iPod's battery is part of the unit... it's rechargable... the 20 GB one is supposed to last 15 hours fully charged... minis up to 18 hours... guess it depends how much scrolling you do. now does it eventually lose juice the more often you use it and charge it... well i guess it's probably like a cell phone. you can charge it via your computer, wall socket or car adapter.
honestly, I didn't do research on it and I've only had it for 3 weeks. i was buying a laptop and they threw in the mini free and i just upgraded to the 20 for about $100 more.
my younger sibs have told me people are getting jumped for their mp3s these days... hmm, was that the same with walkmans back in the day?
yeah, i think you upgrade by buying a whole new one... my sis is jealous mine is in color