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Batteries in parallel

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  • Batteries in parallel

    Can you really wire a battery bank of deep-cycle gel (or AGM) batteries in parallel to act as "one big 12V battery"? I always thought that car batteries (even deep cycle) would discharge each other somehow, and that's why battery isolators are required in marine applications where you want to charge both batteries.

    In my boat I can select Bat1, Bat2, or briefly, BatAll and both batteries get charged via a battery isolator between the alternator and the batteries, but this has a voltage drop due to the diodes used.

    If I have 4 AGM or gel batteries, but only a 12V charge controller, can I really connect the charge controller to 4 batteries connected in parallel?


  • #2
    You can but it is a poor practices.
    MSEE, PE


    • #3
      So if I want to stil with 12V should I buy 6V batteries and go in series, or is there some kind of battery manager device that will rotate amongst them without a significant voltage drop? I google'd around a fair bit, but I must not know the right key words to look for when it comes to proper construction of a battery bank!


      • #4
        Dave you buy the amp hour capacity needed so as to only make one single string. If you need 1000 AH at 12 volts, quit thing 12 volt batteries and start thinking 4 and 2 volt cells in series to make 12 volts or whatever voltage is needed, 2 volt cells goes as large as 4000 AH.

        FWIW 12 volt systems are for toys and RV's
        MSEE, PE


        • #5
          I'm only trying to bank and store enough power during the day to run a phone system and security system, not trying to MIG weld off-grid.

          So really, if I could get 150W of amph panels here in Seattle to a 3A average for 10 hours a day (optimistic) that's only 30 amp hours anyway. That'd all fit in a single AGM battery anyhow.


          • #6
            Does not matter what the size is, one single string of the appropriate Amp Hour rating. If you can do that with 12 volt batteries fine and dandy. Just avoid parallel strings if possible.

            FWIW I am a former Telecom power engineer and we used 48 volt battery plants at up to 20,000 AH .
            MSEE, PE


            • #7
              Do you know the actual power consumption daily for the phone and security system?

              Once you know this you can work out the battery capacity required, this will determine your battery set up.


              • #8
                What type of a security system? If it is using RF devices then hardly anything.

                If on the other hand you are using light barriers and cameras significant power is needed.