Okay here we go:
I have 2 Switch from the first generation.
Both have CFW first it was ReiNX now it is Atmosphere. (Don't ask why because the battery problem is important in this thread!)
I play on both Switch Zelda BOTW and the battery (still) holds for 3 - 3.5 hours continuous play!
I also play in my breaktime EVERY day at work for 45 minutes (different games) and from 100% it goes down to 80% - 75% after play.
I then power it on at home maybe 4 - 5 hours later and when it was on 80% it has 74% left.
What do i use in Atmosphere: Standard DL (github!) + nsdebugger.KIP
Screen brightness is at 50% and "Adjust brightness" is on
Sound is at constant 75% (most of the time i use headphones)
Don't know what you are doing but i hope that will help
P.S. it is nice that some make packages with "Install and use" but sometimes it is overloaded with things you'l never need
Try to install the bare minimum and IMHO the battery problem will be gone!
(If not there could always be a HW problem!)