The Adventure H20 6K portable chargers from myCharge (on Amazon for $39.99) are built to withstand any condition. One of them will provide the power you need while being distant in therain, mud, extreme heat or cold. Even, all of those conditions combined.
The charger is encased in an insulated, ruggedized exterior. The case protects the battery from extreme weather temperatures. They are completely waterproof with an IP67 rating. That means the chargers will function after taking a dip in the lake. I toss one into the bag on my bike without concern, even on the rainiest days.
The built-in carabineer attaches to a my messenger bag when traveling. I take the Adventure H20 6k with me into the North Cascades.