Battery percentage gone wild [SOLVED]

  • Hello,

    I'm building my first battery powered sensor. A simple temperature sensor using a Pro Mini with a DS18b20.
    The temperature reading works just fine, but the battery reading is all crazy.
    I have wired it just like the example.

    When connected to the serial programmer the reading seems fine. The reading jumps up and down but never goes above 100%. I guess fluctuation is due to the powering from the FTDI, and the battery only connected to radio and tap point to A0.

    But when on battery power I got readings every sleep cycle of 120-300%. This is my code:

    // ------------------ MYSENSORS ------------------
    // Enable debug prints to serial monitor
    #define MY_DEBUG
    // Enable and select radio type attached
    #define MY_RADIO_NRF24
    // Set the frequency to channel 110 (2,510MHz), to prevent interference of WiFi channel 14 (-2,495MHz)
    #define MY_RF24_CHANNEL 110
    #include <SPI.h>
    #include <MySensors.h>  
    #include <DallasTemperature.h>
    #include <OneWire.h>
    unsigned long SLEEP_TIME =  5 * 60000; // Sleep time between reads (in milliseconds), 60000 ms = 1 minute
    // ------------------ TEMPERATURE ------------------
    #define COMPARE_TEMP 1 // Send temperature only if changed? 1 = Yes 0 = No
    #define ONE_WIRE_BUS 3 // Pin where dallase sensor is connected 
    #define MAX_ATTACHED_DS18B20 1
    OneWire oneWire(ONE_WIRE_BUS); // Setup a oneWire instance to communicate with any OneWire devices (not just Maxim/Dallas temperature ICs)
    DallasTemperature sensors(&oneWire); // Pass the oneWire reference to Dallas Temperature. 
    float lastTemperature[MAX_ATTACHED_DS18B20];
    int numSensors=0;
    bool receivedConfig = false;
    bool metric = true;
    // Initialize temperature message
    MyMessage msg(0,V_TEMP);
    // ------------------ BATTERY ------------------
    int BATTERY_SENSE_PIN = A0;  // select the input pin for the battery sense point
    int oldBatteryPcnt = 0;  // remember last measured percentage
    #define V_MIN 2.7  // 0% Battery voltage
    #define V_MAX 3.3  // 100% Battery voltage
    void before()
    void setup()
      // requestTemperatures() will not block current thread
      // Setup 1.1 V internal reference for analoge GPIO
    void presentation()
       // Send the sketch version information to the gateway and Controller
      sendSketchInfo("Hottub", "2.1");
      // 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++) {   
         present(i, S_TEMP);
    void loop()
      // read analog pin
      int sensorValue = analogRead(BATTERY_SENSE_PIN);
      // calculate battery voltage
      float vBat  = static_cast<float>(sensorValue * (V_MAX/1023));
      // calculate % left of defined interval
      int batteryPcnt = static_cast<int>(((vBat-V_MIN)/(V_MAX-V_MIN))*100.);  
      // debugging
      #ifdef MY_DEBUG
        Serial.print("Sensor value: ");
        Serial.print("Battery Voltage: ");
        Serial.println(" V");
        Serial.print("Battery percent: ");
        Serial.println(" %");
      // If battery % has change send to controller
      if (oldBatteryPcnt != batteryPcnt) {
        // Power up radio after sleep
        oldBatteryPcnt = batteryPcnt;
      // Fetch temperatures from Dallas sensors
      // query conversion time and sleep until conversion completed
      int16_t conversionTime = sensors.millisToWaitForConversion(sensors.getResolution());
      // sleep() call can be replaced by wait() call if node need to process incoming messages (or if node is repeater)
      // 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>((getControllerConfig().isMetric?sensors.getTempCByIndex(i):sensors.getTempFByIndex(i)) * 10.)) / 10.;
        // Only send data if temperature has changed and no error
        #if COMPARE_TEMP == 1
          if (lastTemperature[i] != temperature && temperature != -127.00 && temperature != 85.00) {
          if (temperature != -127.00 && temperature != 85.00) {
          // Send in the new temperature
          // Save new temperatures for next compare

    How can the value ever be greater than 100% and only when powered with battery and not with the serial programmer?

    What am I missing? I have been struggling with this for 2 days now, and can't come up with any more ideas to try!

  • Hardware Contributor

    @strixx - try to measure the voltage on A0 and see what you get.

  • @sundberg84 Short reply:
    Doesn't matter what the actual reading is.

    #define V_MIN 2.7  // 0% Battery voltage
    #define V_MAX 3.3  // 100% Battery voltage
      // read analog pin
      int sensorValue = analogRead(BATTERY_SENSE_PIN);
      // calculate battery voltage
      float vBat  = static_cast<float>(sensorValue * (V_MAX/1023));
      // calculate % left of defined interval
      int batteryPcnt = static_cast<int>(((vBat-V_MIN)/(V_MAX-V_MIN))*100.);

    In my head this code should never be able to set batteryPcnt higher than 100. Unless sensorValue can be greater than 1023.

    Long reply:
    Yesterday I was measuring the voltage with a multimeter and ended up recalculating the divider over and over again. With the same result over and over again. The voltage at the tap point was not the same as the math told me it should be.
    But then last night it hit me. The internal resistance in my multimeter is not high enough to give me an accurate reading with such high resistors in the divider. (I studied electronics 25 years ago. The knowledge is there but buried deep in my mind)

    In one of all these recalculations I came up with the code snippet in this reply. Simply to make shore that what ever reading you get you will never present a value over 100%.

