For outdoor escursions, I like the looks of the Power Brick Portable Power Bank Charger. It was made for activities like boating, fishing, hunting, camping, biking or long hikes.
The Power Bank wraps a charging battery inside a plastic case that’s durable and waterproof. It has a non-slip surface and a loop that makes it easy attach to backpack or belt.
The charger’s specs list it as having 2200 mAh actual capacity, which is enough to provide a full charge to most Apple or Android phones.
The Power Bank is available in a gray or red case for $24.99 at Amazon.