When the indicator light on the right earcup is illuminated, that means that the power is on. A flashing indicator light on the power switch tells you that the battery is in a low-charge state and will soon need recharging (the battery has less than five hours of life remaining when the flashing starts).
Note: When replacing a battery, make sure you charge the new battery for a minimum of two hours. And to maximise battery life, make sure that you fully charge the battery before storing it for an extended period of time.