Modern Vespa

Recently replaced the stator on my 1981 P200E after an intermittent ignition issue nearly left me stranded. Problem is gone and I recently replaced a dead battery with a Weize YTX5L-BS sealed battery. This battery is a smaller form factor and 4Ah rather than the 5.5Ah of the old Yuasa.

Checked the voltage today because the old battery did not last very long and I suspected a fault with the charging system. 12.7V at rest. New battery and new stator. Voltage at idle about 13V but rose to 15V+ as the revs increased (shut it off since this was high enough to present a problem).

Seems like the voltage regulator is broken. It should clip the voltage around 14.4V - 14.8V but that is not happening. I have a spare unit and will see if it makes a difference or try an aftermarket unit.

Question is whether the lower Ah rating on the battery could play a role in the high voltage. The delta is only 1.5Ah and this is a kickstart bike that only runs taillight, signals and horn off the battery. I don't see why a low Ah rating would cause high voltage readings - should just mean less available amperage but then this bike cannot be applying much if a load, probably only a couple of amps at most. That's the reason I got the smaller battery (& price!).

Your voltmeter could be reading a little high.
I have a Fluke meter that reads pretty much the same, but I have never seen a brand new, fully charged battery that has been resting that reads more than 12.4. The 15V reading at higher throttle may actually be 14.5V, which would be normal.
Also, is your headlight fed by the battery or stator/regulator on your P200?

Interesting. I have a couple of spare single-yellow regulators in the garage and I swapped one out to see if it would make a difference. Now the battery reads 12.6V at idle and no higher than 12.8V when the motor is wound up. Now that suggests a deficit of voltage from the stator (which is new and clearly worked with the other regulator).

I feel compelled now to try the third regulator I have and see whether there is a difference. More to come.

Also, the battery capacity, as far as I know, does not influence its charging ability; so, since your P200 is not equipped with e-start (like mine) a smaller battery will work.
And reading the electrical diagram, the headlight circuit, as originally designed, works off the stator/regulator AC side, so the battery has no role in its charging.
That type your regulator/rectifyer you have is the same as mine, and is extremely reliable. Mine has worked fine since new, over 38 years, but perhaps your first one may be defective. It is also very important that it is grounded well to the frame in order to perform its function.
I suspect that, since you replaced it with the second one, you cleaned the mounting screws holes without realizing it, and your readings are more normal, now.

Update. I have now run all three of the regulators with the engine at idle and steady in the midrange.

On two of the regulators, the voltage reading on the battery steadily creeps upwards as high as 15.05V and 16.42V, respectively, before I shut off the bike. (Strangely, the little Battery Tender voltmeter attached to the pigtails read only 12.8V). Even after shutoff, the voltage crept upwards for a few seconds.

On the third regulator, the voltage sticks at 12.6V with the engine running and taillights illuminated (by the battery, you you noted).

These are obviously totally inconsistent outcomes. The two results suggest that the regulator is not clipping the voltage as it should, whereas the third suggests that the stator is not delivering AC power OR the regulator is not flattening to DC. What is especially unusual is that, for the two regulators, the voltage creeps up very slowly, .01V at a time, even with big swings in the engine speed. Something is not right here.

Unless you have already done it, I would pull the flywheel, remove the stator and closely inspect all its wires, windings, and their connections for insulation heat damage.

Already done. The stator is brand new with less than 1,000 miles on it. It is providing plenty of A/C current since the headlight works very well. I also believe that the rectifier is working, since it pulses the taillight with low battery. It seems to be the regulator circuit.

Are your regulators this model?

https://www.scootermercato.com/Scooter-Parts/Regulators/185824

This is what I had on my PX150E, with single yellow wire stator, and it always worked great. I just had to replace my original stator, which had crumbling wires and no ignition.