WatchOS Battery
After updating to watchOS8 I notice that my battery drain so fast. I'm currently using Apple Watch SE
Apple Watch
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If the battery drains too fast after an update, see all the replies to:
Battery Drain since watchos 7.4 - Apple Community
WatchOS 7.5 battery drain - Apple Community
FWIW, in some cases, unpairing the watch, completely erasing it, then pairing and restoring the backup will resolve the battery draining issue after an update. (Simply unpairing and pairing again may not work.)
Unpair and erase your Apple Watch - Apple Support
Also, some people have found that simply waiting a few days after an update resolves the battery draining problem.