Do Motorcycles Have Alternators? Understand the Modern Electrical Systems and How to Maintain Them

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Motorcycles have alternators whose primary function is to generate power for all electrical appliances while keeping the battery charged during operation. It’s part of the motorcycle charging system, which takes over from the battery as soon as the engine starts up and revs at normal idle speed.

The battery supplies the initial power to boot up the electronic controls like ECUs, fuel pump, lights, displays and most importantly the starter motor, which turns over the engine to crank it. This draws a huge amount of power from the battery, which it needs to top up after each cold start, and that is where the alternator comes in.

In Summary, Here Are the Different Types of Motorcycle Alternators and Their Characteristics

Alternator Type /CharacteristicPermanent MagnetBrush-TypeBrushless
ConstructionUses fixed magnetic fieldUses brushes and slip ringsUses electromagnetic commutation
Low RPM OutputHighVaries with designHigh
Reliability and DurabilityVery reliable with fewer moving partsRequires regular maintenance for brushes and slip ringsMore reliable than brush-type alternators
Power OutputSmooth output but may drop with an increase in temperatureStable but may fluctuate if brushes are worn outStable
CostCheapestLow costExpensive and may require specialized controllers
Maximum Power300-500 watts200-400 watts500-1,000 watts

Where Are Motorcycle Alternators Mounted and How Do They Work?

An alternator stator in the final mounting position inside its crankcase. In this design, the rotor rotates on the outside of the coil.
The new Harley-Davidson’s New Milwaukee-Eight Big Twin engine features an external alternator with a staggering 18 poles in order to generate more juice as low as 850 RPM idling speed.

Motorcycle alternators are less obvious than their car counterparts because most motorcycles, except for large cruisers and touring bikes like the Honda Goldwing, don’t have the space to fit a one-piece automotive-style alternator. Incidentally, these bikes have an insatiable appetite for power with a horde of electronic accessories, which require a more powerful energy source

Every other motorcycle has a three-piece alternator consisting of a rotor, stator, and regulator/rectifier. These components make up the charging system, despite being mounted separately due to space limitations.

All Motorcycle Alternators Work Pretty Much the Same Way

An alternator is an electro-mechanical device that converts engine rotations (mechanical energy) into electrical energy (in watts). The working principle of these machines is guided by Faraday’s law, which states that when a conductor is placed in a varying electromagnetic field, an electromotive force (emf) is produced and if the conductor is a closed loop, a current is induced.

Put simply, if a coil of conducting wire is moved relative to a nearby magnet, electricity is produced in the wire whose voltage and current is proportional to the rate of change of the magnetic field (the speed of rotation for an alternator). This means higher revs produce more power in a motorcycle charging system.


A new rotor assembly replacement part for motorcycle alternator.
Motorcycle alternator rotor with splines in the center for the crankshaft, which rotates it. On the side, tabs read by the CDI pick-up coil, which controls the spark timing.

The rotor is essentially the part of the alternator that rotates with the crankshaft on which it’s mounted. It is the magnet half of Faraday’s law and could use permanent magnets or an electromagnet (uses a small current from the battery to set up a magnetic field).

In the latter types, the excitation current (providing initial magnetism) may be delivered to the rotor with physical carbon brush contacts or using magnetism, leading to three types of alternators.

Brush-Type, Brushless and Permanent Magnet Motorcycle Alternators

Brush types suffer heavy wear at the brushes and may be inefficient as friction eats away at the brushes. In contrast, brushless motors have significantly less moving parts using magnetic fields to transfer the energy without physical contact. They are more durable and efficient as a result although much more expensive.

A third type of motorcycle alternator uses permanent magnets in its rotor and doesn’t need an excitation current to start generating power as magnetism is always present by nature.

Side Note: In some motorcycle engines, the magnetic rotor also supplements the crankshaft, flywheel while in others it forms part of the flywheel. When this is the case, the alternator assembly may be called a flywheel magneto.


Part image of a generic stator for use with motorcycles. This stator has a total of 18 poles and a three phase output carried by three yellow wires.
A 18 pole motorcycle stator with three phase output configuration.

The stator is the part that contains the coils of wire in which the induced (generated) current flows. It’s typically made of laminated soft iron poles on which thin copper wires are wound and remains stationary.

Rather than one giant coil, there are often several coils, usually up to a dozen, each wound on a pole. It doesn’t matter whether the magnetic rotor revolves on the outside or inside of the stator; as long as there is relative motion between the two, electricity is generated. And because the magnets have a north and south polarity, the current induced in the stator coils repeatedly changes direction as they revolve.

