ALL ABOUT ALTERNATORS – Part 1: Inboard Engines
Like all things connected to electricity, I have learned to keep my proverbial “finger out of the socket.” For example, whenever I was doing any kind of work on any of the many boats I either owned or ran throughout the years, I made sure I not only had my battery switch turned to the “off” position, but just as a precaution, removed and taped up the cables. It’s the way I worked around boats whether they were sitting in the water on the dock, or up on the hard getting ready for a seasonal launch.
Given the often-complex nature of the systems found on today’s boats, expert advice and insight is often necessary, and that’s why I went right to the proper sources when the topic of alternators showed up on my editorial assignments radar screen. I spoke to equipment techs at Dometic’s Balmar Alternator division and also contacted Victron Energy that manufacturers Lithium Iron Phosphate (LiFePO₄) batteries, chargers, inverters and DC to DC converters.
Marine alternators differ from automotive alternators in several important ways. They are designed to withstand harsh marine environments that include saltwater exposure, humidity, vibration, and continuous operation under heavy electrical loads. Many marine alternators also include spark-protected components to reduce the risk of igniting fuel vapors in enclosed engine compartments.
How Alternators Work – Inboard Engines
Your inboard engine’s alternator converts mechanical energy from the engine into electrical energy. It is driven by a serpentine belt connected to the crankshaft. As the engine runs, the belt spins the alternator pulley, causing the internal rotor to rotate within a stationary component called the stator. What happens next is one of the great mysteries of physics. Well, that is, for most of us who, try as we might, cannot fathom either the derivation nor the application of the following: B = (μ₀ I) / (2 R). However, as I understand it, this represents the magnetic field at the center of a circular, current-carrying loop. In other words, the magnetic field generated by the rotation of the alternator’s internal rotor about the stator. Now, with that settled, let’s move on.
The belt-driven pulley creates a magnetic field generated by the rotor spinning in the stator’s copper windings. In turn, this produces AC electricity. However, as boats use DC current, a set of diodes housed inside the alternator, known as a rectifier, will automatically convert the AC to DC. Then, a voltage regulator acts to control the alternator’s output, usually maintaining a charging voltage between 13.8 and 14.6 volts for a typical system.
Troubleshooting
Alternators, like so many other things onboard, can have problems. While they rarely fail, or show signs of operational failure without some warning, boaters who can recognize some early signs of possible problems developing can head off a complete charging system malfunction.
One of the most obvious signs is a battery that no longer stays fully charged even after several hours of operating the engine. After testing, the battery, or batteries, may remain weak or quickly discharge once everything is shut down. If multiple batteries show low voltage levels, and they are relatively new or in good condition, it may be time to inspect the alternator. Should you have a battery warning light or charging system alarm at your helm, this also may indicate the system needs to be checked. Other indicators of trouble include dimming cabin lights, flickering nav screens, radios resetting, as well as complete electronic shutdown as the voltage in the batteries drop. These warnings will become more noticeable when high-demand equipment such as refrigerators, windlasses, or bow thrusters are operating.
One of the early maintenance lessons I learned was to listen to the usual sounds your machinery makes when underway and in operation. As with your engine and generator, so too can unusual alternator noise indicate trouble. A squealing belt can easily be adjusted and its length carefully inspected for nicks and gouges and replaced should any be present. However, a grinding or high-pitched whine could indicate worn bearings inside the alternator. Let this go on, and complete mechanical failure will often follow.
Finally, should you become aware of a burning smell, this could signify overheated wiring, damaged insulation, or the failure of the alternator’s internal components which creates a distinct electrical odor. Should this be the case and should conditions allow, shut down, throw the hook, and call for assistance. If you have alternative propulsion like sails, use them, or on boats with twin engines shut the offending engine down and cautiously limp home.
Some Reasons Why Alternators Fail
The environment is a major contributor to alternator problems and operating in saltwater is perhaps the most demanding. Regardless of even direct water exposure, as remote as that might be, the salt-laden air will hasten corrosion on terminals, connectors, and internal components. This deterioration will increase electrical resistance and reduce charging efficiency.
As alternators produce considerable heat during operation, especially during charging deeply discharged batteries, proper ventilation is essential for optimum performance. In addition, constant heavy charging can affect and shorten the service life of diodes, voltage regulators, and insulation.
One of the more obvious places to look at is the belt which can become loose and slip. This will reduce the speed of the spinning alternator rotor and affect the charging output. If the belt is too tight and left unchecked, it will overload the bearings. As far as a belt that shows obvious signs of wear or cracks, complete failure is likely imminent and the belt must be replaced as quickly as possible.
