When charging your head lamp, the reading light LED's will continue to accumulate and flash as the charge increases. When all 4 LED's are flashing this indicates the head lamp is fully charged.
After a full charge, the flashing LED's may turn off after a short period of time, usually five minutes. When unplugged from the charger, you can check the charge level of your head lamp without turning it on by quickly pressing one of buttons.
If your head lamp does not seem to be charging, please check out this article.