See full version: Electrical Resistance on Boats: Keep That Voltage Drop in Check
Figures 2-4 show a simple experiment to illustrate voltage drop caused by resistance. In this case the voltage is produced by a 12-volt power supply that provides a constant 14 volts DC – about what you would find at your battery terminals with the engine running and the alternator charging the battery. here
Now do the math to come up with a percentage value. 13.2 -11.8 = 1.4 volts. This circuit has “lost” 1.4 volts to resistance. Is that too much? Well, in this case divide 1.4 by the measured source voltage of 13.2 (1.4 / 13.2 = 0.106). Move the decimal two places to the right and you get 10.6 percent drop in that circuit. That’s excessive for the circuit. here
Fig. 3 - With the searchlight connected by adequate-sized wires and switched on, the circuit shows an acceptable five percent voltage drop, most caused by the light running normally. here
Again, remember that without a specification to use as your guide, ohm values are pretty irrelevant.
Excessive electrical resistance is the biggest single enemy onboard boats, at least as far as electrical system performance is concerned. In order to combat resistance, you, as a boat owner, need to know what causes it, how to measure for it using your multimeter, and how to mitigate it.
Also Know, how do I know if my boat alternator is bad? more
Why do I have voltage between neutral and ground?
Symptoms of voltage drop You usually can't see the corrosion inside a connection or the damaged wire that is causing the problem. Ground-side voltage drop, a commonly overlooked cause of electrical trouble, can cause most of these symptoms. Any circuit or component is only as good as its ground.
Another simple test is to turn on your boat lights and see if they get brighter after you start the motor (a voltage increase). If the motor cranks after a day on the water but then not the next time out it's likely a bad battery. If you have the patience you culd disconnect one side of the battery after coming in.
Whether the culprit is corrosion, a loose connection, or undersized wire, the results are the same: The flow of the current is restricted, and not enough electricity can reach the equipment in question. The trick is to find the source of the excessive resistance. Again, in saltwater environments the problem is almost always found at a termination point, so start at the switch and work your way out toward the cabin light. But common-sense detective work also comes into play. For example, if you find an excessive voltage drop in a navigation light circuit, start at the outside light fixture, looking for the result of water intrusion that causes terminal corrosion. [links]
Fig. 2 shows the measurement with the probes of an auto-ranging multimeter touching the positive and negative terminals of the power supply. In Fig. 3 we’ve connected a 12-volt searchlight to the power supply with wires about the same size as those between the socket plug and the light. With the light switched on, we see a voltage drop of about three quarters of a volt, and we can assume that virtually all of this is coming from the use of the light itself. (A light, after all, is a resistive load.) Let’s do the math again: 14.01 – 13.28 = 0.73. Then 0.73 / 14.01 = .052. Move the decimal point and we get a 5.2 percent drop. That’s acceptable. here
Poor installation, meaning, among other things, loose wire, low-quality wire, and faulty cable termination points, is probably the most common cause of excessive electrical resistance on boats. Using incorrect terminals for the task at hand, or loose friction-type connectors, or the wrong tools to make crimp connections, will make it much more likely that your circuit will be compromised. This explains why the ABYC (American Boat and Yacht Council) E-11 Electrical Standard has much to say about wire sizing, acceptable terminal types, and pull-test requirements for friction and crimp connectors.
Figures 2-4 show a simple experiment to illustrate voltage drop caused by resistance. In this case the voltage is produced by a 12-volt power supply that provides a constant 14 volts DC – about what you would find at your battery terminals with the engine running and the alternator charging the battery. here
A good multimeter is the boatowner’s main diagnostic tool for troubleshooting DC electrical circuits. (All photos: Doug Logan)
The quality of electrical connections on your boat can be verified using a simple digital multimeter set to the DC volts scale. Why measure volts instead of ohms? There are two main reasons. First, although ohms are indeed the unit of measurement for resistance, it’s impossible to know what the exact measurement values should be unless someone gives you a specification to work with. Engine manufacturers, for example, might provide ohm values for things like coils and engine instrument sending units, and there are often values to be found in shop manuals and engineering standards—but no one provides resistance values for most of the electrical circuits aboard boats. Second, it’s easy to measure for excessive voltage drop, which is a direct indicator of excessive resistance. In addition, acceptable maximum levels of voltage drop are given within ABYC and USCG standards, so we do have a good set of general values to utlize.
Check out this brief primer on boat batteries, their voltage needs and your charging options.
If you own a larger boat, then there’s a strong chance you’re actually operating on a 24-volt charging system. Big boats have more intense power needs and using a 24-volt system is more efficient at meeting the demands of lighting, bilge pumps and other electrical systems than a 12-volt system. more
The simplest portable boat battery charger is a solar design, which you can mount on a pole or on any flat part of the boat that won’t get covered in spray or grime during your trip. Keep in mind this type of charger is typically only good for keeping a trickle or “float” of power flowing into your battery. While it will keep things topped off, it won’t recharge a larger battery that’s been drawn down after a long period of motor-free operation. here