Battery percentage gone wild [SOLVED]

[Strixx]

@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.

[zboblamont]

This post is deleted!

[gohan]

Do you have a capacitor on the A0 pin?

[rozpruwacz]

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.

[gohan]

He is using internal reference

[Strixx]

@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.

[zboblamont]

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

[Strixx]

@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.

[zboblamont]

@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...

[gohan]

yes, just unplug the vcc cable

[kimot]

I always think about this battery percentage. Where it gives its limits.
Then I see some project - "NodeManager"
GitHub
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)```

[gohan]

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

[zboblamont]

@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... ;)

[Strixx]

@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%.

[Strixx]

@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%

[gohan]

use jumper wires

[Strixx]

@gohan Thinking outside the box... :joy: will do..

[Strixx]

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!

[gohan]

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

[Strixx]

@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.