As mentioned before, internal components will wear out in due course. Bearings, brushes that lose electrical contact, and rectifiers and voltage regulators can fail after years of service. Problems with batteries can also damage alternators when they try to charge severely discharged or faulty ones. The process forces the alternator to operate at maximum output for extended periods of time thus creating excessive heat and stress.
Just because you are not well versed, there are some simple things you can do to make sure your alternator is properly operating. The first step you want to take is to measure the battery voltage with a digital multimeter. If you have never used one of these worthwhile, must-have tools, there are plenty of available websites that will lead you, step by step, through its many operational features.
For example, if you are measuring DC voltage for a battery, you simply turn the dial to V for voltage, touch the black probe to the negative (-) terminal and the red to the positive (+) terminal, and read the voltage on the screen. With the engine running, battery voltage should increase between 13.8 and 14.6 volts. Should the battery level remain where it was, your alternator may not be charging.
As stated before, it is important to examine the drive belt for nicks, tears, cracks, glazing, or any other signs of wear and tear including improper tension. Should anything appear wrong, have the belt changed as soon as possible. As part of your machinery space inspection, have a close look at the battery terminals, ground connections, and alternator wiring for any signs of corrosion or loose connections. Use a battery terminal cleaning brush to remove any noticeable residue build up and follow up with why it is happening with a qualified marine technician who, at the same time, can perform a load test to confirm whether the alternator can maintain proper voltage while all the electrical equipment is running.
The question of whether to repair or replace an alternator can usually be made by an experienced and qualified specialist in rebuilding this specific piece of equipment. Replacing worn bearings, brushes, rectifiers, and voltage regulators can sometimes be more economical than buying a new alternator. Should the housing be corroded with stator or rotor windings damaged, or the entire unit have a complete failure, replacement is the only course of action. If you have upgraded your boat’s systems, including larger battery banks and added other power components, you may also have to include a higher output alternator.
Regarding alternator compatibility when using Lithium batteries and battery banks, and their installation in onboard electrical systems, Balmar’s Timm McVey, Tech CSR III, explains, “In sizing your alternator to your power needs with conventional battery chemistries, the alternator should be capable of outputting about 25% of your house battery banks amp-hours. For instance, a four hundred amp hour house battery bank should have about a 100-amp alternator. This is a rough rule of thumb, but conventional batteries have an acceptance rate as they can only accept so much and even a larger alternator will not help much with the charge time. That all changes with a Lithium battery system and just as a note of caution here, if you are thinking of retrofitting, make sure you enlist the services of a certified ABYC marine electrician with a strong background in this type of work. Call it personal preventive maintenance both for safety and economics.”
McVey went on to explain how, with lead-acid, there is a natural limit to the charging current as the battery fills up because as the voltage rises, so does the internal resistance. With Lithium batteries, there is very low internal resistance with a very high acceptance of charging current until the battery is nearly full which is why they charge faster.
The alternator and lithium battery bank don’t have to be made by the same manufacturer, but they do need to be electrically compatible. This can be accomplished either directly or through supporting equipment such as a DC-to-DC charger or a lithium-capable alternator regulator. Lithium batteries have a Battery Management System (BMS), to prevent over voltage, low and high temperature charging protection, and fault detection. In short, the BMS is the batteries control center, making sure the system operates safely and efficiently. It is important especially on boats, where charging reliability and protection of onboard electrical systems are critical that the batteries and alternator are properly matched. Ensuring the alternator is compatible with the Lithium battery bank’s BMS is important for both proper charging, and for safety, so always check with equipment manufacturers and/or a certified technician before any changes to the installation.
Alternators are an important part of every boat’s electrical systems as they charge batteries that are used for all types of onboard functions. To keep them working properly regular preventive maintenance should include a careful inspection of belts and making sure all electrical connections and terminals are clean. Make sure your batteries are in proper condition by using a multimeter thus preventing your alternator from trying to recharge ones that are heavily discharged every time you start things up. As your engine’s alternator is one of the most important electrical components on your boat, maintaining its optimum operational life is critical to preventing breakdowns while underway.
Balmar: https://balmar.net/ +1.256.384.7367 or techsupport@balmar.net
ABYC: abycinc.org
Victron: victronenergy.com
All about alternators Part 2-outboard engines
Learn how modern outboard alternators charge lithium battery banks, what can go wrong, and how to keep your boat’s electrical system balanced.
