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Old 21-03-2023, 18:07   #1
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Isolating 2 Parallel Batteries

Hello to this forum and need some advice. I have a new 2 bank shore charger and 2 batteries in parallel. How can I isolate the batteries to charge each bank separately? I have a battery switch with 1...Both...2...Off. Would I switch it to Off or switch it to 1 or 2 to isolate these batteries just when charging with the 2 bank charger? Thanks for any advice you can give.
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Old 21-03-2023, 19:18   #2
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Re: Isolating 2 Parallel Batteries

If the batteries are, in fact, in parallel then the battery switch has no effect regarding their connection to one another.

Please clarify how the batteries are connected (or not):
  • to one another
  • to the battery switch
  • to the battery charger
  • to anything else in the boat
Alternately, you could describe the outcome you wish to achieve and what you have to work with.

So far I think you have two batteries (unknown type and voltage), a two-bank battery charger that presumably matches the type and voltage of the batteries, and a 1-2-both-off type battery switch.

I also think you are trying to charge each of the batteries separately with the two-bank charger.

If that is correct then
  • Tie all of the negatives on the batteries together. They do not go to the switch.
  • Connect the #1 cable on the switch to the positive post of whichever battery you want to be #1.
  • Do the same with the #2 cable and the #2 battery.
  • Connect the output of the switch to your positive bus.
  • Connect the #1 positive and negative charger cables to the positive and negative posts of the #1 battery.
  • Make the corresponding connections on the #2 battery.
  • Put the battery switch in any position except both. "Off" has definite advantages; "1" or "2" may be slightly less advantageous. "Both" negates the entire point of the exercise (that's not to say the batteries won't charge).
  • Verify that the charger is set to the correct voltage and battery type
  • Plug in your charger.
If you have any other devices that you've not told us about then the answer may be (wildly) different.

This is not in any way a comprehensive tutorial on how to connect a battery switch and/or a shore charger. It is not the only way and it is not even necessarily the best way to do so. You should consider getting some local (perhaps even professional) help if you are not sure of what you are doing. You can hurt your vessel, yourself, or others if you don't know what you are doing.

Other posters: please chime in if you see anything wildly inaccurate in what I've said.
"If I were in a hurry, I would not have bought a sail boat." Me
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Old 21-03-2023, 19:25   #3
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Location: Annapolis, MD
Boat: Sabre 34-1 (sold) and Saga 43
Posts: 1,387
Re: Isolating 2 Parallel Batteries

Most two bank chargers are not really that sophisticated. They have a single charge wire internally, that then goes through two diodes to feed the two charge wires. In addition, the rated capacity of the charger is a total capacity, which can go to either or both of the charging outputs.

If your batteries are normally hardwired as a pair in parallel, you could either run the two charge wires from the battery charger one to each battery, both to a single battery, or just run one wire. The net effect would be the same for any of those arrangements (using both outputs does not get you any extra capacity).
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