There are quite a few ways to increase the battery life on N900, do a search to find out.

Here are a few ideas to get you started.

1. Underclock using Titan's Power Kernel
2. Switch off Wi-Fi when not in use
3. Disable all widgets and apps that auto update
4. Switch off 'Switch to WLAN automatically'
5. Install Autodisconnect to disconnect your net connections when not in use