Hopefully my experience with AW S5 is helpful to those of you who are encountering battery difficulties. I went through a similar phase with my AW S5 aluminum GPS where battery was extremely poor and I was thinking of returning the watch. However, now my battery is at least as good if not better than my S4.
Backdrop. Also have a Series 4 cellular. Battery experience very good to excellent. Got a new iPhone 11 Pro and decided to install everything from scratch and not restore from my previous XS backup. Did the same thing with Series 4 watch. Guess what? Battery life was horrible on series 4 for a few days. Then kept on charger for a full day and back to normal the following day.
Cue to 5 or 6 days ago. Bought Aluminum AW S5 GPS. Set up as new watch in addition to my AW S4. First day of use began at 6am. Watch battery died at 2pm. Second day of use began 6am, battery died at 3pm. Then fiddled with settings to ensure fall detection off, noise reception off, etc. but kept AOD on. However, I did not use it on the third day. Kept it on charger entire day, suspecting it may still be syncing and settling in similar to my S4 when was reset. Next day began at 6am and battery was at 25% by 9pm. Not too bad.
Yesterday, updated iPhone iOS to 13.1.1. Today, began using at 5:30 am and battery is now at 57% at 8:30 pm. I am thrilled. AOD on all day long.
I suspect the recent battery experience is not a failing of AW S5 or even AOD but rather unfortunate timing of IOS 13/13.1.1 coinciding with 'settling in' of new watches.