A battery video guide, earcup guide and button-board guide show separate routes for battery life, cushion wear and failed volume controls.
What to check before buying
Battery and cushions
iFixit links a video procedure for the QC45 battery and a separate earcup replacement guide. Worn ear cushions affect fit and isolation without proving an electrical fault.
Controls
The volume and play/pause button guide covers a PCB used across QC35, QC35 II and QC45. A failed button should be diagnosed as a control-board issue before assuming the headphone battery is the cause.