Battery drain
After update to 9.3 on watch 8, the battery is draining 90% in 6 hrs! How to fix
Apple Watch Series 8
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Force restart your watch i.e. press and hold both the side button and Digital Crown for at least 10 seconds, then release both buttons when you see the Apple logo.
Then make sure to turn off background app refresh and check walkie talkie is not accidentally turned on.
Make sure iPhone is on iOS16.3.
Test the watch for a day.
Then if still no better
Unpair the watch and then pair it again.
Unpair and erase your Apple Watch – Apple Support (UK)
Set up your Apple Watch – Apple Support (UK)
If it still discharges unpair it and pair it again without restoring the backup.
Check that the iPhone is not using any security apps or VPN that can affect apps on the Watch. You don't need any of them and they can be incompatible with iOS16 on the phone.
see also
Apple Watch - Battery - Apple Apple Watch - Battery - Apple (UK)
Batteries - Maximising Performance - Apple Batteries - Maximising Performance - Apple (UK)