This replacement battery (supplier code EPO-CS-SXC550SL) is a rechargeable lithium-polymer cell for the Sennheiser PXC 550 and Sennheiser MB 660 wireless headphones, which EPOS also sold as the ADAPT 660. The CS-SXC550SL designation is Cameron Sino's code for this battery, and it replaces the original Sennheiser battery part AHB413645PCT (507271). Retailers list it at 3.7 V and 650 mAh (2.41 Wh), with dimensions of about 45.4 x 33.8 x 4.3 mm and a weight of around 13 g. Replacing a worn battery restores the headset's playback and talk time when the original cell no longer holds a charge, extending the life of an otherwise good business or travel headset rather than replacing it. Fitting involves opening the earcup and connecting the battery's plug to the headphone's circuit board, so it is best done by a technician or someone comfortable with small electronics repairs; handle lithium cells carefully and never puncture or short-circuit them. Only the battery is supplied. Check the headset model before ordering.