What to look for when purchasing iPhone external battery?
 

Next month I’ll be taking a 5 day trip to NYC and I’ll be using my built-in iPhone 4 Google Maps GPS a lot, so I’m in the market to buy an external battery. So with that said, what should I look for when purchasing one? I’ve 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’s the difference when they advertise both?