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Thread: Draw on twin batteries.

  1. #1
    Administrator Gary's Avatar
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    Draw on twin batteries.

    I have two batteries in my van, which are connected and charged in parallel.

    My question is:

    - Is it best to connect my inverter (rated at 2500 watt continuous, so very large amp draw) to just the one battery?

    - Or is it best to connect the inverter to positive on one battery and negative on other?

    Obviously the cabling linking the batteries is a suitable size.

  2. #2
    Forum Fixer OffRoadDave's Avatar
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    I'd connect it to the largest battery since it will be able to supply more amps for longer, and let the cables between the two batteries carry the rest of the current drawn from the second smaller battery.

    Sent from my Nexus 6P using Tapatalk

  3. #3
    Administrator Gary's Avatar
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    Quote Originally Posted by OffRoadDave View Post
    I'd connect it to the largest battery since it will be able to supply more amps for longer, and let the cables between the two batteries carry the rest of the current drawn from the second smaller battery.

    Sent from my Nexus 6P using Tapatalk
    Both batteries are identical.

  4. #4
    Forum Fixer OffRoadDave's Avatar
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    It doesn't really matter which way you do it then. The linking cables will be carrying the same current, just in different directions depending on which way you connect them.

    The only suggestion is make the cables to the inverter long enough to reach both battery terminals, just in case you need to pull a faulty battery out of circuit and run with only one temporarily.

    Sent from my Nexus 6P using Tapatalk

  5. #5
    Administrator Gary's Avatar
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    Quote Originally Posted by OffRoadDave View Post
    The only suggestion is make the cables to the inverter long enough to reach both battery terminals, just in case you need to pull a faulty battery out of circuit and run with only one temporarily.
    Thanks Dave. Yeah the cables will easily reach either battery, so I should be right if one plays up.

  6. #6
    Junior Member ANT's Avatar
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    I’m gonna disagree. It will matter a lot. If you draw unevenly from batteries in parallel then they will always try to equalise their voltage. This constant lack of equilibrium will cause them to both draw current for charging as well as put out charge for use at different rates. If they are linked in parallel this will create heat and potentially a fire. It will also degrade both batteries rapidly. Batteries in parallel should be used as if they were one battery. I’m not an auto electrician but I have nearly burnt a caravan to the ground by not knowing enough about 12v. A bit of research will go a long way.

    Bottom line: it’s your van!

  7. #7
    Forum Enthusiast DrewXT's Avatar
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    As ANT says, you should to treat both batteries as a single battery with 2 cells, or you'll end up killing one of them.

    Used to be very commonly seen when I was a rep for Exide.

    Batteries should be in parallel if they're 12v, with near positive and far negative posts connected (if that makes sense)...

    Same site discharge, as it allows better balancing, and more efficient charging.

    I had to rewire my entire camper as it was a horrible 12v mess when we bought it.

    Your parallel connection should be in the largest practical cable to reduce resistance as much as is possible between the cells

    Sent from my Nexus 6P using Tapatalk
    Last edited by DrewXT; 20-10-17 at 10:38 PM.

  8. #8
    Administrator Gary's Avatar
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    Quote Originally Posted by DrewXT View Post

    Batteries should be in parallel if they're 12v, with near positive and far negative posts connected (if that makes sense)...
    Yes it does make sense.

  9. #9
    Administrator Gary's Avatar
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    Quote Originally Posted by ANT View Post
    Iím gonna disagree. It will matter a lot. A bit of research will go a long way.

    Bottom line: itís your van!
    Hense the reason I asked.

  10. #10
    Forum Fixer OffRoadDave's Avatar
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    Quote Originally Posted by ANT View Post
    Iím gonna disagree. It will matter a lot. If you draw unevenly from batteries in parallel then they will always try to equalise their voltage. This constant lack of equilibrium will cause them to both draw current for charging as well as put out charge for use at different rates. If they are linked in parallel this will create heat and potentially a fire. It will also degrade both batteries rapidly. Batteries in parallel should be used as if they were one battery. Iím not an auto electrician but I have nearly burnt a caravan to the ground by not knowing enough about 12v. A bit of research will go a long way.

    Bottom line: itís your van!
    Im not sure what you're disagreeing with, Gary has stated the batteries are in parallel and are linked together with sufficiently sized cables.

    In this setup, it doesn't matter if he connects to battery A, battery B, or the positive to battery A, and the negative to battery B or vice versa, the same current will be flowing through the two cables linking the two batteries together in any of the scenarios I mentioned.

    The key is to make sure the cables linking the two batteries together, are of a size that can carry the current drawn by the inverter, this will keep the batteries balanced.

    Sent from my Nexus 6P using Tapatalk

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