What's wrong with my mp3 player?

I have a Philips mp3 player and it has 8gb, so I have plenty of space left. When I go to listen to it (when it’s fully charged) it only lasts about an hour and a half before it goes dead again, and it’s really getting annoying because then I get really bored in my car while we’re driving some place. Can you tell me what might be wrong with it and how I can fix it please, because I pretty sure mp3 players are supposed to last way longer than that when you charge them