Q: I’ve got an iPad 2 and an iPhone 3GS which both came with their own chargers but both use the same data cable. Can I use either charger to charge either device?
A: Apple’s USB chargers for iPhones and iPods are generally interchangeable and even old chargers for the fifth-generation click wheel iPod can still be used to charge an iPhone or iPod touch.
The iPad is a special case, however, as it requires a higher charging current. Although the adapter included with the iPad looks identical to other Apple chargers, if you read the fine print you’ll see it’s actually a “10W USB Power Adapter,” providing 2.1A of charging current. By comparison, the standard iPhone charger only supplies 1A of current. What this means is that you can’t realistically use the iPhone charge an iPad—it simply doesn’t provide enough juice. You won’t harm the iPad or the charger by plugging it in, but you’ll be presented with a “Not Charging” message in the top right corner of the screen.
Note that the iPad charger can be used with an iPhone or iPod touch, however, which may save you the trouble of having to pack an extra charger when travelling. The iPhone, like any other electronic device, will only draw the current that it actually needs and can handle, so when using the higher-current charger, the iPhone still only draws 1A.