fungus:
Divinitous:
Any possible alternative to a voltage divider....In theory: 10 times more resistance in your divider should mean 10 times less voltage drop (more or less).
You could try putting bigger resistors in there, eg. in the megaohm range. The Arduino ADC input has 100 megaohm resistance so there's still some margin to work with.
I'm experiencing the problem before the voltage divider even gets to the arduino. With the arduino removed from the circuit the Vin still loses ~1v. I think the best solution would be the opamp as someone has mentioned.