Better battery reporting

[A Former User]

Hello,
A modification I implemented starting from the "battery sensor" sketch, in order to improve battery % reporting; wrapped up in a small library. Please be kind with my code, I'm merely a hobbyist.
Why:
1- LiPo cells can't get under 3.3v without damage risk, AA cells are leak proof down to 0.7v, I wanted my battery reporting to implement this: LiPo: 100% = 4.15v, 0% = 3.3v // AA: 100% = 1.5v, 0% = 0.8v
2 - Voltage divider precision relies on resistors accuracy, I wanted a way to "calibrate" my divider in order to get proper battery reporting
3 - For fun
Please note that the voltage divider is not the same for LiPo and AA battery (explained in cpp's comments)

Feel free to copy, paste, modify, criticize!

Test sketch

#include <BatteryManager.h>

const int BatteryType = 1; // 1 for 1S LiPo, 2 for 2xAA
const uint8_t BatterySensorPin = A0;
const float VoltageDividerCalibration = 1; // default 1, can be calibrated by measurement

BatteryManager bms;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  bms.begin(BatteryType, BatterySensorPin,VoltageDividerCalibration);
}

void loop() {
  // put your main code here, to run repeatedly:
  Serial.print("Voltage: ");
  Serial.println(bms.reportVoltage());
  Serial.print("Percent: ");
  Serial.println(bms.reportPercent());
  delay(3000);
}

header:

/*
* Battery Manager
*/

#ifndef BatteryManager_h
#define BatteryManager_h

#include <Arduino.h>

class BatteryManager {
	public:
        BatteryManager();
                void begin(int BatteryType, uint8_t SensePin, float Compensation);
		int reportPercent();
		float reportVoltage();
	private:
		int _BatteryType; //1 for 1S LiPO, 2 for 2x AA
		uint8_t _SensePin;
                float _Compensation;
		float Battery_Min_V;
		float Battery_Max_V;
		float Battery_Sense_Pin_Resolution;
};
#endif

cpp

#include <Arduino.h>
#include <BatteryManager.h>

BatteryManager::BatteryManager() { }

void BatteryManager::begin(int BatteryType, uint8_t SensePin, float Compensation)
{
    _SensePin = SensePin;
    _Compensation = Compensation;
    pinMode(_SensePin, INPUT);
    // Battery use the 1.1 V internal reference
    #if defined(__AVR_ATmega2560__)
        analogReference(INTERNAL1V1);
    #else
        analogReference(INTERNAL);
    #endif
    int firstRead = analogRead(_SensePin);
    // check battery type and feed related variables
    if (BatteryType == 1) {
            /* LiPo voltage need to be measured up to 4.2V hence using internal ADC ref of 1.1V,
            *  divider across battery needs to be 1M, 330K, 
            *  which allows a range of 0 to ((1e6+330e3)/330e3)*1.1 = 4.433 Volts
            *  resolution: 4.4333/1023 = Volts per bit = 0.004333659
            *  sense point can be bypassed with 0.1 uF cap to reduce noise at that point
            *  LiPo is full @ 4.2V and risks damage below 3.3v
            *  => LiPo usable range is 3.3 to 4.2 volts so the goal is to report
            *  100% @ 4.2 volts
            *  0% @ 3.3 volts
            */
            Battery_Min_V = 3.3;
            Battery_Max_V = 4.15;
            Battery_Sense_Pin_Resolution = 4.333659; //mv per bit
    }
     if (BatteryType == 2) {
            /* 2 x AA Alkaline voltage need to be measured up to 3V hence using internal ADC ref of 1.1V,
            *  divider across battery needs to be 1M, 470K, 
            *  which allows a range of 0 to ((1e6+470e3)/470e3)*1.1 = 3.44 Volts
            *  resolution: 3.44/1023 = Volts per bit = 0.003363075
            *  sense point can be bypassed with 0.1 uF cap to reduce noise at that point
            *  AA is normally leak proof down to 0.7V, I add an extra 0.1V to avoid any issue
            *  => AA usable range is 1.5 to 0.8 volts so the goal is to report
            *  100% @ 2x1.5 = 3 volts
            *  0% @ 2x0.8 = 1.6 volts
            */
            Battery_Min_V = 1.6;
            Battery_Max_V = 3;
            Battery_Sense_Pin_Resolution = 3.363075; //mv per bit
    }
}

int BatteryManager::reportPercent() {
    // get the battery Voltage
    int sensorValue = analogRead(_SensePin);
    Serial.println(sensorValue);
	float batteryV  = sensorValue * Battery_Sense_Pin_Resolution * _Compensation / 1000;
	// map to 0 -- 100 according to boundaries
	// need to multiply voltages by 10 as map function processes long variables
	return map(10*batteryV,10*Battery_Min_V, 10*Battery_Max_V, 0, 100);;
}

float BatteryManager::reportVoltage() {
    // get the battery Voltage
    int sensorValue = analogRead(_SensePin);
    Serial.println(sensorValue);
	return sensorValue * Battery_Sense_Pin_Resolution * _Compensation / 1000;
}

[Michiel van der Wulp]

@thierryd Nice idea.

Two remarks:

You write in the cpp file:

if (BatteryType == 2) {
/* LiPo voltage ...

You probably mean here "AA voltage"

The AA batteries you talk about are Alkaline (not rechargeable)?
The values are different for NiMH rechargeable (which I always use).

[A Former User]

Thank you for your answer, indeed there's a mistake in the comment, I'll edit that right away!
Yes for AA i mean Alkaline, I implemented the "battery type" feature even if I'm not yet using it, as I currently only power my gadget with LiPo cells. But as it is explained well enough (I hope), anybody can add his battery type in the code! I guess for NiMH boundaries would be 0.9v to 1.3v

[skywatch]

I chose a different approach to this problem (but only with 1 power supply).

That was to replace the resistor pair of 1M0 and 470K with 9M1 and 0.5M preset (trimmer). This reduces the current flow by about 90%+ and by adjusting the preset I can 'calibrate' it to 100%. Changing batteries doesn't seem to require retweeking if the same type are used.

I even did a little sketch to load to the arduino to calibrate the battery reading before loading the main sketch.

So far it is working well for me.