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, https://en-us.sennheiser.com/accessories-dw-batt-03-dw ), 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.