Hi I am an engineering student at OSU and wanted more battery life while studying and pulling all nighters on my iPhone 4. So I added a second battery! I had to fab a new back for the phone but the upside is the back it now aluminum and doesn't need a back cover, just maybe a bumper so it is smaller than if it had an otter box. Here is the final product! It looks a little jankey but I did make it with a dremel, a file, a hammer, and some stong epoxy. Let me know if you have questions or would like me to make one for you! It is fully reversible for warranties as well. It gets about twice the battery life as the original. Enjoy! Edit: The start of version two are the last two!