Battery sensor measure for li-ion cells?

[LastSamurai]

Great information here! I was building my own method for getting battery percentage for my 2AA battery powered node and tried to understand the calculations. Here is my piece of code. What do you guys think? Everything's right?

// define values for the battery measurement
#define R1 = 1e6;
#define R2 = 470e3;
#define VMIN = 1.8;
#define VMAX = 3;
#define ADC_PRECISION = 1023;

int getBatteryPercentage() {

  // read analog pin value
  int inputValue = analogRead(BATTERY_SENSE_PIN);
  
  // calculate the max possible value and therefore the range and steps
  float voltageDividerFactor = (R1 + R2) / R2;
  float maxValue = voltageDividerFactor * VMAX;
  float voltsPerBit = maxValue / ADC_PRECISION;

  float batteryVoltage = voltsPerBit * inputValue;
  //int batteryPercentage = ((batteryVoltage-VMIN)/(VMAX-VMIN))*100;
  int batteryPercentage = map(batteryVoltage, 0, maxValue, 0, 100);

  return batteryPercentage;
}

The 2 lines for the batteryPercentage should do exactly the same. Prefer the 2nd one though, it's more readable for me.

[m26872]

@LastSamurai
About this line:

float maxValue = voltageDividerFactor * VMAX;

You should use your Adc reference voltage here, not VMAX. If it's a 2AA node you probably use 1.1V internal ref.

[LastSamurai]

Thanks! I updated the code (had some errors in the define statements and now got this:

#include <SPI.h>
#include <MySensor.h>  
#include <DHT.h>  

#define CHILD_ID_HUM 0
#define CHILD_ID_TEMP 1
#define HUMIDITY_SENSOR_DIGITAL_PIN 3

// define values for the battery measurement
#define R1 1e6
#define R2 470e3
#define VMIN 1.8
#define VMAX 3
#define ADC_PRECISION 1023
#define VREF 1.1

MySensor gw;
DHT dht;
unsigned long SLEEP_TIME = 300000; // Sleep time between reads (in milliseconds)
float lastTemp;
float lastHum;
boolean metric = true; 
MyMessage msgHum(CHILD_ID_HUM, V_HUM);
MyMessage msgTemp(CHILD_ID_TEMP, V_TEMP);
int oldBatteryPcnt = 0;
int BATTERY_SENSE_PIN = A0;


void setup()  
{ 
     // use the 1.1 V internal reference
  #if defined(__AVR_ATmega2560__)
     analogReference(INTERNAL1V1);
  #else
     analogReference(INTERNAL);
  #endif

  gw.begin();
  dht.setup(HUMIDITY_SENSOR_DIGITAL_PIN); 

  // Send the Sketch Version Information to the Gateway
  gw.sendSketchInfo("Humidity", "1.0");

  // Register all sensors to gw (they will be created as child devices)
  gw.present(CHILD_ID_HUM, S_HUM);
  gw.present(CHILD_ID_TEMP, S_TEMP);
  
  metric = gw.getConfig().isMetric;
}

void loop()      
{  
   int batteryPcnt = getBatteryPercentage();

   Serial.print("Battery percent: ");
   Serial.print(batteryPcnt);
   Serial.println(" %");

   if (oldBatteryPcnt != batteryPcnt) {
     // Power up radio after sleep
     gw.sendBatteryLevel(batteryPcnt);
     oldBatteryPcnt = batteryPcnt;
   }
  
  // totally random test values
  gw.send(msgTemp.set(42, 1));
  gw.send(msgHum.set(42, 1));

  gw.sleep(SLEEP_TIME); //sleep a bit
}

int getBatteryPercentage() {

  // read analog pin value
  int inputValue = analogRead(BATTERY_SENSE_PIN);
  
  // calculate the max possible value and therefore the range and steps
  float voltageDividerFactor = (R1 + R2) / R2;
  float maxValue = voltageDividerFactor * VREF;
  float voltsPerBit = maxValue / ADC_PRECISION;

  float batteryVoltage = voltsPerBit * inputValue;
  float batteryPercentage = ((batteryVoltage-VMIN)/(VMAX-VMIN))*100;
  //int batteryPercentage = map(batteryVoltage, 0, maxValue, 0, 100);

  return batteryPercentage;
}

I am using 2 1.5V aa batteries, but somehow I get a percentage of 136%. Whats wrong there?
Using the map part instead I get 100%, so some values is way to big somehow.

[m26872]

@LastSamurai
If I were you I would:
a. Measure battery voltage to know what to expect.
b. Debug by printing out the raw sensor value 'inputValue'. (0 or 1023 likely means a hw issue).
c. Change VMAX to 3.2, 3.3 or maxValue.
d. Change the line

  float batteryPercentage = ((batteryVoltage-VMIN)/(VMAX-VMIN))*100;

to

  int batteryPercentage = static_cast<int>(((batteryVoltage-VMIN)/(VMAX-VMIN))*100);

[LastSamurai]

Ok I added the change, removed everything but the battery/voltage code and tried it out. Using a serial to usb adapter I got the right values.
Then I readded the network and sleep stuff and suddenly I am getting these 136% percent again. Any idea why? Could sleeping or the network somehow interfere with the measurements?

[AWI]

@LastSamurai Everything is related to VMAX in your case. Have you measured the actual voltage of the batteries..? The type of battery used can explain the 136%

Using the calculation below you will never get an above 100% reading ;)

int batteryPcnt = constrain(map(batteryVoltage, VccMin, VccMax, 0, 100),0,100); // and map to the 0-100% range

[LastSamurai]

The actual voltage was about 2.7-2.8V. So my values should be right?! I will do some more testing soon.

Are you sure about constrain and map? I am pretty sure the map call already maps the values to 0..100. So no constrain needed.

Is it possible that

#define VMAX 3
// has to be
#define VMAX 3.0

[m26872]

@LastSamurai
I think you should note what the map() reference says about constrain and integer math. Try to use mV instead.

[LastSamurai]

@m26872 Thanks! For me thats strange behaviour but it seems you guys are absolutely right!
What do you mean mV? Using VMAX 3000 instead of 3?

[m26872]

@LastSamurai
As input to map(), yes. And that goes for Vbat and Vmin as well of course.

[m26872]

@Tmaster
I'd like to add a few things about the settings a Vmin and Vmax.

If you get a Vbat value outside of [Vmin Vmax] the calculation would give a negative result. Since it's then sent as an uint8_t, it'll not be easy read or meaningful. Due to this it's sometimes a good advice to make the interval wider, use a constraint or handle the exceptions.

Vmin 4.3V is a conservative limit. The APM internal vreg (ldo?) is probably working a lot lower depending on the load. You have to find out yourself.