Measuring battery voltage via ADC or Vref
Keep your voltage source and ground as low impedance power source with twisted pairs to shunt any stray noise better. 
Then add decoupling caps to lower the impedance more just before your voltage divider and keep all leads short in your data collection to improve immunity from stray spikes and use a good ground braid wire that is low inductance or mounted to frame ground.


These are basic EMI practices. Additional shielding, should not be required unless you are right beside the ignition wires.


Alternatively, you can add a small series R e.g. 100 Ohms to 10 uF to filter conducted spikes better, followed by your higher impedance R divider near by.


Any serious automotive design must pass these tests which requires series diode for reverse voltage (accidental loose battery terminal) , series current limiting and MOV Clamp.