Add BatteryMonitor lib

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

libs/hardware/BatteryMonitor/src/BatteryMonitor.cpp

@@ -0,0 +1,53 @@
+#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);
+}