Hi !
If like me you had a severe battery drain after updating to FW 13 (something like 120% in a day with only a 30 min run...)
A factory reset fixed it for me. Unpair from phone, go to the watch settings and do the reset settings and data. (Data might be not necessary, but it's what I did)
I'm at 66% right now and my last charge was 3 days ago with 6 activities (~4h).
It might be the best battery autonomy I had with it in 3 years of use :D
I hope this help !