If you have a spare analog input pin, then a simple solution for any micro is to just wire a precision voltage reference chip to one input and use that value to map the correct range used for the other analog inputs of the ADC. One of many possible examples:http://www.analog.com/en/special-linear-functions/voltage-references/ad580/products/product.html
I plan to use this method to protect my LiPo battery pack. Does it make sense to monitor vcc to determine if there is enough power in my 7.4 V battery connected to Vin ? I hope drop in Vcc signals battery undervoltage but I have doubts.
And if I have two single cell batteries with a tap between them? Is it enough to measure the voltage at the tap without the divider?