I added now later, that although the battery charge level bars dropped from 8 to 1, the battery had been in use 25 hours after the last full charge. I was wondering in the morning already why the charge bars still show 8 although it had been so long after last charging.

So either Mugen has wrong logic in the battery, and it doesn't report the charge level (mAh) correctly, or then there is a hw/software bug in N900.

Anyway, ...well I still wait for a fix for the microSD-card, but battery has improved usability alot because if I have charged battery in the night, I can do about anything (except maybe not play Quake Arena 10 hours) during the day with N900 and the battery will have juice still.
I think Nokia should themselves provide 2400 mAh battery. This seems like a right size for the heavy user.

I won't recommend this 2400 mAh battery yet though, before they prove they have a back cover which makes both microSD-card and camera work OK.
I find it little odd, they do not tell about the microSD-card problem on the web-page.