I have the Galaxy S 3 too. I like to use a GPS program called alpinereplay. It tracks my speed and where I have boarded. Anyways... instead of getting a backup battery, I got an external backup battery. The brand I got is Anker. It is a very nice product and they ship really quick. Ordered Tue and had item by Thurs. The first one I purchased was the 3200. It will take the battery from completely dead to 98%. I liked the product so much I purchased a bigger one the 8400. This one can charge the phone completly 3 times. It may be longer after I brake in the battery. Oh anther thing I like about this external battery is that it has a USB port to charge anything.
I have not used an external battery but I dont think it would be that bad since the Galaxy S 3 is so thin. I put a bumper on my S3 just to make it a little thicker.
I saw that too but I was hoping to go cost effective and I thought it would be inconvenient to have to charge it. Or does it not work like that? Seems more convenient to just pop in the next battery, unless that's how yours works?