Why is my iPod battery not charging, despite displaying charge symbol?

I replaced the battery in my iPod Classic 160GB (Late 2009 model) back in June 2016.

However now my iPod will not charge at all. Charge icon displays when plugged into dock/AC/laptop, however the charge level isn't altered when unplugged later.

No other problems detected.

I've done a reset several times (both when plugged & unplugged), and just did a restore. Still not fixed.

Is it a dodgy battery or a motherboard issue?