The MB Pro 1 and 2 contain a rechargeable lithium polymer battery. With time and use, this battery will degrade, and after 2 to 4 years, it may become necessary to swap it out.
A replacement battery can be ordered from your Sennheiser partner (BATT 03 DW, article no. 504374), and can be installed by the end user, sparing you the need to send your headset in for repair.
For a detailed instructions on swapping the battery of your MB Pro 1 or MB Pro 2, please see the guide attached below.