Beyerdynamic Opus 500 Mk II Notice D'utilisation page 43

This is how to remove the belt clip
3.2 How to insert the Batteries
1. Push the two snap locks on the right and left of the battery compartment and open it.
Remove the batteries. Refer to Fig. 1.
2. Insert two 1.5 V batteries (AA) into the battery compartment observing polarity markings.
Refer to Fig. 2. Then close the battery compartment again.
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Fig. 1 Fig. 2