Samsung Epic 4G is a Sprint version of Galaxy S smartphone. It sports a mighty 1GHz processor and a slide out hardware keyboard with access to WiMAX cellular network. Obviously, using WiMax and Wi-Fi consumes a lot of power and that is why I bought myself its extended battery.
The battery is of same size as the original battery that has 1500mAh. The capacity of the extended battery is 1800mAh.
It fits inside the Epic 4G without any problem despite the fact it is 0.4mm thicker. All-in-all, it fits inside this Android smartphone with no problem at all.
Due to the 300mAh improvement over the original battery, battery life increase is great. Expect around 3 hours of extra usage. I got it from ebay for 23 dollars.