I too just discovered this on my own N800. I thought I accidentally swapped the 770's battery into the N800 and it was just getting old, but when I switched batteries it has the same symptoms.

Today I tried an experiment: Full charge overnight, then leave it in my pocket (standby) all day (which included an 8 hour shift at work - not on wifi or bluetooth.) It's 5 PM now and I am getting battery chimes. I'll try a firmware format to see if there's a software culprit.