The iPad has to have a 2 amp current to fully charge the battery. Most Computer USB ports don't provide enough current, usually 1 amp, to charge and operate the iPad at the same time, if you turn the iPad off and leave the computer turned on for a few hours it will probably charge but maybe not to 100%.. It needs 2 amps, smaller devices are fine with the computer ports, but use the apple 10 watt charger or one that is 2 amps of current. Don't buy anything that does not have the amp output on its label, it would probably be to small. The whole thing is based on the load which is what the iPad would be in this case, it has a bigger battery and needs a lot of current to just operate. So the computer ports just can't handle it. Evidently apple computers do provide 2 amps at the USB ports so they once again were thinking ahead.