Your analog reference is 3.3v. I'd use a larger resistor for R7. With a 10k and 1k, the voltage on R7 with a 9v battery is going to be 0.81v. A 9v battery is dead at what? About 6 volts? That is .54v. 0.27v difference is easy enough for the ADC to distinguish, but a higher resistor size would increase the difference and give you finer resolution. I'd use a divider more like 100k and 47.5k. Using these values would let you read the ADC and divide by 100. (Cool trick Nigel taught me
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Example:
At 9v input, the voltage across the bottom resistor is 2.9v. The ADC conversion of 2.9v with a 3.3v vref is 900.
At 7.5v input, the voltage across the bottom resistor is 2.42v. The ADC conversion of 2.42v with a 3.3v vref is 750