Checking the Marine Alternator or Generator Output

Tech Tips: Checking a Marine
Alternator or Generator Output
By Bill Whitney

Whether your Friendship is a Class ‘A’ or a newer
sloop, chances are good that it has been modernized with some
form of mechanical propulsion to complement her sails. This
could take several forms. From the 1920’s through 50’s it was
usually a gasoline engine, but as technology developed after
WWII these power plants started shifting to diesel, providing a
better safety margin, and as time progressed, less weight. Today
we are seeing advances in battery and battery charging technol-
ogy which have introduced electric propulsion.

We have become used to having this “iron wind”
capability to get us where we want to go when the wind quits or
shifts contrary to our destination. We also have become used to
having the electrical energy from the battery or battery banks.
How much energy the battery banks hold is dependent on how
many batteries there are, their condition, how well engineered
the installation is, and how well fed they are by the generator or
alternator driven by the boat’s propulsion plant. In other words,
how big is “the tank”, how well is it connected, and what equip-
ment do you have to keep it full. Since the first two elements of
the equation are governed by the builder of the boat, and to a
degree by the maintenance it receives, this article will only focus
on the alternators, generators and their regulators that keep “the
tank” full.

Early gasoline and diesel engines were equipped with
generators. Starting in the early 60’s, these engines were
equipped with alternators. What’s the difference? The differ-
ence is both mechanical and electrical and, to explain
sufficiently, beyond the scope of this article. Suffice it to say
that generators produce direct current (DC) using mechanical
switches, and alternators create alternating current (AC) and use
big diodes (electric switches) to convert it into DC. Alternators
are far more efficient and reliable than generators, and have
universally replaced generators in this application.

No matter which you have, generator or alternator, the
key is how much energy it puts back into your batteries. A key
component to this is the regulator that controls the device’s
output. If you own a voltmeter it is fairly easy to check the
health of the components in the system that’s keeping the batter-
ies fully charged. Before you begin testing the system you have
to make sure the battery or batteries are in good shape and are at
least 75% charged. A standard lead-acid battery at rest, that has
not been charged or used for 12-24 hours, should read 12.4 volts
or higher at the battery terminals. If it is less than 12.0 volts, the
battery has a problem and should be replaced. Be sure that the
measurement is made directly on the battery posts. If you take
the measurement through any of the cabling, a drop in voltage
may be caused by a corroded cable connection.

The next step is to check the belt for proper tension. A
loose belt will slip on the pulleys and fail to turn the rotor at the
correct speed, especially when putting out a high current while
recharging the batteries. Be sure to check the belt for wear,
cracks or dryness, Worn or damaged belts should be replaced. A
rule-of-thumb for belt tension is that the belt should not deflect
more than 1 inch on its longest dimension. If it’s too tight it will
stress the bearings of devices driven by the belt.

Now it is time to start the engine and bring it up to a
high idle of 800-1000 RPM. Now check the voltage at the
battery again. After 2-3 minutes it should be between 13.6 and
14.2 volts. If it is less than that, measure the voltage at the
output of the generator or alternator. This is done by putting the
voltmeter’s positive (red) lead on the terminal where the big
cable is attached and the negative (black) lead on the engine
block. If this reading is less than 13.8 volts the generator or
alternator is defective. If it is more than 14.8 volts then there is a
problem with the regulator connections, the regulator itself, the
cables going to the batteries, or the battery switch. In no case
should it be over 15.0 volts. In this case you need to clean up all
the connections or replace any questionable cables. Remember
that you need to check both the positive and negative cable
connections. On at least one occasion I’ve observed that the
only problem was a questionable ground connection between the
generator/alternator and the engine block.

Note that most alternators have the regulator inside the
alternator case where you can’t see it. Some custom installations
have an external “smart” regulator, which controls the alternator
output. In either case the voltages shown above will tell you if
things are working correctly or not.