Let’s not run the system too long (or sleep mode) as you don’t want to loose the battery’s charge.
First off I would run the onboard diagnostics to see if anything pops. Restart your system and press the ***D*** key to enter.
Next I would install this gem of an App [https://www.coconut-flavour.com/coconutbattery/|CoconutBattery] take a snapshot of the apps main window and post it here so we can see it. Make sure the MagSafe charger is plugged in when you do.