I'm getting the same issue. My watch dies, and when I put it on the charger it will usually show around 63% full, and charge to 100%. It then dies after probably about the difference, i.e. a third of the time it would normally have lasted. When I put it back on the charger it's back at 63% full or so. I have never installed a ROM, and it's definitely not overheating here in Ireland. Going into recovery mode and resetting doesn't help.
I have seen in another thread that someone said taking the battery out, which is an involved process (google change lg g watch r battery for the guide on ifixit), and reinserting it works. I might give that a go, but I reckon it's time for a new battery.
Follow-up: Replaced the battery and it's working perfectly! I did get it updated to AW 2.0 before replacing the battery, but it didn't make any difference to the original issue, but it's running sweet and fast too now.
