Do you mean it is marked as 1500 mAh and measures 3.5 volts when charged?
Is it NiMH, NiCad, or LiPo, etc...
Do you actually have 2 of these, or was your originally quoted 7.1 V some sort of error?
If it's less than 5V you will need a SMPS in boost configuration (which is not trivial) and it is likely that there is insufficient power in a single pack to fully charge an iPhone.
If it is > 5V you have other options -- possibly a simple linear regulator (although quite likely a low-dropout one)