My n900 began to drain its battery in very short periods.
I got a new Nokia battery, but the same happens.

Htop shows nothing remarkable other than 90+% of the CPU being taken up by fsck.

Any ideas?

I once had a big battery drain out of nothing, reflash solved. But it's the last resort here, I know.

Can you track the parent process of fsck? On desktop I'd ps axfww but this doesn't work on the N900.