As bad as it feels saying this: Unfortunately, Samsung decided to get away from their own Tizen OS, which was featured on their previous smartwatches, and now they're at Google's WearOS. WearOS is notorious for horrible battery runtime (well, it's a bloated OS with the tons of Google Play Services), and most other smartwatches (ones from, say, Fossil) running WearOS have a hard time lasting even a day when you're doing anything with it. It's just that bad of an OS for smartwatches. And with two days, the Galaxy Watches are actually good for a WearOS-powered device.
I use mine (Watch4 Classic 46mm) to check notifications, track fitness values (workout, water intake, heart rate and what not) with Always-on-Display enabled between 7am and 10pm, and mine lasts 1.5 days when working out using GPS and two days without working out. So, yeah, it has fairly little to do with the size of the battery and very much to do with the fact that WearOS is a power-hog.
I got my Watch 4 pretty much by release, after having had the 2nd generation for a long while. And I too noticed it drainig battery fast.
My solution was;
Turn off Location (GPS)
Turn off NFC
Turn off WiFi (this is the one that drains the most power qccording toy own tests)
I have a mobile plan that includes unlimited mobile data, but even without this, you should be fine using your watch through your phones mobile data or wifi, through only bluetooth on your watch.
Now i get between 2 and 3 days from my Watch 4 , even if I too wish it had a bigger battery.
I have had mine since mid Nov and it would last a day and half. First smartwatch for me, so I thought that was not to bad and got in a habit of throwing it on the charger for a bit everyday. Then on Jan 13th halfway through the day it asked to go to power saving. I charged it to 100% before bed and it lasted 5 hours. I literally did nothing. Did not install any new apps or change any settings. I am noticing I am losing about 10% every hour now even after restarting and trying to turn off most everything.
Maybe a cell in the batteries go bad? Has anyone had Samsung provide a successful working replacement?