Q:
A: Possibly over charging and as the headlight is a powerful wattage bulb it gets the hottest and therefore is mor susceptable to hightr charging voltage Chect that battery voltage at about 2000 to 2500 RPM and if it is above about 13.5 - 14 volts I would suspect the voltage control unit. When the regulator fails the voltage will rise as the revs increase with little or no control so the voltage will go up and up as the revs increase. So if you are getting 16 18 20 or more the bulbs will go The battery will also suffer if you continue without doing something about it. However the battery may survive once the voltage is under control Good luck and safe biking