idling with the light on will drain the battery,if you cant maintain say 3000 rpm the battery will go down,running a lower watt globe will help,or switch of the light at stop lights if its not night time,if you have a trickle charger use it once a week to keep the battery up,,if the battery is old it might be having a hard time accepting the meager charge,if the battery is say three years old replace it if your doing lots of the stop start riding.