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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.
How we picked and tested
To test each battery case, we installed a fully drained iPhone 6 and set it to Airplane mode in the fully charged case. When the iPhone reached a 100 percent charge—or when it stopped charging—we removed it from the case and drained the phone's battery again. Assuming the battery case still had charge remaining, we then installed the phone in the case a second time, noting the phone's battery percentage when the case stopped charging it. We recorded the charge percentages and times for each test, as well as the physical dimensions and weight of each battery case. We repeated each test a few days later and then once more for a total of three tests. We then averaged the charge results.
We also tested each case for subjective aspects of usability, such as how easily we could press the phone's buttons and how the encased phone feels in the hand.
Additionally, we used a micro-USB cable to connect each battery case to a computer to test whether each case allows pass-through charging (the capability to charge the iPhone while it's in the case) and data syncing.
Anker's Ultra Slim Extended Battery Case is the iPhone 6 battery case with the best combination of good performance, price for the capacity, and least amount of additional bulk. It can provide 117 percent of a full charge to an iPhone 6, and its $40 price tag is crazy low for a battery case. Anker originally released the Ultra Slim at a higher price, but the price has dropped considerably, and the company has confirmed that $40 is the new "regular" price. The Ultra Slim offers the best charge value (2.9 percent of a full iPhone 6 charge per dollar, or $34.34 for a full charge) of any of the cases we tested by far. And it adds only 84 grams of weight, and just 6.1 millimeters of thickness.
When it comes to installation, most iPhone battery cases use either of two types of design: sliders or front frame. Anker's Ultra Slim employs the slider design. You remove a short plastic cap at the top to slide the iPhone in place, and then reinstall the cap. Doing so is very easy compared with using the trickier front-frame design of Incipio, Odoyo, and some other cases, which requires you to remove a frame, install the iPhone in the base, and then snap the frame into place around the entire phone.
Anker's Premium Extended Battery Case ($60) is the larger-capacity sibling of the Ultra Slim and a great alternative for anyone more concerned about getting raw power over size or cost—though even in those respects, it's a strong performer compared with the competition. With its 3,100-mAh cell, the Premium Extended can deliver 142 percent of a full iPhone 6 charge.
For $30 more than our runner-up, Tylt has an intriguing alternative. The company's Energi Sliding Power Case ($92) includes a 3,200-mAh battery that, in our testing, delivers a 133 percent charge—so it doesn't match the Premium Extended in terms of absolute charge. What makes the Energi different is its two-piece design: Your phone first fits into a fully protective case, and that case then slides into a separate battery pack. This design lets you use the phone in the smaller case when you don't need the battery, sliding the battery on only when necessary. And that inner case is pretty good on its own, considering that it's not a stand-alone product.
What if you own an iPhone 6 Plus?
The iPhone 6 Plus version of Tylt's Energi Sliding Power Case ($100) is our top pick for Apple's larger handset. It has the same design as the iPhone 6 edition, but it's sized to fit the 6 Plus. In our testing, the case delivered 83 percent of a full charge to a depleted iPhone 6 Plus in just under an hour and a half, better than either of the competitors we tested.
Wrapping it up
Six months after the release of the iPhone 6, we (finally) have a good selection of battery cases for the latest Apple handset. The best of the bunch is Anker's Ultra Slim Extended Battery Case, which at $40 is a screaming-good deal. It provides more than enough charge to keep most people going, and it comes in an appreciably small package.
This guide may have been updated. To see the current recommendation, please go to The Wirecutter.com.