Hi ngeronikolos,

Another way might be to use the Comparator Module of the 628.

After reducing the voltage with a pair of resistors, apply it to 1 input of a comparator.

Then using PWM output on another pin (filtered with a resistor and capacitor), adjust the DutyCycle to control the analog voltage being applied to the other comparator input.

Then when the comparator changes state, you know what voltage it was at.

Not the best resolution, but it should be good enough to tell if the battery's getting Low or not.
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