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EDIT: The way Microsoft performs the Battery Life tests on Zunes
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ZuneInsider reported a few weeks ago :

Zune 4GB and Zune 8GB
Audio: up to 24 hours
Video: up to 4 hours

Zune 80
Audio: up to 30 hours
Video: up to 4 hours

We did all testing using pre-production hardware and software, with the wireless turned off. Audio was tested using 128kbps .wma tracks, and video was tested using 500kbps .wmv videos.

Crave Blog found 22 hours :

Our CNET Labs have just published the results of their Zune 80 audio-only battery drains and the numbers are somewhat disappointing. Microsoft rated the Zune 80 for 30 hours of audio playback with the Wi-Fi feature turned off. Lab testing revealed that the Zune 80 is realistically capable of 22 hours of audio playback with the Wi-Fi feature turned off, or 18.5 hours with the Wi-Fi feature enabled.

GotZune (part of Mobility Site) found 24 hours :

All within a 2 minute period, I powered them all on after fresh full charges. I have the earbuds that go with the player (except I have ultimate ears on the Zune 30) plugged in to them. I am going to run them all until they quit. All the players are on shuffle all songs repeat mode.

• Zune 30 - 12 hours 15 minutes
• Zune 8 -  18 hours and 45 minutes
• Zune 80 - 24 hours and 15 minutes

 As you can see ZuneInsider reports longer battery life for the Zune 4 & 8 players too.

What we can learn from these tests ? When you read battery life differences from site to site, be sure they have used the same way of measuring it (which is almost never the case). Also some kinds of batteries need a few charge/discharge cycles to get the 100% from them. I don't know how important this is for Li-ion battery Zunes and iPods use.

Related articles:
How Microsoft tests Zune battery life
The official Zune 2 battery specs