My iPod Nano 5th G. Won't turn on off the charger!?

I recently just found my iPod since it's been lost, I can't understand why it won't turn on when it's not on the charger? It comes on when I put the charger in, thats fine, an I can listen to music and all, it even shows the bolt on the battery and that it's charging, also shows fine on my computer, it connects an I can sync music but I just don't get why when I take it out the charger it goes off and won't turn back on! I tried looking online on how to reset which I did but no luck not even when when I tried resetting it while charging t still won't come on without the charger connected to it. Unless I'm doing it wrong, I don't get it. It never had that problem, although it's been dead for a really long time but does it make a difference? And the battery doesn't show full, it still shows it charging you know when it moving ? And I can't even tell if the battery has gotten at least half way charged cause it goes right off when I take the charger out. Its been about 7-8 hours that I had it on and off the charger . Can you help me? Does the battery have be completely full to where it's still and green ? Not moving back and forth meaning it's not charged at all? Some gadgets don't show the battery is at least half way. On the computer on iTunes, it says it's up to date