We’ve spent more than 140 hours testing 21 different battery cases (18 for the iPhone 6 and three for the iPhone 6 Plus), and we think the best battery case for most people is Anker’s Ultra Slim Extended Battery Case. It provides an above-average 117 percent of a full charge to the iPhone 6—one full charge plus another 17 percent—and at only $40, it’s by far the least expensive. The result is the highest ratio of charge percent per dollar and the lowest cost per full iPhone recharge out of all the models we looked at. It’s also the lightest and thinnest battery case we tested.
