Battery life of the Forerunner 255 Music

did you allow the screen to light up by wrist movement? I turned off this option for activities, to reduce the battery consumption. 

Is Wifi enabled? Pulse Ox? All those can use extra battery.

Both of these functions are disabled, the gesture is enabled only after sunset. I installed a third-party watch face, maybe that affects it.

Since you mentioned the music model, are you listening to music during these days?

My 2 year old FR255 non-music will use about 5-6% per day if I'm not doing activities and using a relatively efficient CIQ watchface. But, I've also got bluetooth disabled most of the time.

To rule out the third party watchface, I'd suggest just switching to a built-in face for a day or two to see how the battery consumption actually goes. One of the stock digital face layouts can omit the seconds display and has few fields above/below the time. This would give you a baseline of how little your watch can possibly consume per day.

7699399 said:

The battery drains by 15% per day.

Only the first day or every day till it dies?

Switch off gimmicks like garmin share and pulse ox. Normal is about 4-6% a day. Or a one  6 H run with a music