Why does my ipod de-charge so fast?

After my iPod has been fully charged and I skip a couple songs my iPod starts to shut down saying that it needs to be recharged. There have been time where I have also charged it the whole day and it shows that its charging so I don't think that's an issue. It also de-charges really fast when I'm just listening to the thing. I have an iPod classic that's about 4-5 years old and its no where near full. Please help! I'm getting tired of listening to only twenty songs or less before the thing de-charges.