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A Direct Current (DC) Electrical System

August 1, 2016




Paul Esterle has been building or repairing watercraft, of all descriptions, for longer than he cares
to admit, from hovercraft to power and sailboats. Paul specializes in boat improvement and repair
projects utilizing wood, epoxy, and fiberglass.If you have any questions about your boat project,
contact Paul at pesterle@comcast.net.

The original DC electrical system aboard Ternabout was a joke. A two-conductor lamp cord wire ended in the starboard stern locker, where it was supposed to connect to a battery. There was no battery box or other means of mounting a battery on the curved bottom of the locker. A single toggle switch turned the navigation lights off or on while each of the two cabin lights also had their own switches.



The first thing I did was to move the battery location forward to the old icebox. The icebox was too small and too poorly insulated to be of any real use, but it was just the right size for a group 24 battery box and battery. (The group defines the size of a lead-acid battery, as in Group 24, 27 31, 4D and 8D, an 8D battery which weighs almost 160 lbs.!)


The next piece of equipment I needed for my DC system was a power distribution panel. I wanted fuses, switches, a 12-volt receptacle and a battery test meter. I was able to find a Seadog panel at a reasonable price. It had three lighted rocker switches with associated fuses, the battery test switch and associated meter as well as a 12-volt cigarette lighter receptacle, all mounted on a compact aluminum panel.

There were no hollow spaces aboard Ternabout where I could mount the power panel. I decided to cut the mounting hole in the bulkhead between the vee-berth and the galley. This placed the power panel conveniently above the battery box location. The only problem was that the guts of the power panel extended into the vee-berth area.


I ended up making a simple box out of scrap mahogany and plywood to cover the exposed wiring and buss bars. I hinged the box to the bulkhead so it could be opened to access the backside of the power panel.


As mentioned, existing wiring was two-conductor household lamp wiring. The years of use had made the insulation brittle and the stranded conductors corroded. The other problem with the wiring is that much of it was placed between the hull and deck molding and inner liner, making it impossible to access for replacement.


I wanted new wiring so the only solution that I could come up with was to use external wiring conduits. The only suitable materials I could find at the time were 1/2” PVC water pipe and fittings. These went together easily as well as hiding and protecting the wiring. Since then rectangular plastic wiring chases have become widely available at home improvement stores,and if doing this job today, I would certainly use them.


I used marine grade wiring for all the replacement wires. This wire is tinned so it doesn’t corrode as bare copper does. The back of the power panel came equipped with male spade lugs. Wiring up the panel was a simple matter of crimping the female spade lug on the wire and sliding it in place. I did use the crimp connectors with the heat shrink tubing and adhesive sealant. These are a little more expensive but guarantee no water will ever find its way into the connection.


NOTE: Old marine DC systems used red wire for positive and black wire for negative, THIS IS DANGEROUS! AC systems also use black wire and mixing the two could be fatal. Use marine DC safety wire, with red for positive and YELLOW for negative.


I also made up a charging cable for the battery. It is a length of two-conductor wire hooked to the positive and negative terminals of the battery. The other end is a polarized 12-volt connector. My battery charger has the other end of the polarized connector attached to its charging cable. To charge the battery, I lift up the edge of the galley board, pull out the cable and connect it to my charger.


The 12-volt cigarette lighter socket comes in handy for powering 12-volt accessories. I have a 12-volt fan and a portable 12-volt bilge pump. I do have to be careful in using the battery, as I have no way to charge it away from the dock. I plan to add a solar charger at some point in time.


Battery Selection

I mentioned that I elected a group 24 battery for the system. Lead-acid (as well as Gel cell and AGM) batteries come in three different types. The most common is called a starting battery. It is the battery that you have installed in your car and is use to start a motor.


The battery is optimized for that purpose and has thin battery plates inside. These allow a great deal of current to be drawn for a short period of time. After the car is started, the battery is then recharged from the alternator. It is not designed to be significantly discharged; doing so will limit the life of the battery.

A second type of battery is the deep-cycle battery. This type of battery is designed to be discharged more deeply than a starting battery and to survive many more discharge cycles than a starting battery would.

Many larger boats have two or more batteries arranged in banks. One bank would consist of a starting battery used to start the engine. Another bank would consist of deep cycle batteries used to supply DC current for electronics, lights and the like.


A third type of battery is a dual-purpose battery that combines elements of both previous types. It won’t start as big an engine as a starting battery or provide DC power as long as a deep cycle battery. It is a good compromise where there is only room for one battery and there is a modest sized engine and modest DC power requirements.


Since Ternabout’s engine wasn’t electric start, I opted for a group 24 deep-cycle battery.



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