Add BatteryMonitor lib

Authored by @CrazyCoder in https://github.com/CidVonHighwind/xteink-x4-sample/pull/4
Pulled into community-sdk

libs/hardware/BatteryMonitor/include/BatteryMonitor.h

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+#pragma once
+#include <cstdint>
+
+class BatteryMonitor {
+public:
+    // Optional divider multiplier parameter defaults to 2.0
+    explicit BatteryMonitor(uint8_t adcPin, float dividerMultiplier = 2.0f);
+
+    // Read voltage and return percentage (0-100)
+    uint16_t readPercentage() const;
+
+    // Read the battery voltage in millivolts (accounts for divider)
+    uint16_t readMillivolts() const;
+
+    // Read raw millivolts from ADC (doesn't account for divider)
+    uint16_t readRawMillivolts() const;
+
+    // Read the battery voltage in volts (accounts for divider)
+    double readVolts() const;
+
+    // Percentage (0-100) from a millivolt value
+    static uint16_t percentageFromMillivolts(uint16_t millivolts);
+
+    // Calibrate a raw ADC reading and return millivolts
+    static uint16_t millivoltsFromRawAdc(uint16_t adc_raw);
+
+private:
+    uint8_t _adcPin;
+    float _dividerMultiplier;
+};

libs/hardware/BatteryMonitor/library.json

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+{
+  "name": "BatteryMonitor",
+  "version": "1.0.0",
+  "description": "Get battery voltage and percentage",
+  "authors": [
+    {
+      "name": "Serge Baranov",
+      "email": "sbaranov@gmail.com",
+      "url": "https://github.com/CrazyCoder"
+    }
+  ],
+  "dependencies": {},
+  "platforms": "espressif32",
+  "frameworks": ["arduino", "espidf"]
+}

libs/hardware/BatteryMonitor/src/BatteryMonitor.cpp

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+#include "BatteryMonitor.h"
+#include "esp_adc_cal.h"
+
+BatteryMonitor::BatteryMonitor(uint8_t adcPin, float dividerMultiplier)
+  : _adcPin(adcPin), _dividerMultiplier(dividerMultiplier)
+{
+}
+
+uint16_t BatteryMonitor::readPercentage() const
+{
+    return percentageFromMillivolts(readMillivolts());
+}
+
+uint16_t BatteryMonitor::readMillivolts() const
+{
+    const uint16_t raw = readRawMillivolts();
+    const uint32_t mv = millivoltsFromRawAdc(raw);
+    return static_cast<uint32_t>(mv * _dividerMultiplier);
+}
+
+uint16_t BatteryMonitor::readRawMillivolts() const
+{
+    const uint16_t raw = analogRead(_adcPin);
+    return raw;
+}
+
+double BatteryMonitor::readVolts() const
+{
+    return static_cast<double>(readMillivolts()) / 1000.0;
+}
+
+uint16_t BatteryMonitor::percentageFromMillivolts(uint16_t millivolts)
+{
+    double volts = millivolts / 1000.0;
+    // Polynomial derived from LiPo samples
+    double y = -144.9390 * volts * volts * volts +
+               1655.8629 * volts * volts -
+               6158.8520 * volts +
+               7501.3202;
+
+    // Clamp to [0,100] and round
+    y = max(y, 0.0);
+    y = min(y, 100.0);
+    y = round(y);
+    return static_cast<int>(y);
+}
+
+uint16_t BatteryMonitor::millivoltsFromRawAdc(uint16_t adc_raw)
+{
+    esp_adc_cal_characteristics_t adc_chars;
+    esp_adc_cal_characterize(ADC_UNIT_1, ADC_ATTEN_DB_12, ADC_WIDTH_BIT_12, 1100, &adc_chars);
+    return esp_adc_cal_raw_to_voltage(adc_raw, &adc_chars);
+}