Does the iPad draw more current when charging? I seem to be able to charge my iPhone 5 on a variety of USB chargers (car ones included). My iPad retina (iPad 4) seems to be very fussy about which chargers it will actually charge up on. (The charging icon will come on, but the charge won't increase even after two hours). 

My question is, what causes this? Does the iPad draw more current when charging?
The recommended power for an iPad charger is 12W.
The power of the iPhone charger is 5W.

So you should get the iPad charger to charge your iPad.