Maintaining Your Boat’s Electrical System

The battery is the heart of a boat’s 12-volt electrical system.


The battery is the heart of a boat’s 12-volt electrical system. In it, a chemical reaction maintains a  potential difference or voltage, which “pumps” or pushes electrons around whichever circuits are switched on or are “closed.” No charges should flow through an “open” or incomplete circuit; if they do, you’ve got a problem, which we’ll get to later.

The storage battery’s chemical reaction is reversible, which means that it can be recharged, and will be continuously, as long as the engine is running and the alternator or generator is functioning properly. Normally, we take these things for granted, and are surprised when they don’t work-a testament to the reliability of the devices. The complete battery chemical reactions are complex, but can be simplified in a few words, to give us a clue as to how to keep an eye on the battery’s well being: When a battery is being discharged, lead and lead oxide both use sulfuric acid to form lead sulfate plus water, and the negative terminal gives off electrons.

Maintaining Your Boat’s Electrical System

















The electrons travel around the circuit and energize whatever device is turned on. The lead (-) and lead oxide (+) make up the “plates” of the battery. The sulfuric acid is the liquid in the battery case. The lead sulfate forms on both sets of plates, and the water produced dilutes the sulfuric acid electrolyte. Dilute sulfuric acid has a lower specific gravity than concentrated acid. The specific gravity of water is one. That’s why we can use a simple device called a hydrometer to monitor the state of charge of the battery by measuring the specific gravity of the electrolyte solution.

When the battery is charging, the reverse process occurs: Electrons enter the negative terminal and cause lead sulfate and water to change to lead, lead oxide, and sulfuric acid, returning the plates to their original composition and causing the electrolyte’s specific gravity to increase.

The accompanying photograph shows two typical battery hydrometers, a terminal puller, two types of terminal cleaners, and a carrying strap, all available at any auto supply store. The smaller hydrometer sells for less than a dollar and is good enough for a quick battery check. For $2.49 you can get a first-rate glass battery hydrometer with a properly calibrated scale. If you get one of these, spend another $1.89 for an inch-and-a-half by 12-inch PVC sink tailpiece at any hardware store to make a neat protective holster, as shown.

When it’s time to replace a battery, what type should you buy? That depends on what use it will be put to.

There are two basically different kinds of 12-volt batteries based on construction, automotive and marine; and two different types based on use, starting and service. If you see a true marine battery next to an automotive one, you’ll easily recognize the difference. Marine types almost always have a thick, moldedrubber case, screw-type terminals and are robust in all ways, including price. The internal construction also is stout, and different from the auto models, having thicker plates, heavier grids, and fiberglass mat separators between them. Unfortunately, we all have seen thin plastic shell batteries with screw terminals and a polypropylene rope handle, labeled “Marine,” and selling for more than the auto battery right next to it. If it’s not truly marine, I’d stick with an auto battery and set of screw terminal adaptors.

The engine’s starting battery could well be an automotive type, which will give adequate performance for this task. In both car and boat, this battery’s job is to start the engine-period. Once running, the alternator recharges the battery immediately. The service or “house” batteries which run all the rest of your boat’s electrical system, should be of the “deep-cycle” marine type. Deep-cycle batteries will retain their rated capacity through hundreds of recharge cycles, while other types will suffer diminished capacity after only 20 or 30 cycles. Generally, you get what you pay for, and there are not many bargains in quality marine batteries. The only maintenance your batteries require, in addition to adding water and checking with the hydrometer, is keeping the case clean and the terminal connections bright and tight. Use the tools shown in the photograph.

The Breaker Panel

If the battery is the heart, then the breaker panel must be the nerve center of your electrical system. Most standard panels have a vertical row of switches on one side and a row of fuses on the other, with name tags for each circuit in between, or just a row of circuit breakers. Personally, I prefer fuses; they are so simple they can’t screw up. Excess current from an overloaded circuit produces heat which melts the fuse and protects the system by turning it off. It is a cheap and completely fail-safe device. Circuit breakers are becoming common on production boats and are reliable and easy to operate. Circuits feeding a motor, such as a blower, should use “slow-blow” type fuses to allow for the initial surge of current as the motor starts turning. You might have to go to an electronic shop to find these.

