Cellphones and pads are great survival tools, because you can download all kinds of useful information and use it for reference in times of need. But those devices are useless once they run out of power. Power packs can provide multiple charges to prolong the life of your devices, until you find a place to recharge. One of Amazon’s top sellers is the Anker PowerCore 20100 for $40, which promises to provide nearly seven charges for an iPhone7 or five for a Galaxy S6. (Remember to keep them charged up so they’re ready when you need them.)