Are you saying the watch battery is at 5% or the iPhone is?
If it is the watch: There was a thing with Apple Watch and Catalina (of all things - makes no sense since the AW pairs with a phone, but apparently it was a thing) last year that was draining people's AW batteries. See
https://www.waldonell.com/thoughts/articles/did-i-just-fix-my-apple-watch-5-battery-life-issue
So if you have a Mac, you are seeing a lot of CPU activity in Activity Monitor for TrustedPeersHelper, I suppose you could try those things. (I' think I'd try doing a power-cycle of my iPhone and Watch first, though, just to see if that helps.)