Your panel may contain a meter or two. One might be an hour meter to show total engine running time. The others are probably an ammeter and a voltmeter. The ammeter shows when current is flowing. These meters are always center-zero (like in a car); plus means the alternator is supplying current to the battery and carrying the load, minus means the battery is being discharged.

The voltmeter will give you the most useful information about your electrical system. Since voltmeters require a constant drain in the system in order to operate, they usually are wired directly to the master switch with a push-to-test button next to them. You read battery voltage, as supplied to the control panel, every time you push the button. We call the system 12-volt DC, but it actually may be anything between 13.8 voltsat full charge, down to 10.5 or 11 volts in the discharged state. Panel voltmeters usually read zero to 15volts,but only the top few volts tell us anything. If you want to avoid problems and keep on top of things, you may want to replace the voltmeter with a battery charge meter. Spa Creek Instrument Co. of Annapolis, Maryland, sells one that will fit into the same hole and reads the percentage of full battery charge directly, which in turn lets you estimate how long you’ll have to run your engine to recharge the batteries.

Control panels require very little in the way of maintenance but they should not be ignored. Periodically remove the fuses and inspect the end caps for signs of corrosion. I wipe them off with a rag dampened withashotofWD-40.Putthebattery master switch to “off” or pull the positive battery terminal off; then open the panel box and inspect the end terminals and bus bars for corrosion and loose connections. If the panel ever gets wet, wipe it off immediately; otherwise you’ll be inviting corrosion to take over. 

The Wiring
In our homes, the electrical wiring once installed and inspected, is usually forgotten for years or decades and hardly ever causes a problem. Not so on our boats, where systems are subjected to vibration, rolling and pitching, dampness, a hostile salt air environment and temperature extremes. Some units, such as bilge pumps, are meant to run submerged. Components mounted on the foredeck, such as an electric windlass, will be constantly subjected to water. And every wire that runs inside a mast will be chafed and abraded from the vessel’s motion. External connectors for mast wires can develop voltage drops and electrical leaks, in some cases shorts, considering the abuse they are given.

Whenever you spot frayed insulation on a wire, it must be taken care of immediately; it may be the source of an electrical leak that has been draining your battery all season. But if the insulation is damaged, first look to see whether the wire is damaged also. Broken strands effectively reduce the size of the wire and increase its resistance, which causes an excessive voltage drop in the wire; the missing energy will be dissipated as heat along the wire. If the wire has been damaged, it must be replaced with one of at least the same diameter. If only the insulation was damaged, it can be repaired with tape; but you must also stop the chafing from occurring again. This usually means supporting the wire with a plastic hanger. American Boat and Yacht Council safety standards call for all wiring to be supported every eighteen inches along its length.

Most of us are not satisfied indefinitely with the electrical system that came with the boat, even though we may have chosen all the options ourselves. In time, we will find that we need to add a spotlight, a stereo, a freshwater pump, a ‘IV, or some other creature comfort. This is an easy enough task, but it does require some planning and certain minimal skills.

If you have never repaired a washer or dryer, installed a radio in your car, or even repaired a lamp, you might want to let the boatyard take care of the installation for you. If you decide to take on the job, you’ll need a few simple electrical tools. A rechargeable soldering iron is a jewel; it will hold a charge for about six weeks and the small tip gets hot in no time. You will also need a spool of “rosin core” solder; never use “acid core” for any electrical work aboard as it invites corrosive destruction. A crimping tool can be found in most hardware or auto stores. All the holes and notches serve a function. You can cut the wire, strip off the insulation, and then secure a terminal to the end by crimping with it. An assortment of wire and terminals should also be part of your on-board kit.

