I have removed the back and the keyboard nothing there
Hmm, generally it would be under 'replacing coin cell battery' but I didn't find it either in the manual ... though generally the coin cell battery would be easy visible.
I have contacted Packerd Bell a few days ago. But no word from them. Perhaps they use a capaciter to store some energie. I don't know. But if so this means that if the main battery is empty the cmos will lose its configuration after some time.