When your boat is plugged into shore power or running a generator, the battery charger(s) will keep your bank charged up. When conditions don’t require that you run the genset, the alternator provides the charge.
How do you know if the alternator properly charges the batteries? What can you look for to monitor the alternator’s condition and anticipate problems before a failure?
Looking for Trouble
Let’s begin with the alternator belt(s). An improperly tensioned belt can slip in the pulley, preventing the alternator from reaching the required rpm. For conventional V-belts, a good rule is an eighth of an inch of deflection for every inch of span. For serpentine (grooved) belts, you can use a plastic gauge to test for tension, and to make sure the tensioner on the engine is not jammed or broken.
Look for signs of misalignment or wear—a fine black or gray dust in the immediate area will provide an indication. Since alternators create heat as they work, they have fans behind the pulley to pull air through the alternator. If the belt has been slipping, the alternator can be packed with belt dust, which will cause premature problems.
Excessive heat can indicate impending failure. For this assessment, a temperature gun can be helpful. Since temperatures vary with alternator size and brand, checking regularly to establish a baseline will give you the reference you need.
Boaters without alternate sources of charging, such as generator-driven battery chargers or solar, may want to consider carrying a spare alternator. This is especially true if the belt that drives the alternator also drives water pumps on the engine, since a failed alternator pulley could take out the entire engine. Any spare should be checked to be sure it will fit on the engine, and to be sure the pulley is the correct diameter for the belt width.
Turn to the Back
Assuming the belt is OK, make sure there haven’t been any leaks on the alternator. It is amazing how many different leaks there can be on a boat. Water and electricity don’t play together well, so leaks into an alternator can short out the wiring or create corrosion. Look for antifreeze or water leaks, or evidence of rust or corrosion.
Next, look at the back of the alternator. Find the largest stud, usually with a red wire attached. That will be the output positive wire. Make sure it has not become loose or corroded. Some alternators have separate negative posts that will have an equal-size black or yellow wire; check it as well (you can also use it as your ground for the next check).
If the alternator does not have a ground stud, then it is using its attachment points to the engine as its ground. Find a piece of metal on the engine that doesn’t have paint on it to use as a ground. If you can reach the main ground on the block where the battery cable connects, that is even better.
Make sure the battery switches are turned on, and use a multimeter to check for voltage between the alternator output and ground. You should see the same voltage as your battery within about a tenth of a volt or so. If there is no voltage, then follow the positive output wiring. If the engine still has its factory alternator with a built-in internal regulator (more on this below), chances are that the positive wire will run to the starter, where it will connect to the big cable running back to the battery switch. Some engine manufacturers install a fuse or breaker in the alternator output harness that can be checked. If that fuse or breaker is blown or tripped, try replacing or resetting it.
Looking at the back of the alternator, confirm that all the small wires or plugs are well attached and not corroded. In the simplest alternators, these wires turn the alternator on with the ignition, and output a signal for the engine tachometer.
How Does it Work?
To do further troubleshooting, you need to understand what is going on inside the alternator.
The rotor rotates inside a stator. Both these items are coils of wiring around shaped iron. When the belt spins the rotor, it creates a magnetic field that induces current in the windings. Inside a standard alternator is a voltage regulator that tells the alternator how much voltage (and, correspondingly, how much amperage) to output. Also inside is a diode pack (sometimes called a rectifier) that changes the alternator’s AC voltage into DC voltage the batteries can use. For the voltage regulator to work, it must have a wire that senses the voltage at the battery, usually run through the engine ignition switch. This setup allows the regulator to adjust the electrical field inside the alternator to increase or decrease the magnetic field and, hence, the output. As the battery nears full charge, the magnetic field is weakened to reduce output, to keep from frying the batteries.
The next check needs to be done with the engine running, so make sure you don’t have any loose clothing or hair that can get caught in the belts. Start the engine. Measure voltage at the back of the alternator for a minute or so. Then, compare that voltage reading to the battery voltage without the engine running. If the batteries are a little low (perhaps after powering the ship overnight), you might see 11.9 volts. If the alternator is putting out voltage, you’d expect to see at least 12.5 volts and climbing with the engine running. If the voltage remains at 11.9, there is something else wrong.
Rectifiers have semiconductor diodes that let current flow only in one direction. They are sensitive to excess heat and sudden output changes that can happen if the output is suddenly disconnected. Never turn off a battery switch or alternator breaker when your engine is running. Inexpensive devices such as the Balmar Alternator Protection Module can be installed on the alternator to prevent this type of damage.
You can confirm a failed rectifier by checking for AC voltage on the DC output of a running alternator. Set your multimeter to AC volts, and check between the ground and the positive output. If you see more than 0.5 volts of AC, then the rectifier has likely failed. This test has to be performed with the boat not plugged into shore power and any inverters off—each can add measurable AC voltage into the boat’s DC wiring. Because most alternators are wound with three stator windings, there are typically three diode packs. Sometimes, only one or two will be bad, reducing output by a third or two-thirds.
