OK, The battery checker test was to remove a possibly of the checker causing the issue. Carefully disconnect the battery again and plug the checker back in.
As you’ve replaced the battery you either got a bad one or the charging logic on your main logic board is messed up.
At this point I would install [https://www.coconut-flavour.com/coconutbattery/|CoconutBattery] so you can see whats happening while it’s charging and when its on battery alone. Now take a screenshot of the main screen of CoconutBattery which the charger is plugged in and a second when its on battery. Paste them here for us to see [guide|21499]
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In the meantime review this Apple T/N to see if the cord is OK and MagSafe connector on your system and cord are in good working order as well as check that you are using the correct charger for your system [https://support.apple.com/en-us/HT203207|Apple Portables: Troubleshooting MagSafe adapters]
OK, The battery checker test was to remove a possibly of the checker causing the issue. Carefully disconnect the battery again and plug the checker back in.
As you’ve replaced the battery you either got a bad one or the charging logic on your main logic board is messed up.
At this point I would install [https://www.coconut-flavour.com/coconutbattery/|CoconutBattery] so you can see whats happening while it’s charging and when its on battery alone. Now take a screenshot of the main screen of CoconutBattery which the charger is plugged in and a second when its on battery. Paste them here for us to see [guide|21499]