Riders frequently contact us to ask whether they need to replace the rectifier when installing one of our headlight Voltage regulators.

“My headlight keeps blowing and I ordered one of your regulators. Do I need a need a new rectifier too?”

That’s a good question, and to answer it you can do a bit of testing yourself or send us your rectifier.

Here’s what we do in the Shop for a running bike:

You can easily give the rectifier a “dynamic” test using a Voltmeter that can measure AC RMS Volts. Many ancient or low-cost Volt meters cannot measure RMS Volts, so be sure your meter can do so, otherwise, this test is not accurate.

It’s a simple test:

  1. – Set the Voltmeter to measure AC Volts (AC RMS Volts, to be exact)
  2. – Connect the leads to the battery + and – wires.
  3. – Run the engine and measure AC Volts at the battery

Now you might think, “wait a second Pardue, isn’t the battery supposed to be DC Volts? How come you have me checking for AC Volts?”

This is because a rectifier is made of one-way valves (explained at http://www.parduebrothers.com/fullwave ) that do leak a tiny bit of AC power. The job of the rectifier is converting AC power to DC. This test verifies the rectifier isn’t super leaky, dumping AC Volts into the battery. Batteries do not like AC, they need DC.

A good rectifier will pass a tiny bit of AC Volts to the battery, like ½ a Volt or maybe a whole Volt.  Depends on the Volt meter and the rectifier.

A bad rectifier will pass a lot of AC Volts to the battery.  Anything over 1 Volt is really bad. The battery will chemically and thermally experience a quick death on AC.

Note, some inexpensive Volt meters will not properly differentiate DC and AC Volts.  In this case, the Volt meter will indicate many AC Volts when measuring the DC Volts in the battery, but it’s not true. This is easily verified: if the AC Volt meter indicates Voltage with the bike OFF, the meter is not capable of performing this test.


The second test is DC Volts.  This easy test proves out whether the battery is getting charge Voltage from the rectifier.

  1. – Disconnect the meter from the bike, and set it to read DC Volts.
  2. – Re-connect the meter to the battery again.
  3. – Measure DC Volts with the bike OFF.
  4. – Measure DC Volts with the engine idling.
  5. – Measure DC Volts with the engine revved up a little.
  6. – You should measure progressively higher Voltages.


Bear in mind the battery and the lights regulate Voltage on many small bikes. If a bulb is blown, or the battery won’t take and hold a charge, Voltage will soar and blow all bulbs.

Try this load test: If the battery is not capable of running the lights (tail, dash, and neutral lamp) for 5 minutes with the engine OFF but the key switch ON, the battery is not charged or is not able to be charged enough to be viable.  Charge and test again.  Replace it if the test is not passed.

A bad battery will allow the headlight Voltage to rise, even though the headlight is not powered by the battery.  The battery loads the Voltage down for all lights.

The sealed battery is great for trail riding, however, those batteries are famous for failing those who commute on these bikes.  Sealed batteries do not tolerate overcharging, they overheat and swell and stop working if overcharged.  Overcharge happens during sustained high-RPM riding such as on the street.  Trail riders tend to vary the RPM often and run at lower speeds, so it’s not as big a deal for that style of use.

A defective, shorted rectifier that passes AC Volts to the battery, will cook a sealed battery very quickly.  Replacing both is the only cure.

We only run wet cell batteries, just our preference.  Sometimes we do high RPM street runs here in Florida to reach a trail, which will cook a sealed battery in this heat.


If you need more help or just want to see the steps, you can buy our full-color PDF here.


If you find you need a new rectifier after testing, you can find our selection of rectifiers here.


Let us know how to help.