Car battery monitor

i having trouble writing code car battery monitor, controls relay switch alternate supply @ set voltage reading

code: [select]


const int referencevolts = 5; // default reference on 5-volt board

const int r1 = 3900; // value maximum voltage of 18 volts
const int r2 = 1500;
// determine voltage divider resistors
const int resistorfactor = 255 / (r2/(r1 + r2));
const int batterypin = 0; // +v battery connected analog pin 0

void setup()
{
serial.begin(9600);
}

void loop()
{
int val = analogread(batterypin); // read value sensor
float volts = (val / resistorfactor) * referencevolts ; // calculate ratio
serial.print("2nd battery voltage: ");
serial.println(volts); // print value in volts

if (volts > 14.00) //if volts more 14v
    {
    digitalwrite (8, high); //turn on relay join batterys charging
   
  }

    else
  {
    digitalwrite (8, low); //if less 14v turn off relay separate batterys
    delay(3000);
  }}
i have been testing 4.2v battery , voltage divider of 2 470ohm resistors after changing int r1 , r2 accordingly
but it`s outputting in serial monitor negative voltage of -4109 hoping 4.2?
i have grounds connected , voltage divider a0 through resistor +4.2v through other resistor earth. going wrong?

quote from: aviator1 on may 09, 2012, 10:25 pm

code: [select]


const int referencevolts = 5; // default reference on 5-volt board

const int r1 = 3900; // value maximum voltage of 18 volts
const int r2 = 1500;
// determine voltage divider resistors
const int resistorfactor = 255 / (r2/(r1 + r2));



i wonder if problem in int math, magical place works out whole numbers , 1500 / 5400 = 0.

Arduino Forum > Using Arduino > Programming Questions > Car battery monitor

arduino