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Dual Battery Setup ?


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I want to setup my boat with dual batteries. I currently only have one. Does anyone know how this is done, or do i need an electrician?

Is it recommended? I'd rather have the electricial gadgets running off a separate battery. Thanks.

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Majed hey,

Depends ultimately on what you wish the batteries to do for you and how they are to perform that role, you can either 1.set them up in parallel or 2.serial (3. would be a combination using something like a switch).

1. parallel means that effectively you are running the same current but have increased the capacity of the draw (or time it takes to go flat), 2,3, or as many batteries (of same size eg 12 volt) can be set up this way. it allows you basically to say have two 75amp hr batteries for a 12 volt system, totalling 150 amp hours but still pushing out only your 12vlts

2. serial, when in series the total amp-hour capacity of the two batteries together remains the same as the amp-hour rating of either one but the output volt-age is doubled. in our above example we would still have only 75 amp-hours but would now be pushing out 24volts.

3. probably the most common setup found in runabouts, 1st single battery on isolator switch (which allows connection to another battery in parallel or the second battery to run individually), using this idea i would suggest if possible, one battery to start the engine and the other (or other two) to run the house gear/lights/sounder etc when the engine is switched off.

The work isn't so tricky, laying lines etc isn't hard but getting the size gauge of the wire right (and not negating your insurance), wiring the gear up correctly so you don't blow all electrics once switched on etc can be fun. A good book i would recommend (cost me 80) is "Boatowner's Mechanical and Electrical Manual by Nigel Calder"

Hope this helps


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Get a three position battery isolator from one of the boat shops.

Before you go have a look at where the batteries will be mounted and where you would mount the isolator and measure the length between the two. You will be able to get the cables you need premade from the same shop if you know approximately what length you want.

The switch will have a common earth and two termianals for the positive battery leads 1 for the power out. This will allow you to run either battery individually, both together and also isolate them when you put your boat away stopping any residual current draw and stop the batteries going flat.



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