    And as I wrote in the original post. The strange thing is that when the sensor is connected to the serial programmer the sensorValue never exceeds 1023, and therefore the batteryPcnt never exceeds 100. But as soon as i disconnect the serial programmer and put the sensor on battery power, it starts reporting reading of above 100% and all the way up to 300%.
    And it makes it every time. I have tried switching at least ten times with the same result every time.

  • This post is deleted!

  • Mod

    Do you have a capacitor on the A0 pin?

  • what is the reference voltage of the ADC ? You will get negative batteryPcnt values for sensorValue lower than 837, what happens with the batteryPcnt values ? maybe thaey are treated somewhere as unsigned ? You say You use voltage divider but I don't see that your code is handling the voltage divider.

  • Mod

    He is using internal reference

  • @zboblamont Yes. 3V Mini Pro
    @gohan Yes. 0.1uF capacitor.
    @rozpruwacz Internal reference 1.1V. Have a external divider exactly as the example schematics on the home page, to get maximal 1.1 V with two AA batteries.

    As a first step I am trying to understand how the batteryPcnt can ever be higher than 100. With my code that should be impossible. Even if i put 5V on A0.

    vBat should never be able to be higher than v_MAX
    and therefore batteryPcnt should never be able to be higher than 100.

  • @strixx What does your #ifdef MY_DEBUG sequence spit out?

  • @zboblamont Nothing strange. sensorValues between 800 and 1023, and correct corresponding vBat and batteryPcnt according to my formulas. But that's when I have the serial programmer connected.
    I don't know how to get the serial output from debug without having it connected.
    The strange values only appear when on battery.

  • @strixx I think you can disconnect power from the FTDI to the board and leave the battery connected, so long as gnd etc remain connected from FTDI...
    Perhaps somebody can correct me if I'm wrong...

  • Mod

    yes, just unplug the vcc cable

  • I always think about this battery percentage. Where it gives its limits.
    Then I see some project - "NodeManager"
    And there was some functions :

     // [11] the expected vcc when the batter is fully discharged, used to calculate the percentage (default: 2.7)
          void setBatteryMin(float value);
          // [12] the expected vcc when the batter is fully charged, used to calculate the percentage (default: 3.3)```

  • Mod

    as @sundberg84 made in his examples I do an average of the last 4 battery readings

  • @kimot If there were a problem with the sketch it would equally apply when the ftdi is connected with the sensors connected.
    I believe that is why @sundberg84 suggested measuring the voltage at A0... 😉

  • @zboblamont
    The voltage at A0 is at steady 0,8V with batteries.
    Which should give sensor value 744.
    The voltage of the two batteries is 2,88V.

    But as I wrote earlier it must be so that my multimeters internal resistance is not high enough. And is messing up the divider when measuring.
    The calculated voltage at A0 should be 0,89V
    Which should give sensorValue 828

    My sensors right now is reporting 100% then 253% and now 236%.

  • @gohan Cant unplug the Vcc from FTDI. Not without cutting the pin on my Mini.
    And it will not help do average from 4 or even 10 readings. It will still show a messed up percentage. Non of the readings is below 100%

  • Mod

    use jumper wires

  • @gohan Thinking outside the box... 😂 will do..

  • Ok. So connecting the node to serial interface, but without power from it. And power from same batteries.
    The calculations is correct and multimeter is not measuring correct.

    This is debug out put:
    First loop:

    Sensor value: 1023
    Battery Voltage: 3.30 V
    Battery percent: 100 %

    Second and the following loops:

    Sensor value: 877
    Battery Voltage: 2.83 V
    Battery percent: 21 %

    So back to square one. When serial output is connected it works perfect.
    When disconnected it starts reporting all messed up readings!

  • Mod

    Do you have an ldo voltage regulator? Maybe you could try 3 batteries for 4.5v output and use a linear regulator to have 3.3v or use 2 fully charged alkaline and power node without booster, as it could be the booster a bit too noisy.
    My personal favorite solution is using a single LiFePo4 AA battery and vcc library, no buck/booster and voltage divider, everything much simpler

  • @gohan No regulator except the built in on the card. I also tried with two new AA and when not reporting crazy values it is reporting 74% correlating to calculated vBat of 3,14 and the actual voltage 3,24.
    So again; working just fine.

  • Mod

    I meant if you have one regulator to try, I think there may be something related to the booster. Try also a bigger capacitor on the booster output, and a big ceramic one if you have on the vcc of arduino

  • @gohan Well. I think I have found the problem. Investigating now. I will get back during the day when I have tested some more times.

  • @gohan Something doesn't make sense here, if the onboard voltage regulator is removed and battery power is connected to Vcc via a booster it should provide a constant supply.
    If the divider to A0 is fed with the raw battery supply, it should be relatively constant although decaying over time.
    Something is wrong with the circuit I suspect....

  • @zboblamont Yes. It looks like bad wiring. I am testing right now. And it looks like my Arduino clone has some bad markings of the pins.... I am documenting and posting the solution (if it is what I am suspecting right now) later today. Right now I am doing test cycles while doing some paid work.. 🙂

  • Ok. So my strange error was due to connecting GND to the wrong PIN.

    I was connecting GND next to VCC on the side of the Pro Mini.
    Accordning to pics on the MySensors homepage and to the marking of my Arduino that pin is GND.

    But when connecting the FTDI with jumper cables I notice that PIN was marked with CTS.
    So instead I connected GND to the pin one step out.
    This PIN is also marked GND on my Arduino, and on the FTDI. But marked BLK on the pictures on MySensors home page.

    So it seems that both these pins are GND on my board, because everything else was working fine except the reference for analogue pins.

    So topic solved. Thanks for all input. And specally @gohan that made me use the jumper cables and therfore found the error.

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