Motorcycle Stator, Alternator, And Battery Fundamentals

Charging systems—which are comprised of your motorcycle's stator, alternator, battery, and regulator/rectifier—are the proverbial redheaded stepchild of motorcycle maintenance. In every case they're ignored when they're good, cursed when they go bad, and, at best, receive only the most grudging care.

The fact that the majority of riders have never futzed around with their charging system is a good thing, and in fact, none of us should be surprised by that because, current charging systems (note clever pun) are absolute paragons of reliability, particularly when compared to their forerunners.

That being said, it’s always helpful to have a passing understanding of how your motorcycle and its subsystems make it work. So, in an attempt to rectify (another clever pun) the situation, here’s a primer on motorcycle charging system fundamentals.

The charging system consists of three main components: the battery, alternator, and the regulator/rectifier. Your motorcycle battery stores electrical energy for use and acts as a buffer for the bike's electrical system. The alternator creates alternating currents to run the bike and charge the battery. The regulator/rectifier changes, or rectifies, the alternating current produced by the alternator into direct current so it can be stored in the battery and regulates the amount of current produced to prevent overcharging.

You can also consider the wiring that ties everything together, along with a fuse or two and in some instances a relay, as part of the system. As a rule, all motorcycle charging systems are purposely designed to provide somewhat more power than the bike actually requires. That provides an extra margin of reliability and allows you to add some electrical accessories without overtaxing the charging system.

The motorcycle battery's primary job is to start the bike and to supply any extra current the bike requires when the alternator output is lower than the current draw. This normally only happens at low rpm, when electrical demand is high and alternator output low, as it might be when stuck in slow traffic or cruising at low speed.

Lots of high-draw electrical accessories exacerbate the situation and may cause the battery voltage to dip below the point where it can restart the bike. The easiest way to avoid problems here is to monitor the battery’s charge.

The battery also acts as a buffer between the charging system and the electrical components. Alternators are capable of putting out some very high voltages; in many instances more than 75 volts AC can be generated, which is more than enough to fry everything in the system. The battery prevents the alternator from running away, even if the rectifier/regulator fails, though it might sacrifice itself in the process.

Additionally, the battery also protects sensitive system components from being damaged when normal voltage spikes occur, as they do when a circuit is switched on or off.

There are two types of mechanical devices used to create electrical energy. The DC generator was popular on motorcycles developed before the 1960s, before modern accessories, such as the electric starter and turn signals were added. Modern motorcycles use an AC generator or alternator, which produces alternating current. Since DC generators are no longer in popular use, we’ll confine our discussion solely to the alternator.

While there are a number of variations on the theme, all motorcycle alternators boil down to two types. The first and less common is the automotive-type, externally mounted, self-contained “one-piece” alternator, wherein everything needed to create and regulate the current output is housed in one unit. These are typically driven by gears or a small chain from the crank, which allows the manufacturer to gear it up or down as required to achieve the correct rotational speed and current output. Because they’re the most powerful type of alternator, they also create the most heat, which is why they’re mounted externally. Alternators like these are sometimes found on large touring bikes, like the Honda Gold Wing, where lots of generating capacity are required.

Everything else has what’s sometimes called a three-piece alternator, the three pieces being the rotor, stator, and a regulator/rectifier. Although the components work in conjunction with each other, they are stand-alone units that can be serviced as such.

The rotor is a small wheel mounted directly to one end of the crankshaft. Depending on the type of alternator being used, the rotor may have small powerful magnets inset within it, or less commonly a field coil, which is energized by a small amount of current to form a powerful electromagnet. The latter types are called “exciting field coil” alternators and come in brush and brushless styles.

The stator is a series of soft iron poles, usually made with laminated bars, wound with many feet of thin copper wire. Each pole/wire assembly is called a coil, and there may be anywhere from one to a dozen or more of them in the stator, depending on how much output is required.

Depending on the design, the rotor may be the external type, meaning the magnets revolve around the stator, or the internal type, which means the magnets rotate inside the stator. Both types function in exactly the same way.

As the rotor spins past the coils, the rotor's magnets induce a current in the stator's coils. That's called electromagnetic induction, a term you might recall from high school physics. Because the poles of the magnet are oriented north-south, the current reverses itself each time a magnet sweeps through its field, creating an alternating current.

Factoid: Besides its role in generating current, the rotor also supplements the crankshaft's flywheel. In fact, in some engine designs, the rotor is the flywheel, in which case the assembly is known as a flywheel magneto.

