Picture this: A YouTube tutorial on changing a flat tire suddenly goes black just before you get to the part about how to put on the spare. "No! No! No!" you silently shout to yourself, while cursing your utter lack of life skills. Your phone is dead; its 3,000 mAh battery—average for a smartphone—lasts only 12 hours of heavy use per charge on its own, but you can carry 100, 300, or 900 percent more time anywhere you go. Portable lithium-ion power packs will revive your handset in an emergency, or when you're just trying to watch Wonder Woman (yes, again) on a long flight.