Next month I値l be taking a 5 day trip to NYC and I値l be using my built-in iPhone 4 Google Maps GPS a lot, so I知 in the market to buy an external battery. So with that said, what should I look for when purchasing one? I致e 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)). What痴 the difference when they advertise both?