Mid-2014 13in MBP Retina. Same issue. Immediately after install of Big Sur, I got Battery Service Recommended warning and the macbook would not last more than an hour on battery. Charging took long time to bring to full charge. Battery only 2 years old and 550 cycles. Contacted support. They said to bring it to the store. Contacted support again, they said they will not help at all since my battery was previously replaced by SquareTrade warranty who is not authorized. She insisted the problem was my battery and was rude. I wiped the drive, did a fresh install of Catalina and I am back in business. No warnings. Battery health is normal. And I am back to lasting over 6 hours on battery. Completely unacceptable how apple doesnt stand behind their faulting upgrades.