no problem you can charge it with a 6100 ma usb car charger if you want
for charging all that matters is
1. it's 5 volt if it's higher then it will kill the device but if it's higher then 5volts it's in violation of the usb std.
2, it provide enough ma (mili amps) to charge there is no max
ohms law is
amp = volt / resistence
the internal resistence of the iphone desice how much amps it will draw amps are drawn not pushed into a device