Why does my iPod show charged when it's not?

I recently bought a iPod classic 4th gen to repair, bought it expecting to replace the drive but found that it just wasn’t fully plugged in. So the iPod booted, I was able to reformat it and put some songs onto it. The issue I’m now facing is that it won’t fully charge and the small amount of charge it does get it won’t hold for very long. I’ll plug it in and it’ll charge for 10-15 minutes before showing the charged screen and when I unplug it the on screen indicator will show only around 20% charged. I’m assuming it has a bad battery but I just want to make sure that I actually need to buy a new battery and if it’s not some small issue. I’ve also tried with different cables getting the same result.