When adding a new electronic device, first determine the current load the wire will have to carry. Look at the device’s tag for its power rating or wattage. Divide this by 12 volts to get the current in amperes. If you have some spare circuits on the control panel, you’re in luck. If not, you’ll have to either add a new panel board, or find an existing circuit with sufficient capacity to take the additional item. Be sure to connect the new item in parallel with the closest device. This is not the best solution, but it will work with low wattage items such as cabin lights.

If a spare circuit is available, measure the actual distance the wire will have to run from the control panel to the desired location and back. This will tell you how much wire you’ll need and also the size wire to buy. The table gives the rest of the information you’ll need. You have a choice of size, depending on how much voltage drop the device can tolerate. Some devices are more critical than others, so when in doubt use a larger wire (smaller number).

There is no guarantee your boat was wired in accordance with the American Boat and Yacht Council’s scheme; besides, you’ll have trouble finding all those colored wires in a retail store. It’s a good idea to tag new wires (whatever the color) with labels. If the new wires pass within three or four feet of the compass, be sure to twist them together to cancel out the magnetic field that surrounds DC current-carrying conductors; otherwise your compass may be in error whenever the circuit is energized.


This is the part about problems1 told you we’d get to later. Some problems are simple; others may be more complex (and then there are the really difficult problems). For example, a cabin light stops working. Simple. Take off the cover, unscrew the bulb, and see if the filament is still intact. If not, change the bulb. Common sense.

Now your masthead light stops working. Not quite the same is it? Before I climb up the mast I want to make sure I have to. It is possible to check out the filament electrically from down below (I’ll tell you how in a minute). You may still have to go up the mast, but then you will know that there’s no other choice. It’s the same type of problem as the cabin light, but more complex because of the location.

With a relatively new boat, most electrical problems fall into that category; something stops working, so you find the cause and fix it. On older boats the problems get more complicated. Corrosion sets in, moisture does its job, insulation breaks down, wires chafe, strands break, hold-down nuts work loose, resistance and voltage drops increase and electrical leaks develop. Sounds grim, but that’s the way it is aboard. Sometimes these problems can drive you nuts, such as the time my radio-circuit switch gave out. It would receive fine, but couldn’t carry the current load for transmitting. That one took a while for me to noodle out.

You don’t need a complete electronics shop aboard in order to troubleshoot your electrical system. You don’t need a $50 meter to take readings If you have one, fine, but I get along with an old $7.95 Radio Shack kit multimeter. In fact, you don’t need a meter at all to check out a whole lot of problems.

In an auto supply store, you can find a continuity tester and a test light probe for only $1.99 each. I’d recommend you have one of each aboard. But if you run into trouble while off cruising without them, don’t despair. There is an adequate replacement that costs next to nothing, and works just as well. The continuity tester consists of a flashlight bulb and “D-cell” battery. Solder a wire to the side of the base of the bulb (not the tip) and another to the bottom of the cell. Continuity means continuous or complete; it is always checked when the circuit is open or disconnected. Twelve volts will blow the bulb, so make sure the master switch is off. You or your mate will have to hold the cell and the bulb together while testing; but that’s not too hard. If the bulb lights, you have continuity; if it doesn’t, you don’t.

Now let’sgo back to the malfunctioning masthead light. It won’t go on when the switch is thrown. The fuse or circuit breaker is right next to the switch, so I’d start there. If the fuse or breaker is not the problem, then the fun begins.

Next, go to the mast light disconnect, either a plug connector or a terminal block somewhere near them base of the mast. Break the connection and put your continuity tester into the side of the circuit that runs up the mast. If the test bulb lights, the mast wire, socket, and bulb are good. If you don’t get a light, it could be the bulb or the socket connections, but first put one tester wire against the mast and the other into the connector, one wire at a time. Continuity here means one or both wires are shorted to the mast.

