Everyone needs to be aware that not all USB chargers are the same. There are an assortment of specs, and a single charger likely won't support all of them. Check out the Anker chargers and you will find the different models depending on if you want to charge an Apple iPad versus an Android with Qualcomms "Quick Charge". Any charger will support the standard 100/500mA mode. BTW 500mA is not enough to charge an iPad unless it is sleeping.

USB Standard downstream port (SDP):
- "Low power" - 100mA
- "High power" - 500mA

USB "Charging downstream port" (CDP) - Up to 1.5A
USB "Dedicated charging port" (DCP) - Up to 1.5A

Apple proprietary (found in iPads) - 2A

Qualcomm proprietary (found in some Android devices):
- Quick Charge
- Quick Charge 2.0
- Quick Charge 3.0

Tech info on the USB charging standards (but not the proprietary): https://www.maximintegrated.com/en/a...ex.mvp/id/4803