You can still listen to your headphones, even when the battery has died. A 3.5 mm jack is included in the box. Connect the headphones to your smartphone (or whatever device you’re listening to) with the jack and continue listing. The touch controls, however, will not work because the smart phone device does not put out enough power to run the headphone’s internal electronics.
You can listen to music on the headphones while they are plugged in and charging. Sound balance in passive mode may not be as good as in active mode because DSP can't be used. Battery is not replaceable.