Battery power sensor acting strange

[gadu]

So, I hooked up a Pro Mini with a DS18B20 and Battery Sensor resistors from the guide here. I have not yet cut off the LED or regulator and the complete sensor is running on 2xAA batteries.

Everything is working just fine, but today I noticed some strange behaviour where the battery percentage bounched to 87%, back to 83% and up to 89%... is this normal? What could be the reasons behind it?

This is how it looks like

Code I'm using

 #include <MySensor.h>  
 #include <SPI.h>
 #include <DallasTemperature.h>
 #include <OneWire.h>
 
 #define ONE_WIRE_BUS 3 // Pin where dallase sensor is connected 
 #define MAX_ATTACHED_DS18B20 16
 unsigned long SLEEP_TIME = 60000; // Sleep time between reads (in milliseconds)
 OneWire oneWire(ONE_WIRE_BUS);
 DallasTemperature sensors(&oneWire);

 int BATTERY_SENSE_PIN = A0;  // select the input pin for the battery sense point

 MySensor gw;

 int oldBatteryPcnt = 0;

 float lastTemperature[MAX_ATTACHED_DS18B20];
 int numSensors=0;
 boolean receivedConfig = false;
 boolean metric = true; 
 // Initialize temperature message
 MyMessage msg(0,V_TEMP);
 MyMessage msgvolt(1,V_VAR1);

 void setup()  
 { 
   // Startup OneWire 
   sensors.begin();

   // use the 1.1 V internal reference
   analogReference(INTERNAL);

   // Startup and initialize MySensors library. Set callback for incoming messages. 
   gw.begin(); 

   // Send the sketch version information to the gateway and Controller
   //gw.sendSketchInfo("Temperature Sensor", "1.0");
 
   // Fetch the number of attached temperature sensors  
   numSensors = sensors.getDeviceCount();

   // Present all sensors to controller
   for (int i=0; i<numSensors && i<MAX_ATTACHED_DS18B20; i++) {   
      gw.present(i, S_TEMP);
   }
 }
 void loop()     
 {     
   // Process incoming messages (like config from server)
   gw.process(); 

   // Fetch temperatures from Dallas sensors
   sensors.requestTemperatures(); 

   // Read temperatures and send them to controller 
   for (int i=0; i<numSensors && i<MAX_ATTACHED_DS18B20; i++) {

// Fetch and round temperature to one decimal
float temperature = static_cast<float>(static_cast<int>((gw.getConfig().isMetric?sensors.getTempCByIndex(i):sensors.getTempFByIndex(i)) * 10.)) / 10.;

// Only send data if temperature has changed and no error
if (lastTemperature[i] != temperature && temperature != -127.00) {

  // Send in the new temperature
  gw.send(msg.setSensor(i).set(temperature,1));
  lastTemperature[i]=temperature;
}
   }

    // get the battery Voltage
    int sensorValue = analogRead(BATTERY_SENSE_PIN);
    //Serial.println(sensorValue);

    // 1M, 470K divider across battery and using internal ADC ref of 1.1V
    // Sense point is bypassed with 0.1 uF cap to reduce noise at that point
    // ((1e6+470e3)/470e3)*1.1 = Vmax = 3.44 Volts
    // 3.44/1023 = Volts per bit = 0.003363075
    float batteryV  = sensorValue * 0.003363075;
    int batteryPcnt = sensorValue / 10;

    Serial.print("Battery Voltage: ");
    Serial.print(batteryV);
    Serial.println(" V");

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

    if (oldBatteryPcnt != batteryPcnt) {
      // Power up radio after sleep
      //gw.sendBatteryLevel(batteryPcnt);
      gw.send(msgvolt.set(batteryPcnt));
      oldBatteryPcnt = batteryPcnt;
    }
   gw.sleep(SLEEP_TIME);
 }

[daulagari]

I see in your code that before the battery level is sent the temperature is sent:

 // Only send data if temperature has changed and no error
 if (lastTemperature[i] != temperature && temperature != -127.00) {
     // Send in the new temperature
     gw.send(msg.setSensor(i).set(temperature,1));
 }

Could it be that when measuring the battery voltage just after the temperature data has been sent the battery voltage is temporary lower?

A way to test that is to add a delay before sending the temperature.

[kalle]

Maybe this is your problem, which also described at the building page.
"The tap point could be bypassed with a 0.1 uF capacitor to keep the noise level low, at this otherwise high impedance point. "

[gadu]

@kalle aah... that could be the reason!

noobish question perhaps, but what's the tap point?

[kalle]

Take a look here, I hope that make it more clear.

[gadu]

@kalle Crystal clear, thanks!