Of course battery life is greatly affected by the temperature, and below -28C the WT4090 battery really starts struggling.  That's why it's so important that customers use the WT inside the freezer pouch and keep it in use - don't let it suspend for longer than the guidelines in my PMB and in the User or Integrator Guide (can't remember exactly which one right now - but PMB releasing the freezer pouch has full info).  At -20C an agressive Engineering scan profile shows 1X battery life is ~6 hours, and it's less at -25C of course.  (Inside the pouch, in use, it won't go to less than ~ -25C.)  Remember, the 2X battery and freezer pouch for use with the 2X battery are coming soon.  I hope later this month or January at latest.  SEVT Webinar to train you was this past week.  I doubt any customer will complain about battery life in the freezer with the 2X battery!  Still need to keep it in the pouch, though!  (Or inside a coat is even better, if they're doing voice picking and don't need to interact with the WT.)