Example of a simple battery capacity meter using a multimap function with two arrays, one of a basic lipo voltage curve and corresponding capacity percentage.

ww_frank4.4-battery_capacity.ino

#include <Wire.h>
#include <Adafruit_GFX.h>
#include "woody_eyes.h"
#include <Adafruit_NeoPixel.h>

#define PIN 32
woody_eyes eye;

int reading;
float voltage;

float out[] = {14,21,27,35,43,48,56,63,70,77,84,94,100};
float in[]  = {3.67,3.70,3.73,3.76,3.80,3.83,3.86,3.90,3.93,3.96,4.00,4.10,4.20};
float capacity;

void setup() {
	
	pinMode(A10,OUTPUT);
  eye.clearBuffer();
  eye.setTextColor(WHITE);
  eye.setTextSize(1);

  delay(500); // to give the eyes a chance to open

  eye.select(eyeleft);
  eye.begin();
  eye.display();
  eye.select(eyeright);
  eye.begin();
eye.display();

  analogReadResolution(12);

}

void loop() {
  eye.select(eyeright);  // Voltage to the right eye
  eye.clearBuffer();
  eye.setCursor(0, 0);
  reading = analogRead(A10); // RAW Bat voltage
  voltage = float(reading) / 4095 * 4.2;
  if(voltage <= 4.10){
	//shitty attempt to compensate for load voltage drop, makes things look more logical
	voltage += 0.1;	
  }
  eye.print(voltage);      // should be something between 3.3 and 4.2
  eye.println(" RAW");
  //calculate capacity
  capacity = FmultiMap(voltage, in, out, 13);
  eye.print(capacity);
  eye.println("%");
  eye.display();
  delay(5000);

}

// note: the in array should have increasing values
float FmultiMap(float val, float * _in, float * _out, uint8_t size)
{
  // take care the value is within range
  // val = constrain(val, _in[0], _in[size-1]);
  if (val <= _in[0]) return _out[0];
  if (val >= _in[size-1]) return _out[size-1];

  // search right interval
  uint8_t pos = 1;  // _in[0] allready tested
  while(val > _in[pos]) pos++;

  // this will handle all exact "points" in the _in array
  if (val == _in[pos]) return _out[pos];

  // interpolate in the right segment for the rest
  return (val - _in[pos-1]) * (_out[pos] - _out[pos-1]) / (_in[pos] - _in[pos-1]) + _out[pos-1];
}