Synchronous playback with battery-powered playback device

A system comprising first and second playback devices, wherein:

• the first playback device comprises at least one battery, a processor, and tangible, non-transitory, computer-readable memory having first instructions encoded therein, wherein the first instructions, when executed by the processor of the first playback device, cause the first playback device to (i) receive audio content from a network device and forward the audio content to a second playback device for synchronous playback of the audio content with the second playback device, (iii) play back the audio content, (iv) detect that a battery level of the at least one battery of the first playback device has fallen below a predefined threshold, and (v) cease the forwarding of the audio content after the detection of the battery level of the at least one battery of the first playback device has fallen below the predefined threshold;
• and
• the second playback device comprises a processor and tangible, non-transitory, computer-readable media having second instructions encoded therein, wherein the second instructions, when executed by the processor of the second playback device, cause, after the detection of the battery level of the battery of the first playback device has fallen below the predefined threshold, the second playback device to (i) receive the audio content from the network device, (ii) forward the audio content to the first playback device for synchronous playback with the first playback device, and (iii) play back the audio content in synchrony with the first playback device.