4 cases that charge your smart phone

Smart phones are closer than ever to being the one mobile gadget that does everything you need. But of course, the more you do with your phone—shooting and sharing photos and videos, streaming music and movies, and playing graphics-rich games, not to mention calling and texting—the more power it uses, and it's mighty frustrating to run out of juice when you need it. How do you keep your phone alive until the next charging opportunity? One answer: a phone case with supplemental rechargeable batteries built in.

Cases with built-in batteries are always connected to your phone, and they're recharged at the same time as your phone. The cases do add to the weight and size of the phone—but they also add a measure of protection.

We tested two cases each for two of the more popular smart phones. For the Samsung Galaxy S 4, we tried the Mophie Juice Pack for Samsung Galaxy S 4 ($100) and the uNu Unity Series UT-S4-2600B ($70). For the Apple iPhone 5s, we tested the Mophie Space Pack for iPhone 5/5s ($180) and PhoneSuit Elite 5 P5-ELITE-IP5 ($100). We also tested a supplemental battery pack that works with any phone (or tablet) that charges with a USB cable: the Samsung Galaxy Portable Battery Pack ($100).

All the cases did what they claim, providing additional power for those times when the smart phone’s battery has become exhausted. But how well they work depends on how you use them.

How we tested

We set up our phones’ screens so they would not shut off or go into sleep mode, with cellular and Wi-Fi turned off. Then we monitored the phones’ battery levels over time with this reproducible, constant battery drain, with and without the fully charged cases. We also tested recharge times and levels for all the devices. (We didn’t test for the level of added protection the cases provide when the phone is dropped.)

What we found

How consumers use their phones determines the actual amount of time for which the cases provide power, but all the cases claim to about double the time that you can use your phone. Each charges the phone in the case and the case itself, though it takes longer to charge because you're recharging the phone and the extra battery.

So how well did each case do?

The Apple iPhone 5s battery took a little over 6 hours to drain completely. Using the Mophie Space Pack case (below), the fully drained iPhone lasted another 5 hours—about 80 percent longer than the phone alone, but lower than the claimed 100 percent extension. (Both iPhone cases use micro USB charging cables instead of Apple’s Lightning connector.)

After draining the iPhone's battery again, we turned on the PhoneSuit case (below), and the 5s lasted nearly an extra 6 hours—about 90 percent longer, close to the claimed 100 percent battery-life extension.

At the settings we used, the battery of the Samsung Galaxy S 4 took about 12 hours to drain. After it was fully drained, and we turned on the Mophie for Samsung case (below), the phone lasted another 9 hours—an extension of about 75 percent more battery life, close to Mophie’s claimed 80 percent.

As for the uNu case (below), the battery life of the combined Galaxy S 4 and case was about 24 hours, very close to the 100 percent extension claimed. But the uNu blocked access to the phone’s USB connector for some USB cables, since it does not have its own connector, and the case has only a small cutout allowing access to the phone’s USB port.

Finally, the Samsung portable battery pack (below) was able to recharge the Galaxy 4 S to 100 percent power in about 1 hour 40 minutes, even while the phone was on and drawing power. And it was able to fully recharge the iPhone to 100 percent—even while the phone was on and drawing power—in about 4 hours 20 minutes.

Our take

Both Mophie cases and the PhoneSuit case are designed to be switched on to charge a drained phone—and all three have the simple drawback of needing to be manually turned off. It’s not hard to do, but it may be hard to remember: If the cases are accidentally left on, their batteries drain along with the phone batteries. The actual total battery life of your phone life is still longer with the cases, but power users might run out of juice without realizing the backup has been drained already.

The PhoneSuit claims to be designed so that it can charge your phone until its battery is drained, then your phone’s battery kicks in automatically. The same still applies to it, though, smart or not—when needed, it may not be available. Still, these three cases do provide extra power, even if you need to be somewhat conscientious about using them.

The uNu gives you the advantage of not having to think about turning it off and on, but it’s available only for phones with removable batteries—a rare feature in today’s smart-phone world.

The Mophie Space Pack for the iPhone 5/5S is unique in that it adds memory to a phone without any memory-expansion capability that also does not work well in a Windows environment. You access the additional memory through an app: For example, open the app and then take a photo, and it will be stored in the Mophie’s memory instead of the iPhone’s. When the phone is connected to a computer, the case's memory is exposed to the Windows operating system, so operations like drag-and-drop and cutting and pasting work as expected. For this capability alone, the Mophie Space Pack is worth considering.

Bottom line

If you consistently find that your phone is going dead or (for iPhone owners) running out of space for pictures, you might consider one of these cases. They address the need for longer-lasting power, protection from accidental damage, and more memory.

And if your phone won’t fit in available battery cases, then a separate device such as the Samsung Portable Battery Pack is a very plausible alternative for charging your phone, though keeping it charged and available requires more attention than a case that is always connected to your phone.

—Carol Mangis and Bernie Deitrick