Curious: I just got a series 3 last week and use it as my sole phone (hate smartphone form factor, flip phone user since forever). All I use it for is heart rate, phone, messaging, and music (very happy with it).

I've had to charge my series 3 every night. I guess not having it paired with a phone somehow uses more battery power? Technically, I reset my wife's phone with my sim+apple id, paired, reset back to wife's backup and sim. So it has no real phone to pair with, though I guess maybe that causes it to use cellular a lot more than most.