Of all the complaints about the iPhone in all its various models, one of the most common is that the smartphone has a short battery life. The battery life each user gets varies a lot depending on how they use the device and sometimes even humidity can affect the run time of batteries. Whatever the reason the battery life for users is shorter than they like, many people turn to cases with batteries inside to get more use from their iPhone.
There are already several cases on the market today that offer a battery inside for the iPhone 4 and older models. None of them has the same sort of name recognition as Energizer does though. The battery giant has announced a new case specifically for the iPhone 4 called the AP1201. The case has a battery inside that claims to "almost double" the battery life of the iPhone 4 while protecting the device from drops and bumps.
The battery inside the case is charged via USB. The case battery and the battery inside the iPhone can both be charged at the same time thanks to pass through charging design. Energizer claims that the case is good for as many as 500 charges in its lifetime. That sounds a bit short to me, charging it each day will get you only around a year and a half use out of the thing. The AP1201 is in stores right now around the country for $69.99.