Batteries for your Charm
The standard Evo 4G battery (seen here in its fire-engine-red paint scheme) is of the 1500mAh variety. (That's a unit of measurement for the amount of juice a battery holds. A higher number means a bigger battery.) While 1500mAh is by no means small, the Evo 4G is, shall we say, a bit of a power hog. So you might want to consider keeping a spare on hand, just in case.
The Evo 4G extended battery brings more juice -- 1750mAh -- in the same size as the stock battery. That's about 17 percent more juice, all without needing a bigger battery door or adding any bulk to the phone.