My Apple Watch series 8’s battery started draining rapidly all of a sudden with no new update or usage pattern. I’ve cleared all background apps. The battery is barely lasting 4 hours all of a sudden.
Apple Watch Series 8, watchOS 11
Have you tried force resetting your watch by holding in the side button and the digital crown for 10 seconds till you see the Apple logo, then release them?
Failing that I would unpair the watch from your iPhone, reboot the phone then pair the watch back setting it up as new.
Apple Watch battery drain
