MacBook Air battery not charging. Ideas?

I should have noted that I tried all the normal things. I believe that originally the computer was running an older version of OS X and I replaced the battery with a 3rd party battery. When it was replaced, I upgraded to High Sierra and I believe that Apple has changed the Firmware so it won't work with the replacement battery. All hardware test and battery tests confirm that all is good. So I have a brand new battery which seems to be worthless unless I can downgrade to an older OS but with the firmware upgrade, that's not possible in my opinion. Thanks for the reply though. I appreciate it.