Same problem here. I have a macbookpro 13" from 2016. After updating to big sur, the battery is draining very fast while using Chrome and Mail (!!). Even when plugged it's not charging, I have to turn off all apps for the battery to charge and when it does it takes forever to do so. I changed all my cables thinking it was a hardware issue but my laptop was working perfectly before the OS update. Battery health is shown as normal but i have to leave the computer charging all the time if i want to be able to work a full day with video editing or photoshop.