This post was done in partnership with The Wirecutter, a list of the best technology to buy. Read the full article below at TheWirecutter.com.
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.
Why you might want a battery case
Depending on how you use your iPhone, draining its battery during an average day can be easy. If you rely on your phone to last a full day, and you don't have the time (or physical access) to plop down next to a wall outlet, a battery case—which puts a moderate-capacity rechargeable battery inside a bulky iPhone case—can be a smart choice. In the best circumstances, a battery case can double the battery life of your iPhone and then some. And unlike with stand-alone battery packs, you don't need to bring a separate cable or figure out how to carry both devices together. You just slide or snap your iPhone into the battery case to get protection and power in a single unit. If you're looking only for some protection, we can also recommend a regular case.