I gave my son a new Cellular Apple Watch 9 in June 2024 to be used as a standalone watch (without paring it with an iPhone). The feature was called Apple Watch for a Family Member, and now it's called Apple Watch for a Child, but it's the same thing. It worked beautifully until a few days ago. It easily lasted the entire day (until late night) on Cellular with him texting to his friends multiple times per day and even making occasional calls. As of a few days ago, he started complaining about his Apple Watch battery draining too quickly and not lasting through the day. I believe the watch updated itself to watchOS 11.0 (or 11.0.1). I will need to check the version.
Yesterday, I got myself an Apple Watch Ultra 1 (it was on great sale on Amazon) and set it up as a new watch for myself. I realized a few hours after I activated it (without even having cellular activated yet) that it drained 40% in like 4 hours. I was floored. It's almost 4:00 PM, and it's now at 31%. It took it off the charger at 6:00 AM this morning. I was forced to update the software to 11.0.1, as it wouldn't activate on the older software. The update itself failed multiple times, but it's a different story.
Now I've come across this thread. This is a legit issue. I bought the Apple Watch Ultra to get a 2-day battery life (or at least over 24 hours). My original idea was to activate my Ultra as a standalone watch (Apple Watch for a Family Member), which I actually did do this morning. But then I realized that there were a lot of features missing. I couldn't add any credit cards to the wallet on it except for the Apple Card. I couldn't add my work O365 account to it even though there is an option to add an Outlook.com account, but it only supports individual (non-corporate accounts). As I started digging deeper, I realized that setting the Apple Watch for a Family Member would not provide me the features that I needed to have, so I reset the Apple Watch again and activated it "for myself" to be paired with the iPhone. So, granted, the watch was activated twice while on this battery charge, but still, I can see the battery percentage melting away in front of my eyes even after I finished all the activation. I activated the cellular on it (T-Mobile) and made a couple calls while having powered my iPhone down. Maybe I spent about 2-3 minutes on two phone calls combined. There is no justification for the battery discharging.
I've read in this thread about deactivating "Live Activities" under "Smart Stack" and then restarting the watch, which I just did. I also disabled a bunch of notifications on the Watch from iPhone apps and uninstalled a bunch of Watch companion apps that I didn't need. Still, the battery seems to be draining like there is no tomorrow.