microcontroller - Low Battery Indicator

I'm looking to make a battery life indicator that I can use in conjunction with a micro-controller. I'm hoping that I can then read the current battery life remaining and increasingly put the micro-controller to sleep to preserve battery life.

Answer

To the primary question, as to how to measure the voltage of a battery, the most simple method is to use one of the ADC pins on the arduino.

If you're providing 5v from the battery (unlikely), then you can run the + straight to one of the analog in pins, and use analogRead() - each increment in value (from 0-1023) will represent 4.9mV. So multiply the read value by .0049 to get the volts read.

If you're running higher than 5v (more likely), you can use a voltage divider circuit (see: http://en.wikipedia.org/wiki/Voltage_divider) to bring the voltage you're sending to the analog input to <= 5v. If you're running 12V, you'll want to divide the voltage into roughly 1/3. Then, after multiplying the ADC value, multiply by 3 to get actual voltage.

Thus, if using 12V, and a 1/3rd voltage divider:

#define BAT_PIN 14

float read_batt_volts(void) {

  int val = analogRead(BAT_PIN);


  float volts = (float) val * (float) 0.0049 * (float) 3;

  return(volts)
}

!c