How do you install a second marine battery?

When it comes to wiring and mounting specifically. How do you install a second battery in a boat?


  • It's super easy to install a secondary battery. You'll want a marine battery switch in order to switch between both if you dont plan to run them parallel. Make sure you have the correct guage of wire. Connect your black wire first, Your red wire from each battery should go to each individual terminal on the battery switch. Then take the wire that was connected to the original positive terminal on the battery, and connect that to the remaining positive terminal on the battery switch. This will allow you to switch between both batteries without worriying about a discharge leaving you stranded.

    When it comes to the mounting options. Pick up a universal battery box (make sure your new battery fits) and they are designed to be mounted to basically anything. So utilize the space you've got as long as its readily accessible and keep the switch in a good location near both batteries.

  • @LifeIsGood I found this awesome and very helpful YouTube video. I am definitely a visual learner so this helped me understand installing a second marine battery. Its very straight forward and easy once you know what needs to go where.

  • @InsuranceBrokerDon, @BabyShark82 thank you so much for all the help. That video tutorial made all the sense and I needed the visual to best understand installing a secondary battery in my boat.

  • Here is more info and a full walk through of different ways to install a second marine battery in order to achieve different things.

  • edited November 12

    Is the reason behind adding a second battery to your boat for running more electronics or is it to have backup power, or perhaps both? The only reason I ask is because the wiring could change between each situation. If you install a battery bypass switch, you can isolate between batteries or run them parallel.

  • Here are some wiring diagrams that should help install a second marine battery to your system. I'm not sure if you have one motor or dual outboards but here is a diagram for both.

    Single motor dual marine battery diagram.

    Dual motor, dual battery diagram.

  • @KingFisher808 It would be dual marine batteries and a single outboard. I'd like a starting battery and a running battery for my electronics etc. I'd also like the starting battery to be protected from draining while out on my boat in case I leave something on. It looks like with the battery switch listed above and your first wiring diagram, I should be able to accomplish this fairly easily. Thanks everyone for the advice.

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