I wrote a program BatteryGraph for monitoring battery status on a Nokia N900.

BatteryGraph will monitor the battery statistics and shows the results in a graph. Also monitors average cpu load and network connection.

It's available for download from http://maemo.jeroenwitteman.com/BatteryGraph/
(I'm trying to get it in the maemo extras repository)
EDIT: it is now available in Extras-testing and Extras.

As this is the first release the usual warnings apply, installing this program may harm your device in many ways (though it is running fine on my N900 )

All comments are welcome

Changelog v 0.3.2 - 2010 April 8

• Changed fullscreen toggle to double click and menu option
• Added option to configure drawing of charging bars
• Small layout changes for upcoming Qt4.6

Changelog v 0.3.1 - 2010 March 28

• Added option to export entire graph to png image file

Changelog v 0.3.0 - 2010 March 17

• Changed background daemon to record battery status only on change instead of regular interval
• Changed background daemon to record network connection status, charging status and average cpu load as well
• Changed graph to display the new data sources (network connection, charging, cpuload)
• Changed grid layout
• Added kinetic scrolling
• Added option to choose zoomfactor
• Added option to disable/enable showing cpu load, network connection and charging status
• Added preferences screen
• Added option to limit maximum age of database records
• Added option to select scrollback history size
• Added option to configure colors of the different graph elements
• Added option to configure cpu load scale
• Optimized graph drawing code for kinetic scrolling performance
• Rearranged main screen layout
• Lots of small changes

Changelog v 0.2.2 - 2010 February 21

• Added option to move forwards/backwards in time by clicking on the right/left side of the graph (clicking in the center will toggle fullscreen)
• Added minimum and maximum value for voltage and charge in the graph
• Added database statistics