I have 10 kohm resistor between plus and minus of a 12 V battery. I have connected Automation pHat ADC port two over the resistor. My idea is to measure battery voltage. My Python program is running every 10 minutes and takes maximum voltage from 20 measurements. There are more then 700 results. The measured result varies very much. Minimum is 0 V, maximum is 14,29 V and average is 10,79 V.
With two different voltmeter I have measured 12,4 V.
Is this inaccuracy normal? What have I done wrong?