Mophie has announced Juice Pack Air ($100), a new battery case for iPhone 5. Juice Pack Air comes with modestly more battery capacity than Mophie’s recently-released Helium, packing a 1700 mAh battery compared to Helium’s 1500 mAh. It purports to double the iPhone 5’s battery power, though testing results of its predecessor and competing models suggest that this is highly optimistic. An included micro-USB cable can charge both the…