I've never had luck with the electric hand warmers. They all run out of power way too fast.I've never been a fan of the hot hands for long term "just in case." I've had a couple of times when I've opened them and nothing. I don't know if the factory didn't seal them correctly or if they were just too old. I keep two zippo refillable hand warmers in my bag with some fuel in a sealed box. I also normally keep 2 of the electric hand warmers in there with charging cables. In my hiking pack I also keep a 40watt folding flexible solar panel with them.