Definitely the battery going bad for whatever reasons. Have you been heavily draining it past 20% regularly when you first got the phone?I have been getting very bad sot since the last couple of months, nothing much has changed in terms of my usage, signal strength, etc,. Even tried multiple factory resets, day before yesterday I had to change my phone thrice, I don't even get 3 hours sot on most occasions. Thinking of getting the battery replaced from Samsung, battery has 86% health, I want to how bad is 86%? Or is it normal?
My wife's S8+ needed a screen change so when she took it to Samsung she also had them swap out the battery even though just 1 year old and was still doing ok-ish. But man! the improvement in battery life is huge.

