- 1). Place the second and/or third battery in line with the stock battery in your boat, depending on how much space you have for extra batteries. You want to align the positive and negative terminals on all of the batteries in the same orientation. This way, the wiring runs will be shorter and easier.
- 2). Disconnect the positive and negative terminals from the batteries using your hands. Marine batteries have wingnuts to hold down the terminals, so twisting them counterclockwise will let you remove the terminals correctly.
- 3). Loop the battery cables from each terminal to the next. Meaning, you connect the positive terminal on one battery to the positive terminal on the next. Then secure each terminal down using the wingnut and your hands. When you're done, the negative terminals of the two to three batteries will be connected with cabling and the same with the positive terminals.
- 4). Connect the positive lead for the boat to the positive lead of the original battery and the negative lead for the boat to the negative lead of the original battery using your hands. The boat will now see a 12-volt load at the first battery, but the amperage will be multiplied by the number of batteries in line.