The battery is located behind the speaker without the microphone boom. It can be a bit tricky to get to, so here are some instructions to help you out!
Removing the battery on the Sennheiser MB Pro 2
1) Remove the ear cushion. The cushion has padding and a plastic plate that snaps off or on. You can fit the plastic ear plate back into the cushion if you accidentally separate them. Make sure shiny black circle on ear plate is facing out before cushion is attached to the headset
2) Once the cushion is removed, you will see a latch that releases the battery door on the opposite side. You will need to flick the latch up towards the headband to remove it
3) Once you’ve pressed the latch, the battery door with the Sennheiser symbol on it can be removed. The door is on the opposite side of the speaker, One end of the door has a long tab (used for the latch). When you put the door back on, make sure that the latch tab is towards the bottom away from the headband
4) With the door removed you can see the battery. The white coupling where the wires meet the speaker is kind of difficult to get to
5) Putting the battery back in, you want the metal lines on the coupling to be facing down away from the headband
With the battery back in, the door should snap back on easily.
Time for a new Bluetooth headset?
Many Sennheiser Bluetooth Customers have made the switch to the Leitner LH375!