    Let's burn this some more.

    Assume we have a 100AH battery and a 50AH battery wired parallel. At the end of the wire, we have 150AH available something to suck on. The consumer knows nothing other than we'll say 12 volts is available to him.

    Now that is 10A consumer. In theory, we have 15 hours of use available to him. When he sucks, does he suck more out of the big battery, more out of the small one, or does he suck 5A out of each? He don't know. He's just sucking 10A. IMHO (I'm a structural, not a EE), he's sucking 5 out each and the little battery will die after 10 hours and the big battery will die after 20.

    Really got no skin in the game. I'm just bored. I did notice that one of the big lithium suppliers has proposed a 50AH and a 125AH cranking system for those that aren't comfortable with 1 - 125AH
    With a 50AH and 125AH in parallel, the batteries will supply current proportionally. The 50AH will supply 50/175 or about 28.5% of the load, while the 125AH will supply 125/175 or about 71.5% of the load. Charge received is divided at the same ratio. The batteries would remain at very close to the the same state of charge at all times. This assumes they are the same technology and in the case of lithiums that the BMS settings are the same.
    If you have integrity, nothing else matters. If you don't have integrity,
    nothing else matters.​