My experience with the chemical warmers is that they have a one season shelf life. Put some in a kit & expect to replace them next fall. It might help to store them in their own hard case & don't fold the packaging. Apparently moisture leaks through the package and the chemicals slowly go bad. If its soft, its still good. Hard ones have gone dead.

They are great for getting a couple when they may/will be used such as late fall hiking or ball games.