Is your N900 powering off unexpectedly ? when you:
* run with your device in your pocket
* close the keyboard
* shake your device (it's not a feature)

well, don't be like me, i opened the device and lost one screw, checked the ribbon between the screen and the motherboard... etc

but the answer was easier:

just check that your battery pins are narrow enough to hold tight the N900 connectors to the battery , if not you can buy a new battery or use the tiny screwdriver and try to narrow the space between the pins.

worked great for me