ADC resolution after voltage divider
For an ADC with a high impedance input then a resistive voltage divider is the best approach in terms of simplicity. I'd put a 100nF cap across the lower resistor just to give the imperfect (and sometimes glitchy) input impedance of the ADC a better chance of an accurate reading especially if sampling is multiplexed among a few other analogue inputs.

You might need to make the range greater than 12 volt because on charge the voltage might look closer to 14 volts.

If your ADC is 10 bit and you choose a 0 to 15 volt range then resolution is 15/1024 = 14.6 mV per bit.