Modern Vespa

2 years ago, I replaced my stator (alternator).
1 year ago, I replaced my voltage regulator.
This was my 3rd voltage regulator in 20 years (~45,000 miles).
My battery is 3 years old.



A year ago, with new voltage regulator, with engine ON, I recall charging voltage was around 13.5V even during idling.



But in the last two days, I had trouble starting my scooter. With engine off, my battery read only around 12.2volt. Scooter started with jump. Used one of those portable battery jump. Now with engine ON, charging voltage is only around 12.2volt.




These are the tests that I did.

1) I could tell each of the three diodes in the voltage regulator are working (infinite Ohm in one direction and finite small Ohm in the other direction).

2) I tested this (#1) using ground to diode, and negative to diode.

3) I tested that ground wire is connected to the scooter body (electrically negligible Ohm).

4) With engine ON, each phase of the stator (yellow wire) is putting out around 6.5 volt AC. Could someone comment. Is 6.5 volt normal?

5) But the "combined" DC output from the voltage regulator is only around 12.2volt




Even with #1, #2, #3, can the voltage regulator still be broken?

Should I trickle charge my battery overnight? Does voltage regulator use battery's output voltage as some reference voltage?

I would suggest that a three-year-old battery is inherently suspect, and would remove that from the equation before pursuing component failures. With the scooter started and hooked up to the jump battery, see what the voltage is as you rev the engine.

The voltage regulator uses ground as a reference, not the battery voltage.

Thank you, Juan! Very helpful.

Rickster

At idle, right after you start it, 12.2 is within acceptable range. When you rev it up, the voltage needs to rise. At about 4000 rpms, if i remember correctly, it should be above 14 volts.
If it's not charging, the voltage when you rev it won't go up, and will go down gradually while you ride.
it takes a long time to run a battery dead, and there are a lot of people who keep it on a tender and only take short rides during the day that go years with a broken charging system.