Anker Power Bank (20K, 87W, Built-In USB-C Cable)

A dead phone means no tickets, no map and no way to find your friends, so carry the weekend's power in with you. At 20,000 mAh this has room for several phone top-ups across a long weekend, and the USB-C cable is built into the body, so there is no cord to forget at home or lose in the tent. It can charge three devices at once, handy when everyone's battery dies at the same time. Weight is the price: 439 g is a lot heavier than a 10,000 mAh bank, so it lives in a bag rather than a pocket. If you only need one top-up a day, a smaller bank will save your shoulders. Charge it fully before you leave home.