It recommends to make sure the SteriPEN is up to a temperature above 32F. They recommend NiMH batteries as the best choice for cold weather. If in cold weather store the batteries in a warm place for better performance.
Answered on 3/11/2011 12:00:00 AM by Customer Care from Sierra Trading Post
Water tends to solidify in less than 32' conditions, and that would be the limiting factor.
Answered on 4/11/2012 12:00:00 AM by Lars from PNW