Ipod Battery Trouble

Ok so my boyfriend gave me this Ipod and it was perfectly fine when he had it. But now I have it and I put some songs on it and left it on the charger for a while I took it off and it only lasted for about 30 mins. So then I left it on the charger for about 4 hrs and I took it off and I only got to listen to it for about about 20 mins like soon as I started playing the first song it took a chunk out of the battery bar then in the last 5 mins that it stayed charged it had about half a batery bar then I t just said no Battery power left connect Ipod to power so I reset it and it came back on and had like no battery power and immediately cut off then I figured I still wasn’t leaving it on the carger long enough so this time I hooked it up at about 10pm and left it on the charge til about 8am the next day. and the same exact thing happened. My boyfriend says that I should restore it but its like 700 songs on the thing and I dont have hem on my computer anymore so I dont want to restore it then he said I should take it to the Apple store but I don’t know. BTW its an Ipod mini.