Dear all.
i found a way to improve the life on the n900 battery, usual my n900 it take just 24 hours before it will be dead but now it take 2 days with 12 hours working on my device.
all you have to do is recharge your N900 from the USB cable from your PC and when the led turn to green remove it directly don't keep the led green to much time.
this way did work on my device and am sure that it will work on ur device too.