The logic board which hold the charging unit is what goes bad. I dont know what causes it but i have seen a few cases where the battery just wont charge. its a hit or miss but the logic board would be the first place i would check. in all my cases it was just a matter of replacing the logic unit. BEFORE YOU go and order a part and try to fix it just look inside the charing port of your iPod and make sure there nothing in there.. sometimes its just a piece of lint stuck in there. =) http://www.ifixit.com/iPod-Parts/iPod-Video-Logic-Board/IF195-001-1