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Alternators in Outboard Motors (Boat Motors That Charge Battery)

Modern boats cost a whole lot more than they used to. It is partly due to the natural rise in prices caused by inflation; however, a significant factor is a massive increase in the advanced technology used in boats and engines.

All modern outboard motors which produce more than 5 horsepower have an alternator or stator built into the design. With owners installing a greater number of high-draw electrical equipment on their boats, manufacturers will ultimately have to replace the stators with alternators.

Before the 1960s, most cars and boat engines used electrical generators driven off the engine crankshaft to recharge the battery. As car engines grew in size and compression ratios, increased manufacturers were forced to replace the inefficient generators with alternators. Outboard motor manufacturers followed this trend very soon afterward.

outboard motors

Quick Tip

If you have a newer generation outboard motor rated above 5HP, it is probably fitted with a stator or an alternator. As more electrical equipment is included in the design of a boat, the manufacturers will be forced to change from stators to full alternators.

What Do Outboard Motors Contain?

Most outboard motors have a charging system.

If the outboard motor has an electric starter and it was built in the last 20 years, it will have an alternator or a stator installed as standard. Both these devices charge the battery when the engine is running.

The only exceptions are the smallest (2-2.5hp) engines that do not have an alternator or stator.

Older engines or smaller units that don’t have an electric start may be equipped with a magneto or generator to create sufficient electrical current to power the coil and energize the spark plugs.

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As modern boats come equipped with an even greater number of electrical consumers, the need to sustain the battery becomes imperative.

Modern boats regularly come equipped with.

  1. Autopilot.
  2. Bilge pumps.
  3. Depth gauges.
  4. Electrical sockets for charging cell phones and other equipment
  5. Electric tilt and trim devices.
  6. Electric steering systems.
  7. Fire & smoke sensors
  8. Fridges/cooler boxes.
  9. Fuel tan gauges.
  10. Lights.
  11. Navigation /GPS devices.
  12. Sound systems and amplifiers.
  13. Starter motors.
  14. Touch Screen Displays.
  15. Trolling motors.
  16. VHF/UHF Marine Radio.

These are just a few examples of the possible devices which are regularly installed.

While Suzuki and Yamaha (and older models of Johnson, Evinrude, Mariner, Force, and Tohatsu)  outboard boat engines use stators, with the increasing draw required by the new features, alternators are gradually becoming the standard.

Stators are essentially one part of an alternator. They are installed inside the engine and directly powered by the flywheel’s outboard motor.

 When the engine is running, magnets on the flywheel spin around a stator and produce current. They are attached to a regulator and rectifier diodes, which ensure the current converts to DC  format and does not exceed the circuit’s requirements.

The result is that an alternator produces more current suitable for high-demand boating applications.

How Does An Alternator Work?


An alternator works as follows.

  1. An alternator has a rotor that is driven off the engine’s crankshaft.
  2. The rotor spins within the crankshaft.
  3. A series of magnets, placed in alternate north/south and south/north positions around the circumference of the rotor.
  4. A stationary set of copper wires is positioned around the rotor’s outside in what is called a stator.
  5. As the rotor spins the magnets, it makes an alternating electric current.
  6. The alternating current is then passed through the rectifier diodes on the alternator, which converts it to Direct current.
  7. After the current is converted to direct current, it passes through a voltage regulator, ensuring that only 12 volts flow in the circuit.
  8. With the number of magnets and wires, the alternator produces high currents of electricity.
  9. Alternators produce more electricity at low revolutions than a generator does.
  10. Alternators have a sensor on them that measures the energy available in the battery. If the battery is fully charged, the alternator will stop generating electricity.
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Alternators vs. Electrical Generators

suzuki outboard motor

If the batteries did not discharge, an alternator would not be required.

The alternative to an alternator, generator, or stator could eventually use renewable energy such as solar or wind power. Presently neither of these technologies can cost-effectively replace the existing electrical generation methods of regular outboard motors.

Before the 1960s, motor car engines were equipped with a generator run off a pulley attached to the engine’s crankshaft. These devices had several drawbacks, for example.

  1. They did not produce enough current.
  2. No sensor cut the circuit once the battery was charged, so batteries were often damaged.
  3. They produced a current equivalent to the engine speed. It meant if a car drove at night, the brightness of the headlights was determined by the engine’s speed.

The more complex, high compression ratio engines needed a 12-volt battery (instead of the previous six-volt unit) to turn the starter motors over.

