On the front of the transmitter there is the battery charging indicator light and when the battery is charging this indicator will light up AMBER. NOTE: Once the battery is charged the AMBER light will go out.
The batteries are initially charged at manufacture but Sennheiser has no way of knowing how much the batteries will be depleted from the time they leave the factory to the time a customer sets them up so we recommend the 16 hour charge to be on the safe side. Initially the charge may be quite short but once you start using the headphones the charging process will take longer and the charge light will stay on for longer.
Initially when the headphones are brand new the charging light on the transmitter may appear intermittently. Usually it will take a few charging cycles (ie: using the headphones and charging the headphones) for the charging electronics in the headphones and transmitter to get into sync and the issue will resolve itself.
When the headphones have been in use for awhile occasionally the headphones can develop an electrical glitch where the LED indicator light and the internal electronics get out of sync so the light will either not come on as expected or remain lit after fully charging or be intermittent. This may happen when products are disconnected from power for a period of time, the batteries have recently been changed or a power surge has occurred. It usually takes a few charging cycles (ie: using the headphones for a few hours and then charging them) for the lights and internal electronics to sync up. If you are experiencing this issue simply use the headphones and charge the headphones in a normal manner and the indicator light and internal electronics will get into sync and the issue will resolve itself.