Next month Ill be taking a 5 day trip to NYC and Ill be using my built-in iPhone 4 Google Maps GPS a lot, so Im in the market to buy an external battery. So with that said, what should I look for when purchasing one? Ive noticed a lot of these advertise the mAh (milliampere-hour), and then some will also advertise the amperes themselves (i.e. 5200 mAh (2 amps)). Whats the difference when they advertise both?