Apple Watch series 4 battery
My Apple Watch series 4 is only 8 months old. The battery used to last over 48 hours. Suddenly it doesn’t last even 12 hours.
To understand Apple Watch performance and its relation to your battery.
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 the replies to:
Update 6.2.1 Battery Draining Too Quickly… - Apple Community