Dual Battery Setups.


When attempting this type of project, there are several safety factors and issues that you must be aware of. It is possible to construct this project incorrectly, with the result of having a dangerous boat.

The US Coast Guard publishes regulations covering the engineering, construction, and installation of these systems. These regualtions are Federal Law, and can be found in 33 CFR 183. The reader must comply with all aspects of 33 CFR 183, as well as any other applicable law, regulation, or standard.

The reader of this web page acknowledges that they bear all risk associated with constructing such a project, and agree that the author, web host, and any other party are not responsible for any action or consequences the reader takes as the result of reading this information.



The first topic to discuss is a review of how automotive alternators and voltage regulators work. While this is not a white paper on the operation of automotive engine-based charging systems, a short reminder of how they work is appropriate.

The alternator is a device that runs off the engine which produces AC voltage. This voltage is rectified to DC and controlled by a voltage regulator. The regulator can be either attached to the alternator or a separate item. The regulator also senses the battery voltage and adjusts its own output to charge the battery as needed. In effect, the regulator is also a battery charger. This issue will have more importance once battery isolators and combiners are discussed.

The basic engine/battery circuit is shown here. While this seems simplistic, it provides a foundation for further discussion. Note that many marine engines include the main DC breaker for the boat's DC power (shown in the yellow circle in the photo below). Federal Law (33CFR183.455) requires that a form of overcurrent protection, such as a circuit breaker or fuse, be located near the power source. On smaller boats, the circuit breaker on the engine typically satisfies this requirement.


Often, the boat owner does not feel comfortable with a single battery, and for safety purposes decides to upgrade their electrical system with dual batteries. A dual battery approach provides a spare battery for engine starting, or if wired appropriately, separate batteries for engine and house (everything else) needs, with the house battery having the ability to backup the engine battery.

When deciding on a dual battery setup, there are several general designs that can be used. These are:

We will discuss each type, building on the knowledge of the previous one. Regardless of which type you intend to build, you should read each section in sequence, since important issues are discussed in each area which may not be repeated.