With this app, you'll know your device's battery state.
The app tells you the estimation hours and minutes for :
 - stand by
 - talk time (non available for the iPod Touch)
 - web browsing on 2G/3G and Wifi
 - audio playback
 - video playback
A charging indicator is also available.

 

The informations update every 5% of the battery life (due to a limitation of Apple).

 

Download this useful tool :)