MacBook takes hours to start charging - battery or IO board issue?

I've got a unique issue. My beloved laptop has started to not charge. I plug in the adaptor - it shows no light, and remains dead. However, if I leave it for a few hours, it'll start charging.

It's not a problem with the charger (I've tried 2, and they both work with a different laptop right away), and I've done an SMC reset.

I figure the issue could be either with the battery, or the left IO board. The battery condition is 'replace soon' - but other than that, is there any way to tell what is at fault? Doing a replacement is easy I just want to ensure I'm replacing the right part.