It generated a need to improve the generators, so alternators became the standard.

Chrysler Motor Corporation launched the first car with an alternator in the early 1960s.

Pros Of An Alternator

The benefits of an alternator are.

  • Alternators generate higher energy levels and run more efficiently.
  • An alternator installed on a 350HP or 425HP engine will produce 100 amps.
  • Alternators are regulated to provide the same voltage irrespective of the engine speed. 
  • Alternators will not damage the battery by overcharging it.
  • Modern alternators can be set for the required current requirements of an electrical boating system.
  • Alternators are easier to access and remove for servicing.

Cons Of An Alternator

The downsides of alternators are.

  • Unlike a generator, an alternator cannot recharge a dead lead/acid battery.
  • Alternators require a battery in the circuit because they cannot provide the “surge” current needed to start the engine or power refrigeration systems.
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Most outboard motors (even those without an electrical starter) can charge a battery just like your car’s motor can charge its battery. Large displacement outboard motors do this as a rule.

Alternators vs. Stators


In some of the current low to midrange Yamaha and Suzuki outboard engine models, the stator is still installed as standard.

Pros Of A Stator

A stator has a few benefits to the boat manufacturer, as follows.

  1. Stators are integrated within the engine.
  2. Stators take up less space in an already crowded engine structure.
  3. Stators cost less than alternators.
  4. Stators have fewer components that an alternator which influences reliability.

Cons Of Stators

Alternators are slowly replacing stators for the following reasons.

  1. Stators produce less power than alternators; while this was sufficient for older boats, modern vessels need the high current generated by an alternator.
  2. Most stators only start charging when the engine RPM exceeds a specific value.
  3. A stator would not cope on boats that require large current values.
  4. A stator is harder to remove as it involves dismantling the flywheel assembly.

Why Does The Boat Battery Keep Going Flat?

boat battery

If you have a modern boat engine with a stator or alternator installed, there should be continual recharging of the battery, which prevents it from going flat.

If this has happened to you, follow the steps below to check where the problem lies.

Check The Stator / Alternator

The first and easiest check is to check that the stator or alternator is working.

If you doubt the type of charging system your engine uses, check the operating manual or on the manufacturer web site.

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You will need to start the engine for this test, so make sure that muffs are fitted, and water is flowing into the water intake.

  1. Use a voltmeter and connect it to the appropriate terminals on the battery.
  2. Start the engine and ensure water is squirting out the tell-tale.
  3. When the starter motor is activated, the voltmeter reading will drop.
  4. The reading should start to rise, and if the stator is working, the charge should increase above the reading displayed when you first connected the voltmeter.

To specifically test the function of the stator, you can also perform the following process.

  1. Remove the engine cover.
  2. Use a voltmeter that is set to the ohms (Ω) selection.
  3. Access the wire which comes off the regulator/rectifier and connects the stator.
  4. The wire should have a three-pin plug (representing the three coils) which you can disconnect.
  5. We will assign the titles L1, L2, and L3, running clockwise from the top position.
  6. First, test the natural resistance in the voltmeter by holding the two leads together. The reading should be between 0.1 and 0.2 ohms.
  7. Attach the voltmeter (ohmmeter)  leads to L1 and L2
  8. Repeat the test connecting L2 and L3 and finally connect L1 to L3.
  9. Each of the readings should have been between 0.25 – 0.37 ohms (Ω). Remember to subtract the baseline reading to get something more accurate.
  10. There is a problem with the stator if you get a reading higher than 0.4 ohms (after deducting the baseline) or if you get a signal on the meter showing it is an open circuit.
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Check The Boats Electrical System

If the charging system passes the tests above, check all the electrical consumers on the boat to make sure that they are switched off and not drawing any current.

A common culprit of a continuously drawn current is an alarm activated while you are away. These systems are notorious for flattening batteries.

Check The Battery

If the charging system passes, the next step is to check the battery itself.

Follow the next steps to check the condition of the battery.

  1. Using the voltmeter, check that the battery voltage is greater than 12V.
  2. Some batteries have a health indicator positioned on the top surface. If this displays a problem, it is generally very accurate.
  3. Look at the external battery casing and see if there is any damage.
  4. Check the condition of the terminals and ensure there is no oxidization or damage.
  5. Check the battery box to look for signs of acid leakage.
  6. If the cells have a cap, check the distilled water levels inside. If levels are too low, they need to be filled with distilled water.

If the battery fails these tests, it is probably time to replace it.