Monitoring Lithium Polymer Battery Voltage from Microcontroller
I was hoping to be able to uniformly shift the voltage below 3V using diodes or a voltage divider but from a couple google searches it looks like current fluctuations would make that impossible.  


That sounds like BS, do you have a source on that? A simple resistive voltage divider can be used to linearly scale down the voltage:







simulate this circuit &ndash; Schematic created using CircuitLab

This would drop the 4.2V maximum voltage down to 2.89V, well within the range of the input. The divider output has an impedance of 69KΩ, which should be low enough for the analog to digital converter but I could not find the ADC specs anywhere in order to make sure.