Forerunner 235 Battery is draining since software update

sa_elliott, I think you should back off on your assumption that it's hardware.  Others have posted similar problems since updating to version 8.20.  I have the same problem you describe.  I think your problem isn't unique to your watch, and therefore probably not hardware related.

Another thread was discussing turning off the heart rate monitor to extend battery life.  I'm going to try that until the next software update.

9542241 said:

until the next software update

Wouldn't wait for a new version, old updates are listed with dates in this post: Led not switching off when taking watch off. Last one is from August 4, 2020.

Mine is the same since the update. I use it mostly as a watch and step counter. Plus record the odd run with the dog. For on my bike I use an Edge Explore 530.

I'd only charge it once every 10 days now it lasts about a day which is next to useless. 

Waiting for the next update is futile, there has not been one since 2020.  You can try a software downgrade, there is detailed instructions here and a link to an old update - https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-230-235/104663/wrong-date-and-time/1144213#1144213 , but this did not work for me.