MacBook Air 2017 battery life issues after installing BigSur. Shutting down at 28% and battery life reduced to 3h.

Hi,

I have read some similar posts re. batter issues but unfortunately I was not able to find a reliable solution.

I bought my MacBook, 13'' last year and at first I was amazed at how long it could 'survive' on one charge, up to 10h+ with moderate usage (I am a teacher and use it only for work purposes)

And everything was OK until I installed BigSur...I have no words what a disappointment it has been. Suddenly, my computer slowed down considerably, applications (although I have not many of them)have been freezing and shutting down, even the Pages took ages to react to a simple font change :/

Anyway, the WORST was the battery, though.

1. My battery's life was reduced from (after a year of usage) 8h+ to 2h! It happened nearly 'overnight". And I have no clue why.

2. I have no way of checking the Battery Health as the button/ function has disappeared from the System Preferences Menu (see pic attached)

3. I have no idea how many change cycles have been completed either as the count has also disappeared from the Hardware register menu.

4. Also, recently, my computer started shutting down at 28% (!) and there is no way I can turn it back on without plugging it in.

So basically, in a nutshell, before Big Sur, in December, battery life 8h+, today, barely 2-3h. The usage has not changed, and I am really careful about charging it properly.

Any advice would be appreciated!