https://www.apple.com/batteries/ [This has been edited extensively since the original post to more clearly illustrate the following.] A USB extension cable will lower the current flowing into the iphone battery due to the increased resistance of the cable. The current is reduced, not the voltage. The length of the cable determines what the charge rate will be. The longer the cable, the less current will flow. This allows much gentler trickle charging when using the phone while plugged in and when not in a hurry to charge. Lower charging rates keep the battery cooler while charging and put less stress on it. A 5ft USB extension cable will effectively cut the charge rate of the standard iPhone charger (3.55 watts) by 65% to 1.26 watt. Two of them (10 ft) will trickle charge the battery at 10% of the normal rate at 0.34 watts. What this equates to is that an iPhone, when plugged in, can be trickle charged at a low enough rate with a suitable length of USB cable, that the number of micro charge cycles can be reduced due to the lengthened charge times... It's also a lot easier on the battery. This can be useful when using the iPhone always plugged in and keeping the battery capacity below 100%. Using an USB extension cable is not recommend if the goal is to charge the phone to 100% as quickly as possible. To keep the charge indicator calibrated, the battery still should be discharged once a month to below 20%, then fully charged to 100% while using the standard iPhone charger and cable. >>>>>> Standard iPhone wall charger alone: with 5 ft USB cable with two 5 ft USB cables connected together.