[cloudbonsai]

It seems that the battery lasts 35 hours with heart rate tracking, 1 month with no HR, and 11 months with power saving.

    Run Time

    Using activity functions (heart rate): Approx. 35 hours max. 
    Using in watch mode with heart rate measurement OFF: Approx. 1 month
    Using with power-saving function ON: Approx. 11 months

I'm not sure how well the "solar charging" feature works, though. It's surprising that it does not last longer than Fitbit or Garmin.