UPDATE:

The problem is related to WiFi !!!

When I turn WiFi off the capacity shows 6 hours now, instead of 2 hours with WiFi on.

Funny thing is nothing at all changed with my WiFi setup here.

Maybe one of the neighbours got a new access point which is disturbing reception? There are about 20 access points visible from here.

This is a bit annoying because without WiFi the N800 is not really useful for me.