External Regulators
Boats requiring more alternator output for large battery banks may have been upgraded to aftermarket alternators with external voltage regulators. An internal voltage regulator fixes the finishing output at a set voltage and holds it there, a setup that can over- or under-charge batteries, depending on outside temperature or type of battery.
You can determine if you have an external regulator by tracing the smaller gauge wiring leading from the alternator. If it disappears into the engine’s factory wiring harness, then the alternator is likely internally regulated.
Most external regulators are mounted near the engine on a bulkhead. Alternators create heat as they convert spinning energy to voltage, and putting sensitive electronics in that heat path can dramatically shorten their lives. Moving the electronics to a cooler, off-engine site eliminates the vibration and heat, and allows many more capabilities.
Treating Batteries Kindly
If you have a boat with two or three small lead-acid batteries and you do occasional weekend trips, an internal regulator will serve them just fine in most circumstances, as long as you pay attention to the acid levels. The simple regulator may shorten the life of the battery in some situations, but how much is difficult to measure.
If you get five to seven years out of a battery utilizing internal regulation, and you might gain a few more months with external regulation, the installation costs might not make sense. But if you have a large battery bank and frequently cycle to 50 percent capacity (or even deeper with carbon foam or lithium batteries), then treating these batteries as kindly as possible with sophisticated charging is a good investment.
Batteries are better served with more precise voltage regulation as they charge. External regulators break down the charging cycle into segments: bulk, absorption and float. Different battery chemistries also favor different charging voltages; optimizing those voltages can prolong battery life. Several major players, including Balmar, Victron, Mastervolt, Wakespeed and Sterling Power, make quality regulators with varying capabilities.
Deep, rapid charging of large battery banks creates heat in the batteries and the alternator. Batteries also charge at different rates for different outside temperatures. A battery at 32 degrees Fahrenheit will require a much higher charge voltage than a battery at 100 degrees Fahrenheit, where that same high voltage would cause the battery to boil. Feeding the regulator with temperature information from thermistors mounted to the batteries allows the regulator to adjust those voltages.
Similarly, if an alternator has a thermistor mounted to it, the regulator can ramp down output if the alternator temperature starts to exceed safe limits. This can be especially true with large lithium battery banks, which can often accept the maximum alternator output until they are nearly full. A regulator that has the ability to let the engine get up to speed before the alternator begins charging eases starting and initial warming up of the engine.
More Testing
A few more troubleshooting checks can be performed with an externally regulated alternator. Make sure the regulator turns on when the ignition is activated. A blown fuse or poor ground might be the problem. There will also be an output for the field wire to the alternator and a stator wire. Each should be checked for continuity and proper connection.
Malfunctioning temperature sensors can also affect output. The usual troubleshooting step is to remove their wiring temporarily from the regulator. Some regulators have Bluetooth capabilities and will connect to NMEA 2000 for monitoring, or connect to proprietary computer interfaces for diagnosis.
To diagnose an external regulator failure, one simple alternator check, called “full fielding” the alternator, can be performed. Research your specific alternator type for this test, in which you attach a jumper wire between the output wire (which you’ve just verified has battery voltage) and the field terminal on the back of the alternator (after disconnecting the alternator’s existing field wire and taping the end so it doesn’t short). If possible, use a jumper wire that has a 12-volt light soldered in-line. The light bulb creates a small load that slightly drops the voltage from a complete full field, offering a safety factor.
If, with the engine running, the alternator makes the battery voltage rise rapidly, then the external voltage regulator is damaged. This test bypasses the voltage regulator, causing the alternator to charge at its maximum capacity. If the voltage does not rise, then something else inside is damaged.
Don’t leave an alternator full fielded. It will overcharge and damage your batteries.
Solar panels, large loads and varying engine rpm can confuse some regulators that only sense the batteries’ voltage, causing them to over- or under-charge the batteries, or to limit alternator output, wasting charging capability. If the regulator sees the battery at a high voltage, it will slow the alternator output even if the battery could still take full alternator power.
Regulators such as the Wakespeed WS500 utilize a shunt (a device installed in-line in a wire or cable that has a set resistance) in the battery cable to monitor all amperage regardless of what produces it. This setup adjusts the alternator output accordingly. Smaller wires connected on each side of the shunt can then be used to accurately measure amperage. This regulator also has a provision for a minimum tachometer field that will prevent the regulator from completely turning off the field, thereby dropping out the tach, which can be a problem with some external regulators.
Power on Tap
When your battery bank runs large loads like air conditioning or a DC gyrostabilizer, or you spend days at anchor without a genset, you will have a large investment in that battery bank and will want to protect it with a sophisticated charging profile and clear monitoring.
Replenishing the power in our batteries while we motor is central to how most of us use our boats. Greater alternator outputs and more sophisticated regulators can speed up large battery bank replenishment and offer increased flexibility at anchor. Understanding how your alternator works will keep the juice flowing.
This article was originally published in the March 2023 issue.
