Updated MacBook Air to macOS Sonoma, now battery drains quickly

Uninstall Avira by following the developer's instructions. If you have any other anti-virus software, cleaning/optimizer apps, or third party security software installed, then uninstall them as well by following their developers' instructions. These types os apps are not needed on a Mac and will usually cause more problems than they solve because they interfere with the normal operation of macOS. macOS already has great built in security especially when the user also does their part by practicing safe computing habits as outlined in this excellent article which I think everyone should read & practice:

Effective defenses against malware and other threats - Apple Community

Other options are to Shutdown the laptop or leave the power adapter connected.

Frisson411 wrote:

How do you account for the fact that Avira was installed before the Sonoma update and there were nil issues?

The update changed things. After all, that is what updates do or there would not be any updates.

How do you account for the fact that 1000s of people are complaining about Sonoma battery drain, with no mention of Avira?

There are lots of reasons for battery drain. I did mention several categories of apps that wreak havoc in my earlier post and there are dozens or more apps for each. People seem to love those apps. And those developers do love to prey upon peoples fears since it empties people's bank accounts & fill's the developer's accounts.

(PS Avira has been uninstalled on my laptop)

Hopefully that was the source of the problem, but even if it does not completely solve your problem, it will be good for your system.

If you still have issues, then post a complete EtreCheck report so we can examine it for clues.

How to use the Add Text Feature When Posting an EtreCheck Report - Apple Community

Frisson411 wrote:

How do you account for the fact that Avira was installed before the Sonoma update and there were nil issues?

How do you account for the fact that 1000s of people are complaining about Sonoma battery drain, with no mention of Avira?

(PS Avira has been uninstalled on my laptop)

Thousands of users all using the Avira Security Software ?

All the user are like minded and thinking the same way

Windows-like viruses that self-replicate and affect macOS don’t exist because of the underlying UNIX Foundation and permission limitations.

The macOS operating system resides in a sealed and read-only volume that can’t be opened by users or third-party applications.

Once this type of software are installed and embedded into the computer >> Avira

Start Over from Scratch 

Heavy Handed - Yes ?

Effective in removing the AV Software - Yes 

You can spend hours or days hunting down all the bits and pieces of this software and never really get it all off the computer

For Apple Silicon computer >> Use Disk Utility to erase a Mac with Apple silicon.

For Apple Intel computers >>   Use Disk Utility to erase an Intel-based Mac followed by How to reinstall macOS

Always make a Time Machine Backup  before  proceeding 

If going this route - I suggest Not using Startup Assist to migrate everything back.

This will probably Re-Introduce ( Avira  ) back into the Operating System 

Have you uninstalled Avira yet? If not, then it does not surprise me that your laptop is crashing. If you do nothing, then nothing will change.

Plus if you still have problems after uninstalling Avira by following the Avira developer's instructions, then post a complete EtreCheck report here so we can examine it for clues in order to try to help you get your system stabilized.