The rechargeable batteries are located behind the left ear cushion. Remove the ear cushion by rotating a quarter turn counter clockwise.
Ear cushions The ear cushions are secured to the earcups by means of a quarter turn fastener. To remove an ear cushion, turn it in the direction of the arrow until you overcome a slight resistance and lift it off the earcup.
Attention: Only use re-chargeable batteries, never use standard batteries! Insert the rechargeable batteries. Charge the batteries for at least 24 hours prior to first time use. Subsequent charging time is approx. 18 hours per charge. Place the headphones on the charging cradle of the transmitter. The red LED lights up, indicating that the batteries are being charged. The LED remains lit even when the rechargeable batteries are fully charged. The unit has overcharging protection.
Nearly discharged batteries will significantly reduce the sound quality.
It's just that simple.