Mophie Juice Pack Powerstation Pro 

Mophie’s Juice Pack Powerstation Pro ($130) is an all-terrain, weather-ready external battery for those times when a wall outlet isn’t available. The battery is rated at 6000 mAh which is the equivalent of about three full charges for an iPhone or hours upon hours of juice for any USB-compatible Smartphone or handheld. As for ruggedness, the battery is IP65 rated meaning that its completely dust-proof and water-resistant. Link