Battery sensor measure for li-ion cells?

[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.