Ripple Voltage

📖 YOUCANIC Automotive Glossary

Ripple voltage is the residual AC voltage component present in the alternator’s DC output, caused by the rectifier diodes converting the alternator’s three-phase AC generation into DC. A healthy alternator with all six diodes working produces very low ripple (under 0.5V AC measured at the battery with the engine running). When one or more rectifier diodes fail, the ripple voltage increases significantly (above 0.5V AC) because the failed diode creates gaps in the rectification process, producing a pulsating DC output rather than smooth DC.

Excessive ripple voltage can cause dim or flickering headlights, radio interference/whining, erratic gauge behavior, and premature battery failure from inconsistent charging. DIYers can test ripple voltage by setting a multimeter to AC volts and measuring at the battery terminals with the engine running at 2,000 RPM — readings above 0.5V AC indicate a diode failure. An oscilloscope provides a more detailed view, showing the actual waveform pattern that can identify exactly how many diodes have failed. Alternator replacement is typically required when diodes fail, as individual diode replacement is rarely practical.