ipod nano 4th generation doesn’t work without charging

Hello :) I bought an Ipod nano 4th generation used from 2008 but works apparently all well but it only works when I plug it in, once I unplug it the ipod screen is black and just won’t turn on. Im confused because at first I thought it was the battery not being charged but now I have been charging it for 5 hours and it still won’t turn on. I tried resetting many times too but still no luck. Any help?