Same issue here. Had a fully functional laptop with great battery life until I made the HORRIBLE decision to upgrade to Big Sur. Immediately my battery would drain extremely fast and now it says my battery requires service. I contacted apple support. They told me to run diagnostics - https://support.apple.com/en-us/HT202731 and if no problems were found to sign out of icloud and restart to fix the issue. But you can guess what happened? No issues were found, signed out, restarted - problem remains! So they ask me to bring it in for a tech to look at. First appoint is in three days but I know what they are going to say - buy a new battery. NO! I have three days to wait so I guess I should wipe my mac and downgrade it to catalina, seems like some people have had luck with that. Unfortunately I don't have a backup to restore to. This has been a frustrating massive waste of my time. Macbook Air 13" 2015 i7 Cycle Count 648