The basics of motorcycle charging systems

Electrical problems in motorcycles can get annoying to the point that you’re clueless as to what exactly is going on. They’re hard to diagnose, difficult to pinpoint and, if not found on time, can lead to expensive repairs. Then, there’s the complexity of different parts, with the ignition, charging and lighting systems all separate but connected by a mutual component—the battery.

The bike not starting or the lights dimming can mean different things. Most bikers will assume it’s a dead battery, but delving deeper can mean something entirely different.

What is the charging system on your bike?

Three components comprise the charging system on your bike. The battery is at the core of the system and stores the power needed for the different bike parts. The alternator produces alternating current with the engine on and helps recharge the battery. The motorcycle voltage regulator rectifier converts AC power from the alternator into usable DC, then regulates the direct current so it doesn’t exceed a specified voltage, typically 14.5V, so it doesn’t fry any components. 

This is the simple take on things, with the system also consisting of wiring and circuit protection devices to prevent minor faults from turning into something more serious. 

The role of batteries and alternators

The battery provides the juice to start the bike. It also needs to have enough stored current when the alternator is underperforming or going bad so that components work as they should. An often neglected role is the protection batteries provide from overcurrent from the alternator, even if the regulator is fried in the process. Similarly, it’s the battery that protects sensitive circuitry throughout the bike from high voltage. 

Alternators on bikes are of the two-piece AC type, consisting of the rotor and stator, paired with the regulator rectifier. Rotors are alternator parts mounted on the crankshaft that rotate with it to produce power. 

There are three types: older brushed, newer brushless, and permanent magnet rotors. The last two are more common and able to sustain high output even at lower engine and crankshaft speeds. Brushless rotors are a newer design that can provide high amounts of power for bikes with a lot of electrics and a more stable supply of power.

Stators are the collection of coils inside the crankcase. As the rotor spins past the coils, magnets induce an electromagnetic charge, and one with an alternating current alternates between the north and south poles in the magnet. 

Enter regulator/rectifiers 

The large quantities of alternating current need to be converted into usable DC power that the battery and 12V components can use. This is the job of the rectifier. In modern bikes, this component is paired with a voltage regulator, or charging system part, that regulates and modifies the voltage from the alternator, keeping it under 14.5V.

This is done with a sensing element that constantly monitors voltage levels in the battery and cuts the current from the alternator once a voltage limit is exceeded. Since voltage spikes and unused power generate a lot of heat, the rectifier/regulator additionally includes a heat sink to dissipate high temperatures. 

Regulators (of the field coil type) and rectifiers were separate parts on bikes in the 1970s. Still, the shift has been to incorporate the parts into a single unit, connected or part as the alternator itself. This is the origin of the rectifier/regulator name. Bikes with larger displacements use the three-phase type, whereas motorcycles with less power require single-phase regulators. 

While there’s the possibility to repair stators and rotors, the motorcycle regulator can burn out, often due to prolonged periods of high voltage. This risks damage to the battery and all electrical system parts. In cases like these, the only solution is to get an OE replacement specified for your bike. 

Signs of faulty and damaged regulator rectifiers

Regulator rectifiers are placed in areas with good airflow due to the high heat they generate. Heat is the biggest killer of the devices, but loose battery connections, near-depleted batteries and poor grounding in three-phase types can also cause problems. Once the rectifier can’t transform AC power into DC and the regulator is unable to contain overvoltage, you’ll start to notice a few common signs:

• Dimmed or flickering lights—this is one of the most common problems with a faulty regulator rectifier and often the first you’ll notice. Either it’s headlights that erratically change between bright and dimmed on their own, or there’s less brightness and visibility with the high beams. 
• Change in engine performance: abrupt or gradual changes to power delivery, such as sputtering, stalling, or sloppy acceleration, can be caused by a faulty regulator rectifier incapable of controlling the output from the alternator. 
• Dead or depleted battery: a drained battery can be from dozens of different things, but a bad motorcycle voltage regulator is usually the cause (or a faulty alternator) if the battery isn’t getting enough charge. Conversely, the battery can be overcharged or exposed to enough overcurrent to cause swelling or leakage. If you are experiencing hard starts more frequently, it is time to check the electrics, starting with the regulator. 
• Engine and/or battery light – often, one of the first warnings of a poorly performing battery is through the instrument cluster on dashboards or screens. The check engine and battery lights can mean a faulty alternator (the diodes in the rectifier or the stator), a bad regulator, a battery losing charge, or the computer registering faults in power delivery. Additionally, any of the (analogue) instruments and gauges can start to flicker or not work at all. 

Troubleshooting issues and finding replacements

With the engine off, use a voltmeter or multi-meter to check the voltage coming through the battery terminals. This should ideally be between 12.5 and 12.9 volts, but below 13.2 volts. Check the voltage again, this time with the engine idling. The recommended reading here should hover around 14V.

Check the voltage once more, this time with the throttle turned. A reading at or under 14.2V means the battery is charging and there are no issues with the regulator or alternator. Of course, readings that are way over or under this average point to a defective part, and you’ll need a working replacement before other parts are damaged as well. 

Regulator rectifier spares are easy to find. Just look up your nearest bike parts store that caters to the bike you ride. Regulators are different on different bikes (including bikes in different trims) due to the different power requirements.  Choose a replacement for your make, model and production year, and get the part changed by a pro to ensure it works.