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Dual Battery Charging

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  • Dual Battery Charging

    Looking for my options to charge a spare battery. I have a starting battery that is always hooked up during the season. I have a spare deep cycle that I would like to put a charge on every once in a while. What options do I have to do this while running? I might just get trickle charger but hate to be out and need a battery to remember I left it on the charger. I seen ACR but not sure how those work. What you all think? Thanks.

  • #2
    Hook up a duel battery switch with 1, 2, both, and off settings. Easy to install. Then you can use the "1" setting for your cranking battery to start it up, "2" setting to use the deep cycle if you are anchored and listening to music or whatnot, and every once in a while, use the both setting and the motor will charge both batteries while you are under way. Plus, when your not using the boat, the switch can be on off and won't cause any drain on either battery. You can also hook the bilge pump-if you have on with a float-right to the battery so it will always come on when needed, regardless of where the switch is!
    1989 ThunderCraft--Mercruiser 3.0

    2001 25' Crest Tri-Toon with a 2001 Mercury Saltwater 200hp


    • #3
      Good tip on the bilge pump but I don't use it since I don't take on water. But it would come in handy if I did spring a leak and not notice.

      I guess my main concern was charging both batteries at the same time since they were different. I know not to put a load on them but wasn't sure if charging them together would be ill advised.


      • #4


        • #5
          Originally posted by BullsBay View Post
          As per Captain John Wenz

          "If the switch is in the “BOTH” position and one battery is nearly at a full charge and one is flat, the alternator will “read” the higher of the two, and the flat battery will never be charged."

          This above statement by Captain John Wenz is definitely incorrect!!..........Less any differences in internal/ext resistances and possible temperature differences between each battery, for all intents and purposes, when switched into "both", both batteries are now paralleled together, as such, they are now "hard wired" as one BIG battery (2X the capacity if they're same capacity).

          It's therefor electrically impossible for the "alternator to “read” the higher of the two" since, as stated above, beings now they're hard wired in parallel via this new buss bar, there can't be "higher of the two" per-se because there is only ONE battery now. A bigger one but, none the less, just one!! Making his statement quite false!!

          When switched to both, at the second the switch/wiring connects both batteries together in parallel, they are forced to equalize their charge, this happens in milliseconds if wiring and internal resistance is small'ish.

          e.g., if one battery is higher in charge (not capacity) then the other, the higher of the two will always dump(dis-charge) and "equalizes" its charge with the lower of the two.

          If and when the engine starts up, all's the alternator knows is there is a battery out there and it starts to charge it.....One, two or twelve, if they are in parallel, it's either become a big, or it's a medium or a small battery depent on the switchs' line-up, it knows no better, it's always just one battery to the alternator and it charges it as best it can depending on it's new capacity.
          Last edited by sam am I; June 11th, 2019, 11:14 PM.


          • #6
            Trickle chargers are to replace parasitic drains while a battery is in storage. They are not a viable means of charging a deep cycle battery after each use.

            Unless you run the motor for long periods of time, using “both” to maintain two batteries isn’t really a viable option either. An on-board battery charger is your best bet.


            • #7
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              Ok thanks. I am thinking two battery isolator relays. With a three way switch; main on, both off, spare on. Any foreseeable issues?