There are two main types:
1) Separate rechargeable batteries that you charge your phone from; and
2) Batteries that double as iPhone cases.
This is a review of the Anker® 2400mAh MFI Certified Extended Battery Case for iPhone 5s, available on Amazon for about $40. And by the way, Anker gave me this product in exchange for my honest feedback.
This is a good looking product, that comes in a eco-friendly, but satisfyingly elaborate package. The product consists of a battery pack that fits on the back of your phone, an audio jack adapter, and a plastic surround that attaches the battery to the phone and completes your battery/case (two are included, one smoke and one clear).
How it works as a battery pack
Your phone uses its own battery. Then, when you notice it running low, you power on the case and it starts feeding power to your phone. It approximately doubles your battery time.
What it's like as an iPhone case
Considering this as a case, it's a big, long and heavy one. It's black and streamlined and looks good. The case is slightly slippery. That means it's less comfortable and secure to hold, but it's easy to slip into a pocket. The case is the kind that does not cover/protect the iPhone's screen.
The battery/case adds to both the width and length of the phone. The extra width is the battery, of course. The extra length is needed because you can't open your iPhone to connect the phone to the external battery. The only way the battery/case can attach to your phone is through the phone's lightning jack.
So your phone needs the case to plug into (and thereby cover) its whole bottom side. That means that the battery/case also covers the speakers and headphone jack. The case handles the speaker issue in an clever, low tech way: the plastic is shaped in a way that sort of bounces the sound out. The sound from the speakers sounds a little different, but not worse, and the volume is not reduced.
The phone jack problem is handled, though, is handled poorly. You are given a bulky adapter to use. If you forget it (as I did the day I tested this product), you'll have to take your phone out of the case if you want to listen to music. A better method would have been to have the adapter built into the case rather than being separate so that when you fit your phone onto the lightning cable, you also fit it onto the microphone/speaker plug.
One big plus!!!
One great thing about this case is that it connects to power with a USB cable. Then your phone charges itself from the case. So if you're using this case, you don't need an expensive lightning cable.
At about $30, this is a well-priced product that does its job simply and well. This is the perfect product for you if you want a battery that doubles as a case, don't listen to music with your iPhone and like a case style that does not cover the front of the iPhone.
This would be a better product if
- If the audio adapter were built in so that the user doesn't have to schlep the adapter around
- If it came with a $.05 screen cover