Those iPhones were designed to pull more than 1A from the charger. Since the stock charger only supplies 1A, it gets hot (not too much, though) while charging. If we use an iPad charger (or any other capable of supplying more than 1A), it won't get hot, but this time it's the iPhone that gets hot while charging. Does it mean the stock charger is always being overloaded while charging those iPhones? I know Apple lists the iPad charger as compatible with every iPhone on their site. But, is it really safe regarding battery long-term life?