@phantom00 Does your phone's battery only last a day?
Without trying, my crappy iphone (ya know, the ones where Apple purposefully made them run worse with every IOS update and some stuff to do with batteries, and then got caught on and treated everyone like fools by giving them something for free), can at least last a few days.
Wikipedia isn't going to be something like playing a game or watching a video and would take even less battery. Then you just need to put on low power mode and flip some settings to prevent certain stuff from running (differs on OS probably), and then you can make it take even less power to have your phone running. Then simply shut it down between uses (or keep it on sleep if you're in a danger zone), and only use Wikipedia to search for something specific rather than browse it for an hour for something useful.
Then you bring out the power pack after all that. Besides, you can use google to figure out good ways of getting energy, and write notes to store on your phone if you really wanted to be prepared for that impossible event.