This is before all programming. This problem is simple measurement. I am using a simple resistor divider to measure voltage. At this point all measurements are made with a standard digital VOM.
My objective is to measure the battery voltage of my alternate energy system so I can make decisions and take actions based on that. But before any of that can happen I need for this to be resolved.
I made a divider using a 4.7k and two 3.3k resistors as follows.
Input voltage 4.7K + 3.3K for a total of 8K. measure voltage 3.3K ground
When not plugged into the Arduino my divider performs exactly as the math would indicate. 16.54V = 4.83 15.00V = 4.381 12.00V = 3.504 11.60V = 3.388
When plugged in to the Arduino board I get the following. I have tried this on 2 different input pins.
16.54V = 4.03 15.00V = 4.00 12.00V = 3.504 11.60V = 3.388
I don't understand what is going on here. Why is my high end voltage not the same? The input impedance of the ADC is supposed to be 10Mohms as I understand it which should make no difference here.
Would appreciate any insight.