Are sealed lead acid batteries water-resistant?
Water shouldn't be able to get inside sealed lead-acid cells.  The terminals are not environmentally sealed, though.  If you backpack gets soaked, they battery could discharge through the wet cloth.

Cold increases the internal resistance of the batteries.  As a result, you lose more energy on the internal resistance.  You will get less charge back from the battery.  Max output current also decreases*.  At 00C, you should still be able to get few amperes necessary for charging your gear.


(source of image)

* BTW, vehicle starter batteries are rated for Cold Cranking Amperes. So, they are rated for the worst case.