This application is from Nokia OVI store which can be used for your Nokia 5800 to monitor the battery levels. Jus download this application from Nokia OVI store and install this on your Nokia 5800. When this is installed, go to Applications and select Battery Monitor. This application now shows the battery level of your phone not just how much is used and how much is remaining.
This application shows you the estimates on talk time, web browsing and music playback. To explain in detail when the phone is used for talking purpose only the battery will remain for 4hours and 40 minutes (from the screenshot shown below which is taken from my phone). When the phone is used for web browsing only the battery will remain for 3 hours and 40 minutes and when it is used for music playback alone the battery will remain for 1 day and 3 hours. Overall battery level will be 40 minutes .
It has also got the statistics about the battery usage,
And the info screen which shows the version and overview of this application.