My Garmin Instinct is about 2.5 months old. Over the past week, the battery has been going dead in 2 days or less after a full charge. Prior to that, I had to charge it perhaps once every week to 2 weeks. I typically do on average 5 5K runs a week and 3-5 5K hikes, with some shorter 1-3 mile walks sometimes sprinkled in. I recall it lasting on one bar of battery for a few days.
The only routine/setting I've adjusted recently is doing low heart rate training on my runs. I've been doing this for about 3 weeks now, so before the battery performance change. Initially the heart rate warning came up often, but now that I'm adjusting it comes up much less frequently. In the past few days I've checked the battery after my runs and it still showed full bars right after so the drain is not from that.
Where I seem to be losing the battery life is overnight. When I wake up and look, I'm always down a bar or two now. What could be causing this? I'm thinking it could be something like blutooth but I don't know what would have changed to spike that activity.