Monitoring voltage without a known reference
Most (all?) AVR's with an ADC have an internal reference voltage which is regulated to a constant value, independent of the voltage supply. Check your datasheet...in the chip I'm using, it's 2.56V. There's a register that picks between comparing against Vcc, AREF, or internal voltage reference. You'd just set it to compare against internal voltage reference, along with using a voltage divider so the maximum voltage is never over 2.56V.