I was having this problem as well. I got onto Garmin support and explained how I have charged to 100% and losing 60%-80% over night. They had me do the reboot, I removed watch faces (I only have two watch faces installed that came on the watch) etc... It was ok for a couple of days, then started happening again. I again contacted Garmin support. The rep I was chatting with had me return my watch and had a refurbished watch sent to me. It has been doing the same thing. I don't think it was a battery issue, I think I could have kept the original as I think it is a software issue. Is there going to be a fix for this sent out at some time? I rely on this watch to wake me up at 4:15 AM to go swim at the Y (and not wake my husband). I woke up one morning and it was completely dead. I love the watch and just want a software fix for this.
Thanks