Try to tighten the end cap all the way until it stops by twisting clockwise. If it turns on, GREAT! If it doesn't, then try swapping out the extra battery with the positive (+) end first and tighten the end cap by twisting it clockwise.
It depend on what mode the light is in, typically from 4 hours of constant light to 16 hours of flashing light. We recommend the flashing mode for riding since the flashing light will capture the attention of car drivers.