Wood Charcoal and water are not friends. Leaving Charcoal out in the rain or even leaving it outside and exposed to fog, high humidity or dew means the briquets will not light efficiently. To avoid this, always store your briquets in a cool, dry place. Just grab the bag and bring it back into the garage with you, or roll the top of the bag shut and place it in an empty trash can or storage bin—with the lid on to protect it from the elements.
The shelf life of Charcoal is indefinite so long as the product is stored in a cool, dry place. It has a shelf life of 1–2 years if stored properly, meaning the bag is unopened, free of tears or sealed tightly.