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3 batteries 1 alternator

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Old 04-21-2008, 06:04 PM
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Default 3 batteries 1 alternator

Can anyone help me with the wiring layout? I want to add a 3rd battery - I already have 2 on a 1/2/all switch. I beleive they are running parallel now. Can I add a 3rd battery to one side also in parallel or will that double my volts?

Looking for the easiest installation with out doubling volts.


I know isolators work great - which I want to add as well, however I need to extend the amount of time I can run a battery with out switching oand or starting to recharge. Hense I want to run one side as one big battery.

Last edited by JBoat; 04-21-2008 at 06:08 PM.
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Old 04-21-2008, 06:31 PM
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Parallel doubles the amps, series doubles the voltage. Why not just get bigger capacity batteries?
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Old 04-21-2008, 06:57 PM
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Limited room in the hatch - I can fit one more of what I have off to the side, Plus a bought a fresh one so I have the extra sitting around.

Can I run the third in parallel if it is already parallel to the other battery ? Does that even make sence ?
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Old 04-21-2008, 07:39 PM
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I did this in my last boat. I wired it so that the "1" was connected to one battery and the "2" was connected to the other two batteries in parallel (i.e. negitive to negitive and positive to positive).

I installed a high-capacity alternator. My 502mag came with a 65amp alternator, and I upgraded to a 100+amp alternator.

As I drove the boat down the lake in the morning, I would put the Perko switch to "all". When I stopped, I put the switch on "2". I ran the stereo all day on "2".

At the end of the day I knew I had a "reserve battery" to start my engine. I would move the Perko switch to "1", and get back to port/trailer.

Note - If you run your stereo all day and drain the two batteries in parallel, don't try to run the engine on "2". The heavy drain may fry your alternator. Get back to port/trailer on "1", and use a battery charger to charge the two parallel batteries overnight.

Good luck!
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Old 04-21-2008, 08:42 PM
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Run an Isolator on the 2 batteries and it will charge the battery that needs it the most.
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Old 04-21-2008, 09:04 PM
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Yes you can run 2 in parallel. As said before batteries in series raises the volts.You want to do parallel unless you want to fry all of your 12 volt system.
I always heard that running 2 batteries in parallel the weaker one will kill the stronger one, It will kill the longevity of the better battery.
I ended up putting in another selector switch and bigger amp alt. For this you just run a short wire from one all terminal to the other all terminal.Then number one terminal to the 3rd postive battery. Then you just parallel your grounds to the third battery. You are probably seeing the ground wire in parallel on your 2 batteries.
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Old 04-21-2008, 11:09 PM
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If you parallel................MAKE SURE they are exactly the same batteries!!!!!!!!!!!!!!!!!!!!!
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Old 04-22-2008, 01:18 AM
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hey im glad I noticed this thread and I dont wanna hi jack it or anything but i have a question since everyone is on the subject.. so I have a total of 6 batteries with 2 1/2/all switches and on (3 batteries per side) one side drains completely while the other side doesnt I keep it on 2 while achored and playing music then switch it to 1 when im ready to go but now one side wont start and the other side starts fine (twin engine set up) so then i have to use jumper cables to get the other side started what gives? any ideas? thank you so much very any pointers.
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Old 04-22-2008, 06:43 AM
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Fox I am guessing that you do not have a ground wire between your two motors. In other words you have a ground from your starting batteries that runs to your port motor and the house ground is probably running to your starboards motor but you may have removed at one time or broke the ground between the two motors. Hard to know for sure but that is what ussually causes that problem.
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Old 04-22-2008, 12:30 PM
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All 3 batterys are teh same make model and size.

So never run the switch on all. If I do 1 its the single(starting bat) and move to 2 its the dual set up in parallel that I can drain all day.

Did I get this right ? And on the dual ( side 2) set up I am just going pos to pos and neg to neg and leave the rest be ?
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