I plugged my iPad into my Macbook Air to charge and it says “Not Charging”. WTF? Turns out that iPads (sometimes?) only charge when asleep, particularly when plugged in to computer USB ports. That leads to a sort of quantum paradox where the act of observing the thing charging stops it from charging. Stupid.
MacBook Air ports are apparently capable of providing the 2 amps of current an iPad wants to charge. The way to know for sure is to look for “Extra Operating Current” in the USB part of System Information. Oh except the Air won’t put the current out if it’s asleep.
More detail in this article. USB charging is too complicated.