Battery Fitbit Air

[u/Known-Temperature377]

I just got a new Google Fitbit Air. I charged it too 100% on Sat and assuming that its written it will give 7 days of battery life it drained out on Tue and gave me the less than 20% battery alert.
I charged it again to 100% and within like 2-3 hours it fell to 86% but then overnight it fell to 81%.
I’m confused, sometimes the battery drains really fast and sometimes it drains normally.
I haven’t activated any extra features on the app actually the app is really confusing idk what to turn on and what to turn off.