This leads us to the third and final piece of the alternator puzzle…

Motorcycle Regulator/Rectifier

A replacement part for motorcycle alternator regulator. It’s a semiconductor based device with metal enclosure and heat sink.
The metal casing and heat sink design helps the regulator lose the excess power received from the stator as heat.

Yes, it’s that simple, a rotor with varying magnetic field rotates with the engine near a stationary (stator) coil and, bam, you have juice! One itty bitty issue though, the resulting voltage is of alternating current (AC), which is not compatible with the bike’s 12V DC system. And so, we feed it into the regulator/rectifier to convert it to direct current (DC).

Each motorcycle power regulator is designed to limit the voltage in the system within acceptable limits typically 13.8-14.5V. Total power output of a motorcycle alternator is 300W to 500W and may vary according to the requirements of each design. Regulators can be mechanical, shunt (parallel) or series regulators.

Mechanical regulators have physical contacts that open and close to control the voltage injected to the system. Shunt and series regulators are more modern and use transistors and diodes, respectively, to achieve the same goal.

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4 Common Problems with Motorcycle Alternators

Inspecting a motorcycle stator for damage. In this design, the stator bathes in engine oil, but damage is clearly visible on the blackened upper coils.
Excessive heat due to high loads and operating temperatures cause the insulation on stator coils to melt, leading to failure.

Motorcycle alternators face many hardships, not the least of which is being tucked away in the engine crankcase, where they fade into oblivion until they finally snap! And when they do, signs could include a dead battery, dim lights that brighten with higher revs, weak spark or no spark at all, difficulty starting the bike, and lagging acceleration.

Despite their small sizing due to space constraints, they have to deal with angrier engines, revving often, revving upwards of 10,000 RPM, and power everything as the battery is only good for starting. That is really immense pressure, but these systems, as you would expect, really push the limits before tapping out.

Here are four common issues a mechanic can help you fix when your motorcycle alternators stalls:

1. Worn Brushes

Brush-type alternators utilize carbon brushes to transfer energy between the rotor and stator through constant rubbing, which leads to wear and tear. As time goes by, the brushes become depleted, resulting in sparking and power loss at the junction, ultimately resulting in failure.

2. Belt Tension for Motor-Vehicle-Style Alternators

A few motorcycles use external alternators like those you find in cars, and they rely on a belt drive to transfer the engine’s rotation to the alternator’s rotor. If the belt is loose, slippage occurs, leading to reduced power output. On the other hand, if the belt is too tight or worn out, it could cause damage to the bearings, eventually leading to alternator failure.

This brings us to the next common issue…

3. Bearing Failure

Again, external alternators are particularly prone to this issue because their bearings may not receive enough lubrication. When bearings fail, it doesn’t spin as fast, generating less power while producing excess heat and a whining noise.

4. Overheating

Overheating cuts across all alternators. Because of the high speed of operation and high power demands of the motorcycle, alternators generate a lot of heat, which can melt the insulation on the windings. Cooling system failure or inefficiency could cause a gradual build-up of heat.

Bigger alternators under heavier loads tend to produce more heat, which is why they may be placed outside the engine case with a dedicated cooling fan.

My Final Thoughts on Do Motorcycles Have Alternators?

Upgrade your motorcycle's charging system with a sleek chrome left stator, magnet, and regulator.
Stylish chrome stator with magnet and regulator, essential components for powering up your ride with a touch of elegance. #motorcycleparts

Alternators can be quite technical, and I wouldn’t recommend tampering with them unless you are a qualified professional with a good understanding of how they work. Further, it’s not feasible that you try and change out one type of alternator for another in your motorcycle as this could burn fuses and other problems.

However, maintaining a healthy motorcycle electrical system with clean and dry sockets, proper rated fuses, a healthy regulator and battery with clean terminals are a good place to start for a positive trickle-down effect on your alternator’s lifespan.

Also, avoid overloading the alternator with too many aftermarket electrical accessories, like auxiliary lights as well as heated grips and seats, which may not be accounted for in the original design.

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Information for this article was partially sourced and researched from the following authoritative government, educational, corporate, and non-profit organizations:

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About the Author:

Michael Parrotte started his career in the motorcycle industry by importing AGV Helmets into the North American market. He was then appointed the Vice President of AGV Helmets America. In total, he worked with AGV Helmets for 25 years. He has also served as a consultant for KBC Helmets, Vemar Helmets, Suomy Helmets, Marushin Helmets, KYT Helmets, and Sparx Helmets.

In 1985, he founded AGV Sports Group, Inc. with AGV Helmets in Valenza, Italy. For over 38 years now, the company has quietly delivered some of the best protective gear for motorcyclists in the world.

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