DFDR Battery
For info - the DFDR is only a memory stick with a lot of protection and doesn't have a battery. Normally over rear galley and is accessible in flight.
The battery referred to is the DFDR ULB ("pinger") and it was the 'shelf life' that had expired; may still have worked anyway for 30 days when activated. The problem is that the 30 days had expired by the time they got to anywhere near the current search area.