Synchronous playback with battery-powered playback device

1. 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.

2. The system of claim 1 , wherein the first instructions, when executed by the processor of the first playback device, cause the first playback device to transmit a message to at least one of the network device and the second playback device, wherein the message comprises an indication of the battery level of the at least one battery of the first playback device has fallen below a predefined threshold.

3. The system of claim 1 , wherein the first instructions, when executed by the processor of the first playback device, cause, after the detection of the battery level of the battery of the first playback device has fallen below the predefined threshold, detect the first playback device has been connected to an external power source and forward the audio content to the second playback device after the detection of the first playback device connected to the external power source.

4. The system of claim 3 , wherein the second instructions, when executed by the processor of the second playback device, cause, after the detection of the first playback device connected to the external power source, the second playback device to cease the forwarding of the audio content to the first playback device.

5. The system of claim 1 , wherein the first instructions, when executed by the processor of the first playback device, cause, before the detection of the battery level of the battery of the first playback device has fallen below the predefined threshold, the first playback device to forward the audio content to a third playback device for synchronous playback of the audio content with the first and second playback devices.

6. The system of claim 5 , wherein the first 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 forward the audio content to the third playback device for synchronous playback of the audio content with the first and second playback devices.

7. The system of claim 1 , wherein the first instructions, when executed by the processor of the first 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 first playback device to detect that the battery level of the at least one battery of the first playback device is above the predefined threshold, and cause the first playback device to transmit a message to at least one of the network device and the second playback device after the detection of the battery level of the at least one battery of the first playback device is above the predefined threshold, wherein the message comprises an indication of the battery level of the at least one battery of the first playback device is above the predefined threshold.

8. The system of claim 7 , wherein the second instructions, when executed by the processor of the second playback device, cause, after the detection of the battery level of the at least one battery of the first playback device is above the predefined threshold, the second playback device to cease the forwarding of the audio content to the first playback device.

9. The system of claim 7 , wherein the first instructions, when executed by the processor of the first playback device, cause, after the detection of the battery level of the at least one battery of the first playback device is above the predefined threshold, the first playback device to forward the audio content to the second playback device for synchronous playback with the first playback device.

10. The system of claim 1 , wherein the second playback device comprises at least one battery, and wherein the second instructions, when executed by the processor of the second playback device, cause the second playback device to transmit a message to at least one of the network device and the first playback device, wherein the message comprises an indication of a battery level of the at least one battery of the second playback device.

11. A method comprising:

receiving, at a first playback device, audio content from a network device;

forwarding the audio content from the first playback device to a second playback device for synchronous playback of the audio content with the second playback device;

playing back the audio content in synchrony via the first and second playback devices;

detecting that a battery level of at least one battery of the first playback device has fallen below a predefined threshold; and

after the detection of the battery level of the at least one battery of the first playback device has fallen below the predefined threshold:

ceasing the forwarding of the audio content from the first playback device,

receiving, at the second playback device, the audio content from the network device,

forwarding the audio content from the second playback device to the first playback device for synchronous playback of the audio content via the first and second playback devices, and

playing back the audio content in synchrony with the first playback device.

12. The method of claim 11 , further comprising transmitting, a message from the first playback device to at least one of the network device and the second playback device, wherein the message comprises an indication of the battery level of the at least one battery of the first playback device has fallen below a predefined threshold.

13. The method of claim 11 , further comprising after the detection of the battery level of the battery of the first playback device has fallen below the predefined threshold:

detecting the first playback device has been connected to an external power source; and

forwarding, via the first playback device, the audio content to the second playback device after the detection of the first playback device connected to the external power source.

14. The method of claim 13 , further comprising after the detection of the first playback device connected to the external power source, ceasing the forwarding of the audio content to the first playback device.

15. The method of claim 11 , further comprising before the detection of the battery level of the battery of the first playback device has fallen below the predefined threshold, causing the first playback device to forward the audio content to a third playback device for synchronous playback of the audio content with the first and second playback devices.

16. The method of claim 15 , further comprising after the detection of the battery level of the battery of the first playback device has fallen below the predefined threshold, causing the second playback device to forward the audio content to the third playback device for synchronous playback of the audio content with the first and second playback devices.

17. The method of claim 11 , further comprising after the detection of the battery level of the battery of the first playback device has fallen below the predefined threshold;

detecting that the battery level of the at least one battery of the first playback device is above the predefined threshold; and

transmitting, via the first playback device, a message to at least one of the network device and the second playback device after the detection of the battery level of the at least one battery of the first playback device is above the predefined threshold, wherein the message comprises an indication of the battery level of the at least one battery of the first playback device is above the predefined threshold.

18. The method of claim 17 , further comprising, after the detection of the battery level of the at least one battery of the first playback device is above the predefined threshold, causing the second playback device to cease the forwarding of the audio content to the first playback device.

19. The method of claim 17 , further comprising, after the detection of the battery level of the at least one battery of the first playback device is above the predefined threshold, forwarding, via the first playback device, the audio content to the second playback device for synchronous playback with the first playback device.

20. The method of claim 11 , wherein the second playback device comprises at least one battery, and wherein the method further comprises causing the second playback device to transmit a message to at least one of the network device and the first playback device, wherein the message comprises an indication of a battery level of the at least one battery of the second playback device.