It sounds like you have to calibrate your battery.
- Plug in the power adapter and fully charge your PowerBook's battery until the light ring or LED on the power adapter plug changes to green and the onscreen meter in the menu bar indicates that the battery is fully charged.
- Allow the battery to rest in the fully charged state for at least two hours. You may use your computer during this time as long as the adapter is plugged in.
- Disconnect the power adapter while the computer still on and start running the computer off battery power. You may use your computer during this time. When your battery gets low, the low battery warning dialog appears on the screen.
- At this point, save your work. Continue to use your computer; when the battery gets very low, the computer will automatically go to sleep.
- Turn off the computer or allow it to sleep for five hours or more.
- Connect the power adapter and leave it connected until the battery is fully charged again.
Thanks for all your feedback. I geuss I should take better care of my battery if I don't want to exchange it any time soon.
Though, I dug up my original charger and charged the MBP at home using the charger untill fully charged with the lid closed. This time when I opened the lid, I got 8 h of battery time.
Dont know why, but it seems that the MBP does not get fully charged using the Cinema Display cable set. So, if I expect to be running on battery, I should just make sure I use the original charger instead and I'll be OK!