For your possible interest, here is part of an article written by Hadley Markoski of Small Dog Electronics (http://www.smalldog.com/) that provides info on the charging of your iPhone or iPad. Note especially the last paragraph.
Jim Hamm
"all devices with lithium ion batteries have on-board charge controllers that regulate the charging. Many modular lithium batteries have protection circuits built into them by default. So no matter how much charge current is available, the controller will always have the last say about how much of it hits the battery. This is why you can charge your iPhone with a larger iPad charger. Just because the larger charger is rated for 10-watts (2 amps) doesn’t mean it’ll be delivering that all the time. It means it can deliver up to that if necessary. There is some portion of the iPhone’s charging profile where the extra power can be safely used, but it’s only some portion of the charge process, not all of it. This is why your iPhone will charge faster, but not twice as fast.
Can you go the other direction and charge your iPad with the iPhone charger to extend it’s battery life by charging it more slowly? No. This will not work because the iPad’s larger battery requires the extra power to charge it. The battery cannot be properly charged without the full 10 watts being available. This is why you’ll often see USB chargers say that they can or cannot be used to charge tablets. All tablets, not just Apple’s, typically require 2 amps. USB ports on most computers follow the USB bus protocol which means they can only output 0.5 amps or 0.9 in some cases. USB-C can output up to 3 amps. This is why your iPad may not charge when plugged into your laptop (though it can still transfer data over USB).
These charging rules basically apply across Apple’s entire line of products. You can always charge a smaller device/battery with a larger charger, but not a larger device with a smaller charger. Obviously this only works if the voltages are the same. You cannot charge your iPhone with the charger for your MacBook because the voltages are different (as well as the plugs)."