What Can You Do With A Multimeter?

If you're like most boaters, you probably have a multimeter or DVOM (digital volt ohmmeter) stashed in a drawer aboard your boat. But do you really know how to use it? Or is it something you put on board just because a magazine article suggested you really need one? In this article, I'm going to repeat that suggestion but also explain why this tool is such an important part of your onboard tool kit.

Getting Started

Of most importance to boat owners is the ability to read and interpret voltage and amperage readings. Testing for electrical continuity has significance as well, but measuring ohms of electrical resistance is of limited value unless you have been provided with a specific value to measure for. I recommend acquiring a meter that has an amp clamp integrated into it and also a meter that employs "self-scaling" so that you don't have to concern yourself with whether you're on the correct range for what you are trying to measure. Let the meter figure that part out for you.

One meter that I've used quite extensively that fits the bill is the Blue Sea Systems model 8110 Mini Clamp Multimeter AC/DC. It has a good range at 0.01–400 amps for both AC and DC and 0.001–600 volts for both AC and DC. It also offers a broad range for measuring ohms of resistance, but without a specification to work with, this will not be useful to the average boat owner. Of more use on the ohms scale is the audible continuity alarm. This feature is great for checking fuses, especially low-amperage fuses with hard-to-see filaments or incandescent light bulbs to see if there is electrical continuity through the filaments.

Getting Connected

With an amp clamp-style meter, measurement is relatively easy: Use the clamp around a single current-carrying conductor and take a reading. It's important to clamp it only around one wire if multiple wires run in the same jacket. Also remember to select whether you're measuring AC or DC current. With AC, the orientation of the clamp is irrelevant. With DC, look at your clamp carefully; depending on the clamp model it will have either a plus symbol or sometimes an arrow indicator on one of the clamp jaws. This mark indicates which side of the clamp needs to be oriented toward the source of DC power. If the meter only has an arrow marking, it may be indicating the direction of current flow. Don't assume that's always the case. Read the instructions provided with your meter; the arrow may be pointing toward the source of DC power.

As for voltage and continuity testing, you'll be using the red and black leads provided with your meter. The black lead gets plugged into the terminal indicated by "COM," and the red lead plugs into the terminal marked "+" or "V" and "Ω." Remember that for both voltage and amperage measurements, you'll need to select AC or DC.

What Can Your Meter Tell You?

Most boat owners won't think to use a multimeter until something electrical on board stops working. The reality is that using a multimeter to get a sense of whether failure may be just around the corner could save you a lot of grief. One of my books covering electrical systems, Powerboater's Guide to Electrical Systems, 2nd Edition, is loaded with tips on things to look for and specifics on how to do it. Incidentally, don't let the book's title fool you. The same tips and suggestions work just as well on sailboats. We'll cover five of the suggestions right here.

1. Charging system output. Testing the output from your boat's alternator and/or shore power-supplied charger is a good idea to ensure that you don't have a voltage regulator problem and that the charger is calibrated correctly for the battery technology currently installed. Often batteries get upgraded without regard for the settings on the charger. This plays heavily in maximizing battery cycle life.

For the alternator test, go right to the battery. In multiple battery installations, make sure battery switches are set for the battery you are checking. It's a good idea to discharge the battery a bit before testing by running some accessories for 15 to 30 minutes. Test the battery voltage before going any further. You want to see at least 12.2 volts (approximately 50% charge).

Start the engine and run up to about 35% to 50% of normal operating RPM with your multimeter connected to the positive (red) terminal and negative (black) terminals on the battery. Your meter is set to the DC volts scale. Your voltage reading should be in the 13.3 and 14.8V for a 12-volt system with a flooded cell deep-cycle battery, but to be certain, verify recommendations from the battery manufacturer.

Readings significantly different from the battery manufacturer recommendations indicate either a faulty voltage regulator or, if adjustable, one that is set incorrectly.

An additional test here is to turn on all the DC loads on your boat while leaving your meter connected and see if your alternator can keep up. The minimum voltage you should read with all the DC loads turned on is 0.5 volts more than that initial open-circuit voltage reading across the terminals. If it's less than that, the alternator current output is inadequate for the existing DC loads on board. While you're at it, try your amp clamp around the DC positive battery cable. Again, turn on all your DC accessories and see how many amps your alternator can deliver.

