Kammo
3
It is very likely that the battery is defective. It is not normal for it to drop to 84% after a year. Mine is at 94% after a year and a half. It is best to contact an authorized Apple service. The battery can be replaced. I’m not sure about the warranty, but I think they replace the battery for free if it drops to 80% or lower within a year. If it is out of warranty the battery is between $150-250, depends on the macbook model.