How to Use Two Marine Batteries in a Boat

 Marine batteries are generally lead-acid and deep cycle types are used in trolling motor because of their strength and their large storage capacity. A bank of batteries, which is two or more stacks connected together, can be connected to one of two ways.

Use Two Marine Batteries in a Boat

Use Two Marine Batteries in a Boat

  • Decide what you need from two marine batteries you are fitting in your boat. You may be installing them to provide greater voltage than is available from a single battery, or you can install them to increase the amount of time current 12 volts is available for you without charging.
  • Connect the batteries in series, which means attach the cables connecting the positive pole of one battery to the negative terminal of the next, if you want an increased supply of volts. Caravans usually use 12 volt devices and wiring, designed to accept only 12 volts, so this would not be the suggested connection method.
  • Attach the batteries in parallel by connecting the cables from the positive pole of one battery to the positive terminal of the next and the negative pole of one battery to the negative terminal of the next if you want an increased ampere-hours delivery. Typically, a boat uses the power of 12 volts to operate lights, outlets and equipment, such as water pumps and entertainment systems, for several hours, so this would be the suggested connection method.
  • Connect the batteries with four gauge wire, red for positive and black for negative. Use proprietary battery terminal connectors at both ends of each wire length and tighten the connectors with a wrench. These materials are readily available in all auto parts stores.
  • Make sure the batteries are held in place against movement when the boat is moving. Straps and fasteners are available from most auto parts stores.

Tips and Warnings

AMP hours is a technical term describing usable current over time. If your battery bank can provide power for six hours to equipment that attracts 8 amperes per hour, you have a bank battery of 48 Ah (6 x 8 = 48). The more amps-hours you store, the more you can camp in your boat van without 12-volt power loss.

Always use batteries that are identical in size, type, age and capacity when connecting a battery bank. Incompatible batteries will cause a discount or overcharging to decrease the life expectancy of the battery and increases the risk of overheating and explosion.

Always wear appropriate protective clothing when working with acid-filled batteries and be aware of the risks of gasses emitted by batteries.


Connecting a marine battery differs little from connecting a car battery, although marine batteries use a slightly different type of post, so this is faster and easier to exchange a flat battery for a new one. This is important because the marine environment can be very difficult on batteries, and replacement is needed more often than it is in cars.

How to Connect

  • Remove the wing nuts that are threaded on the battery terminals by turning them to the left. Although these are usually made from corrosion-resistant materials, they can get caught up in place occasionally. If this happens, try turning the nuts with a pair of adjustable clamps. Be careful not to force the nut and bend off one of the tabs. If the wing nut does not light, put a few drops of penetrating oil or a silicone spray on the posts and let it sit for a while before trying to remove them again.
  • When the wing nuts have been removed, lift the battery cables. The battery cables in a boat will have lugs mounted on the ends so that they slide over the battery posts.
  • After you have replaced the battery, slide the battery cables over the studs. Check that the red cable is on the (+) positive terminal, and the black cable to the negative (-) terminal after.
  • Put the wing nuts on the posts and tighten them by turning them to the left.

Tips and Warnings

When removing the battery cables, take the negative cable first. This will avoid the possibility of shorts.

How to Install a Marine Battery Selector Switch

Adding a marine battery selector for your boat will help guard against a flat battery. The selector allows the power of a battery to use, while the other charges the battery and stays at full capacity. Switching from one battery to another reduces the risk of accidental grounding. By alternating power drain between two batteries, the total power drain is distributed so that neither the battery loses its charge. Battery switches can be purchased at most marine supply stores.

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Hi! I'm Liam Oscar, and this is where I spend most of my time attempting to publish valuable articles for my readers! This site contains all of the information that I believe we all need to know about batteries, chargers, and other accessories.

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