I have the same issue. After update to the Big Sur, my battery capacity is significantly reduced. I called the Apple service. The service guys is so dumb. He just ask me to change the battery. I told him this happens right after I update my system and my batterray was really good. I only had this mac for over a year. He said things happens in life like battery fail. He is just so dumb like a idiot. I am asking for solving battery issues but he talks about philosophy. Just ridiculous. I think Apple just want to make more money.