Powering the Arduino with a battery and reading the battery voltage

Question

I am trying to display the battery voltage as it is being used to power the Arduino.

I have tried powering the Arduino through the USB and the battery voltage is read accurately when compared to the multimeter reading.

But when the battery is powering the Arduino and being read, it keeps displaying 5.00 V, which it should not be.

Just wondering what might be happening and how I can go about resolving this issue.

I have been trying to research some answers, and came across something about an 1.1 V reference voltage, but I'm not entirely sure what they mean.

Here is my code for reference:

#include <SoftwareSerial.h>
SoftwareSerial BTserial(10,11);

int battPin = A1;

void setup() {
  pinMode(battPin, INPUT);
  BTserial.begin(9600);
  Serial.begin(9600);
}

void loop() {
  int voltReading = analogRead(battPin);
  float volts = (voltReading/204.6);
  BTserial.print(volts);
  BTserial.print(";"); 
  Serial.println(volts);
}

I am using a 3.7 V Li-po battery. It is connected to A1 and also to Vin.

Answer 1

AFAIK, Arduinos are set up to use their operating voltage as the analogRead reference voltage by default.

So what I think is happening is that you are trying to measure the battery voltage with the battery itself as a reference voltage.

This will, of course, always show "full scale".

You could use a voltage divider to bring the measured battery voltage down to about 1 V (max) and use the Arduino's internal reference of about 1.1 V for measuring.

Note that the 1.1 V internal reference is stable, but not necessarily exactly 1.1 V. You will have to measure it first.

Answer 2

You need to divide down the battery voltage into the 1.1V max range, and use the Internal bandgap voltage reference for the ADC conversions.

Connect the Battery to VCC, not Vin, you don't want it going thru the regulator.

Answer 3

If you want measuring a "battery level" in mAh for example, you must measure Voltage AND Current, so INA219 is a better deal (it is a I2C control).