Apple Watch series 4
My watch will no longer hold a charge. Does it need a new battery?
Apple Watch Series 4, watchOS 9
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Follow these steps:
If your Apple Watch won't charge or it won't turn on - Apple Support
If you still need help, contact Apple Support.
Your best option may be to make an appointment at the Genius Bar in an Apple Store near you and let them look at it:
Genius Bar Reservation and Apple Support Options - Apple
To understand Apple Watch performance and its relation to your battery, see:
Apple Watch Battery and Performance - Apple Support
“Battery life” is the amount of time your device runs before it needs to be recharged. “Battery lifespan” is the amount of time your battery lasts until it needs to be replaced. Maximize both and you’ll get the most out of your Apple devices, no matter which ones you own. See the "Tips for Apple Watch" section in:
Batteries - Maximizing Performance - Apple
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.
For repair & service: