If the helmet has not been charged in a while the battery will park itself.
There is a way you can wake up the battery. Plug the round plug into the DC jack about half way of the light you are having trouble with and move it side to side a little creating a blinking charge light. When the charge light goes out for a second and dimly comes back on you can leave it plugged in. You will not need to do this every time you charge the helmet, only if it has been completed discharged.