I have an Apple Watch Series 4, 44mm, I bought it in early February this year. I've synced it with my phone running iOS 13 beta, but the problem hasn't happened all the time I've had beta.
My Apple Watch used to last two days at a cost for how much I use it. The first day I would go down to around 68% and let it sit overnight and wake up to about 60%. Then I finished the second day off with about 20% battery left.
For about a week, I charged the Apple Watch at 30% until I realized I was charging it every day. There was no gradual battery deterioration. One day I realized that it only lasted one day.
I've never dropped it or had water damage or anything. My phone has had iOS 13 beta since it came out, but my issue started recently.
Any advice or answers will help, please.