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.
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.