I just got a Kensington dual car charger. It has a 2.1 amp USB slot, and a 1.0 amp USB slot. I did a little bit of testing this morning on the way to work. I plugged my iPhone 4S into the 1.0 amp slot, and after 4 minutes the iPhone gained 3% of charge. Then I plugged the iPhone into the 2.1 amp slot, and after 4 minutes the iPhone only gained 3% of charge. I was hoping the iPhone would charge twice as fast in the 2.1 amp slot, but that didn't happen. Does the iPhone only take 1 amp, even in a 2.1 amp slot, or what?