I have a Chinese 150cc scooter with a regulator/rectifier that is supposed to regulate the DC output to about 14.5V. I'm seeing around 15.5V (when not at idle) under normal operating rpm.
However, when I add an extra 15 watt load to the DC circuit ( 15 ohm power resistor added across the battery terminals) the voltage is properly regulated in the mid 14V range at all rpms, where it should be!
Why is this happening, and is it okay (for the regulator) to operate with this extra load on the circuit?

 

It is ok but, instead of wasting the energy as heat through a resistor, you might consider using the headlamp instead. Most countries require the headlamp at all times as a safety fracture to increase your visibility at all times.
Many voltage regulator circuits require some minimum load to stabilize the voltage regulation. I expect your scooter's was designed with the headlight as the minimum load. Note: most countries that require the headlight don't even allow an on/off switch - that's why I am guessing you are not in the US or Europe (there may be an exception for schoolers under 175cc).

 

It is ok but, instead of wasting the energy as heat through a resistor, you might consider using the headlamp instead. Most countries require the headlamp at all times as a safety fracture to increase your visibility at all times.
Many voltage regulator circuits require some minimum load to stabilize the voltage regulation. I expect your scooter's was designed with the headlight as the minimum load. Note: most countries that require the headlight don't even allow an on/off switch - that's why I am guessing you are not in the US or Europe (there may be an exception for schoolers under 175cc).

Thanks for your reply MrSalts!
My headlight is on the AC circuit and always on, so it's already being "used" to regulate that circuit. Your suggestion is a good one, and I do plan to add "additional lighting" to the scooter to replace the resistor as a load. I also think, as you state, that the regulator just needed a bit more load to regulate properly.