Surely the % fat reading tells you either the percentage of fat your body has - 18% is pretty low, really lean people I know are only 20% and I haven't checked mine in long enough for it to be totally irrelevent, so that's awesome for you! OR equally it might represent the percentage of the calories you burnt from fat stores rather than glucose (glycogen) stores (ie how much of your workout was anaerobic or whatever type of CV it is that burns fat)? But yeah, go have a look at the manufacture's website, or... I can't believe I'm suggesting this... read the manual? (I never read manuals... ever).
This might be the clue as to why your I-pod broke!!
Squint