2. AC ripple. A very simple but telling test for alternator condition is to switch your voltmeter to the AC volts setting while the leads are still connected to the positive and negative posts on your battery. You should get a very low AC volts reading in the range of 0.000x. Anything above 0.4 volts AC is indicative of a rectifier diode inside the alternator that is getting ready to fail and leave you high and dry.

3. Battery load test. Although not a perfect test, you can perform a load test on your batteries by measuring voltage at the terminals while running a heavy load like an anchor windlass or starter motor (without starting the engine). Normal shop procedures for load testing dictate a load equivalent to 50% of the cold cranking amperage rating for the battery being tested. That requires a load of greater than 300 amps for even the smallest of marine batteries. You need to isolate the battery being tested from any others in the system to ensure that the data you gather is specific to the battery you are testing. So you may need to disconnect any cables connecting to paralleled batteries in a bank. Obviously, you'll need to have the battery being tested connected to the boat's DC loads.

The other challenge here is that if you are going to use the engine starter motor as the load, you will need to figure out how to ensure the engine won't start while you are performing the test. If you are using loads like an anchor windlass or electric winch, you're all set. With gasoline-fueled engines, you should be able to disable the ignition system by unplugging a gang plug that supplies power to the system. For diesels, you'll need to find out how to shut down the fuel injection system while you perform the test. I recommend consulting the engine manufacturer for advice on how to accomplish this before you attempt it on your own. There are just too many variables here to generalize.

In any event, by running the windlass, electric winch, or starter motor for 10 to 15 seconds while watching your multimeter set on the DC volts scale, you want to see no less than 11.1 volts as a reading. Less than that indicates that the battery you are testing is losing capacity and getting ready to let you down. Of course, the battery should be fully charged (12.6 volts or more) before you attempt the load test.

4. AC false ground. One of the more common wiring errors we find on board can usually be traced to a boat that has been serviced by a land-based electrician. It's not their fault; they probably did what they were trained to do. The term "false ground" refers to a connection between an AC neutral conductor and an AC grounding conductor. Here in the U.S., that means the white and green wires in the system. False grounds create an extremely unsafe situation that can cause AC leakage current to exit the boat via the bonding system that interconnects underwater metals. Swimmers beware!

To test for this, make sure the boat is unplugged from shore power. Next, plug your meter leads into any onboard AC outlet. (If it's switched, make sure it's switched on but not powered up by the shore power or an inverter or AC generator during the test.) Power on when using your meter's ohm scale will at the least trip a fuse inside your meter or, in a worst case, burn out your meter.

With your meter set to the ohms test function, indicated by the omega symbol, plug your meter's red lead into the larger of the two slotted sockets (AC neutral) and the black lead into the rounded socket (AC grounding). You should get an "OL" reading on your meter if all is well. If you get a resistance reading and a possible audible continuity alarm, there's trouble on board. At this point you need to contact an ABYC-certified marine electrician to locate the false ground and get it squared away. The false ground represents a serious safety issue and can go unnoticed until a serious accident occurs

5. Shore power voltage. Most all boats with installed shore power systems will be equipped with a voltmeter on the main distribution panel, but many small boats with just an installed battery charger and single duplex outlet may not. Excessively low voltage at dock receptacles is quite common and can wreak havoc with some equipment.

The industry standard is plus or minus 10% of the nominal rating. So for a 120-volt system, readings below 108 volts AC or above 132 volts AC (very rare; it's more common for it to be too low) are considered a problem. Keep in mind that the values will change throughout the day as demand on the dock varies. It's easy to check with your meter set to the AC volts scale. If the boat has a duplex receptacle on board, simply plug your red lead into the smaller of the two vertical sockets and the black lead into the larger of the two. Take a direct reading. Your reading will be the same as that at the outlet on the dock.

In closing, let me just say that the multimeter hidden in that drawer is one of the most valuable tools you own. In terms of what you can discover by using it, we've just scratched the surface in this article.