Voltage regulator (regulator-rectifier)

On most motorcycles the voltage regulator is combined with a rectifier in one unit, the regulator-rectifier. The alternator (stator) produces alternating current that rises with engine speed. The rectifier turns it into direct current, and the regulator limits the voltage so the battery is charged correctly and the electrics are not damaged.

Faults are common, often caused by heat or corroded connectors. Symptoms of undercharging include a battery that keeps going flat or dim lights; overcharging may boil the battery, blow bulbs or damage electronics.

Check the charging voltage with a multimeter at the battery with the engine running at raised revs and compare it with the manual; typical values are around 13.5–14.5 V. Clean and grease the connectors, and consider an upgraded MOSFET regulator on bikes known for failures.