i listen to my 1G nano everyday on my stereo dock at work, at least 4-5 hours daily. over the past couple years i've noticed the battery life decline substantially when i use it undocked, with headphones. i only get 1 hour of playback undocked.

my stereo dock (ihome) charges my nano everytime its docked, so i'm thinking the constant charge cycles must've messed up my battery? i only assume this cuz my friend, who got the nano at the same time as me and only docks it to sync or charge, doesnt suffer from the big decline in battery life.

my question is, given this info, should i be worry about treating my iphone the same way as i did my nano (docking in my stereo to play (and charge) everyday, even though the battery meter is full), in addition to using it as a mobile phone? or should i just stick to docking it only when i need to charge or sync?

or is my battery theory totally off?