Now you’ve got a messy job, but think about it before you tear things apart. You have to go aloft to replace the bulb or fix the wire, or else get someone else to do it for you. If it’s a wire problem, the old wire will have to go, but first attach the new one to it and feed it into place as the old one is removed.

Going back, if the tests at the mast were good, the fault has to be between the switch and the mast base. With the mast wire still disconnected, use the 12-volt bulb tester and touch its wires to the connector on the panel side while your partner turns the switch on. If it lights, you have current to this point. If it does not, your problem must be upstream of the mast.

Now the troubleshooting begins. Touch the test wires across the battery terminals to see if it is dead. Touch one test lead to the system’s ground, usually the engine. Take the other lead and systematically work your way from the battery’s positive terminal to both sides of the main battery switch, the fuse holder, the circuit switch and any terminal blocks. A brightly lit bulb indicates the circuit is good to that point; a dim bulb indicates a higher than normal resistance in the line, a problem to be dealt with. If the bulb doesn’t light, your circuit is incomplete, due to a broken wire, bad connection, or short upstream of that point. Your eyeball inspection of the system is often as good as the electrical tests you are making, so pay attention and look for defects.

The worst electrical problems are intermittent ones; sometimes they are there, others times not. These are typical of loose connections, or leakage caused by moisture, or a chafed wire that leaks or shorts only when it flops against a grounded piece of metal. A wire behind a bunk may leak electrically only when it is moistened by a deck water leak; a wire in the bilge may leak when there is water in the bilge, and not when it’s dry.

Here’s how to know for sure whether or not you have a current leak. Make certain that all the switches are off. Then pull off the positive battery terminal. Touch your test lamp between the positive post and the battery cable connector. If it glows brightly some switch is stuck on or there is a short. If it glows dimly, there is a fairly large leak. No glow means either no leak or perhaps just a small one.

Here’s where a test meter is more useful than the bulb. Put the scale on DC volts, O-15 range, and touch the red lead to the plus post and the black lead to the loose connector. Zero means no leak. Any leak at all, no matter how small, will give a 12-volt reading. If there is a leak, switch to the DC amps scale and try to measure the current; if you get no reading, go down to the milliampere range. There has to be some current if you got a 12-volt reading, but it might be insignificant.

To measure the magnitude of the leak in ohms of resistance, switch to the “R” scale and now connect the test leads between the negative post and the loose connector (you don’t want any battery current present in the wire while making this reading). The lower the ohms reading, the greater the leak. Under 10 ohms usually indicates a switch hanging up and not breaking the circuit (or you forgot to turn it off). Any value up to 1,000 ohms can cause a lost current problem and serious battery depletion. More than 1,000 ohms usually is not serious. Over 10,000 ohms (near infinity on the scale) there isn’t much that can be done that’s just one of the consequences of living in a damp environment.

If you have a sizable leak, start sorting through your circuits one at a time until you find the problem. The continuity meter and 12-volt meter are your tools here. The job can take half an hour, or it can take all day, but be methodical and soon you’ll find the hot spot.

Maintaining Your Boat’s Electrical System


Darrell Nicholson
Practical Sailor has been independently testing and reporting on sailboats and sailing gear for more than 50 years. Supported entirely by subscribers, Practical Sailor accepts no advertising. Its independent tests are carried out by experienced sailors and marine industry professionals dedicated to providing objective evaluation and reporting about boats, gear, and the skills required to cross oceans. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser who has been director of Belvoir Media Group's marine division since 2005. He holds a U.S. Coast Guard 100-ton Master license, has logged tens of thousands of miles in three oceans, and has skippered everything from pilot boats to day charter cats. His weekly blog Inside Practical Sailor offers an inside look at current research and gear tests at Practical Sailor, while his award-winning column,"Rhumb Lines," tracks boating trends and reflects upon the sailing life. He sails a Sparkman & Stephens-designed Yankee 30 out of St. Petersburg, Florida. You can reach him at