Alternators put out a boatload of current—far more than any DC generator and in a much smaller package—but there’s one problem. Alternators produce alternating current, but the battery, which is the foundation the charging system is built on, can only store direct current. The solution is to incorporate a rectifier, which is a device that changes the alternator’s AC output to DC, so it can be stored in the battery.

Early on, the rectifier was a stand-alone device. Alongside it, most charging systems also used a regulator to control the alternator’s output. A lot of British bikes used a Zener diode, which in essence was nothing more than a preset electrical switch that monitored voltage. If the voltage was under 14 volts, the Zener turned off and all current was sent to the battery. When the electrical demand was low and charging voltage approached 15 volts, the Zener turned on and routed most current to ground. It wasn’t a great system and is no longer used, but it did work.

Other systems (BMW, for example) used a mechanical voltage regulator to keep tabs on their alternators. Over time, the rectifiers began to incorporate the regulator’s function, which resulted in a nice compact unit that could control every aspect of the charging system’s regulation.

Factoid: Because the rectifier/regulator is constructed of semiconductors, it radiates a lot of heat, so they’re normally constructed with printed circuit boards mounted in a finned aluminum case and mounted someplace where cooling air can dissipate the heat.

When the key is turned on and the starter button pressed, current flows from the battery to the necessary components, like the starter motor, ignition system, and fuel-injection system. Starting the engine uses some of the battery currents, and the drain may be intensified by electrical accessories or a low idle, which doesn't spin the alternator fast enough to recharge the battery during warm-up.

As the bike is ridden and engine rpm pick up, the output of the alternator also increases. The current is conducted by the wiring harness to the regulator/rectifier. It reaches the rectifier as alternating current, where it passes between one and six diodes—depending on the type of system and its capacity. The diodes, which are electrical doors, only allow current to move in one direction, so only the positive voltage reaches the battery and the bike’s electrical components.

The most common way to control the alternator is by using a thyristor in conjunction with a voltage sensing circuit to regulate the DC voltage. When the DC hits a predetermined level, typically 14.5 volts or so, the sensing circuit tells the thyristor, which is nothing more than an on/off switch, to turn on. When it does, it connects the stator coils to ground, which kills the alternator output. As soon as the voltage drops below a certain level, usually around 13.0 to 13.5 volts, the thyristor turns back on and the alternator output picks up.

Factoid: If all of this turning off and on seems like it takes time, you're right; under normal circumstances, the cycles take place thousands of times a second, so fast that only a semiconductor can make happen, which is why mechanical voltage regulators have fallen by the wayside.

In a nutshell, the alternator puts out current, which charges the battery and runs our electrical accessories, and everything else is just a detail. So what kind of maintenance does the electrical system require?

Of utmost importance is the battery. As we’ve discussed many times, battery terminals need to be kept clean and tight, and if you’re using a flooded cell battery, the water level should be checked and topped off on a regular basis. You also need to keep your battery charged. Batteries with low initial charges force the charging system to work harder than normal; a hardworking system puts out a lot of heat, which is always detrimental to a charging system’s health. If you don’t ride the bike much, keep it plugged into a battery tender.

As far as your motorcycle’s regulator/rectifier goes, there’s not much you can do. I like to look at the connections whenever I can; loose or corroded terminals create high resistance, which overheats the connector and leads to failure. Overheated connectors will often melt or discolor, which is proof positive that your electrical system has been overloaded and if an overheated connector is caught in time it can usually be repaired before any other problems occur. If the regulator/rectifier is mounted where you can get at it, make sure its mounting bolts are snug and that the cooling fins aren’t obstructed by debris.

Lastly, whenever I service my bike's battery, I throw a voltmeter across the terminals just to make sure everything is up to snuff. A decent voltmeter can be had for less than $20. With the bike idling, measure the DC voltage across the battery terminals. It should be close to 13 at idle and rising as you increase the engine speed. At anything over a fast idle, the battery voltage should measure 14.0 to 14.5 volts.

Triumph regulator/rectifiers don’t have a great reputation. Suffice it to say the damn things fail on occasion, and even when they’re working, they’re not all that great. Part of the problem is that they often charge inconsistently, so the battery may maintain enough of a charge to start and run the bike yet never become fully charged.

Initially, I was going to install a new and expensive OEM rectifier, but another popular upgrade is to install a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) rectifier. Compared to OEM, they are roughly half the price and twice the rectifier of the OEM unit. In the interest of full disclosure, I should point out that the MOSFET upgrade is popular with lots of ADV and sport-touring guys, and the forums are full of info about them, so don’t think this upgrade pertains only to Triumphs.