Hi Norm,

Just checking I assume you did bring the motor to maximum rpm to check the voltage.
as we know
If the engine is running at a low RPM, the output of the alternator may not be sufficient to raise the battery voltage significantly above its current state. At low RPMs, the alternator may produce less voltage and current, which may result in little or no charging of the battery.

To charge the battery effectively, you'll want to ensure that the engine is running at a higher speed to allow the alternator to produce the necessary voltage for charging.

So most likely you've done the above and still not charging ,you would think that just leaves
a problem with the battery or the regulator ,assuming the wiring is correct and no problem with the motor earth to frame,

a few steps to troubleshoot the issue:

Check Rectifier/Regulator Wiring: Ensure that all connections between the stator, rectifier/regulator, and the battery are secure and free from corrosion or damage. A poor connection can prevent proper charging.

Check the Rectifier/Regulator: Even though you have a new rectifier/regulator, there is a possibility that it could be faulty. Test the rectifier's output with the engine running. You should see a DC voltage reading of around 13.5 to 14.5 volts at the battery terminals if the rectifier is functioning correctly.(this step was done / possibly bad regulator)

Measure DC Voltage: Use a multimeter to measure the voltage across the battery terminals while the engine is running at a higher RPM (around 3,600 RPM). This can help you confirm whether the rectifier is functioning. If you see little to no increase in voltage, that indicates a problem.

Check Battery Condition: Ensure that the battery itself is healthy. If the battery is old or has internal issues, it may not hold a charge even if the charging system is working correctly. Consider testing the battery's condition separately (e.g., using a load tester).

Inspect the Stator: Although you are getting the expected AC voltage from the stator, it can still have issues. Inspect the stator for any signs of damage, such as burnt windings or internal shorts that may not be immediately visible.

Ground Connection: Confirm that the ground connections from the battery to the frame and engine are solid and free of corrosion. Poor grounding can sometimes affect the charging circuit.

Test under Load: If possible, measure the output while the mower is under load or at higher RPMs, as some systems may require a certain load before they begin to charge effectively.

Cheers
Max.