I leave mine in my unheated barn with open stall doors all winter and have had no problems. Also had an Elcon 2500, which is basically the same internals as a Alta standard charger, on my conversion which while stored in my garage at night and for charging, was my daily driver for 7 years so lots of routine cold temps and daily use when I got home. Live in Boise so 10 or so days in single digits, 1-2 days below 0F and a couple of weeks continuous below freezing.
No problem with the charger being in cold temps, just don’t want to charge the bike if the battery cells are below freezing.