HeltecV4: Use ADC_2_5db attenuation for battery reading

The resistor divider (390kΩ/100kΩ) keeps the ADC pin voltage in the
0.61-0.86V range across the full LiPoly range (3.0–4.2V). ADC_2_5db
(~1250mV full-scale) uses ~70% of the ADC range at full charge, giving
better resolution than the default ADC_11db (~20%) while maintaining
headroom over ADC_0db (~950mV) to handle chip-to-chip ADC variation

variants/heltec_v4/HeltecV4Board.cpp

@@ -63,6 +63,7 @@ void HeltecV4Board::begin() {
 
   uint16_t HeltecV4Board::getBattMilliVolts()  {
     analogReadResolution(12);
+    analogSetPinAttenuation(PIN_VBAT_READ, ADC_2_5db);  // divider puts pin at 0.61-0.86V so this attenuation should cover it with some room to spare
     digitalWrite(PIN_ADC_CTRL, HIGH);
     delay(10);
     uint32_t raw_mv = 0;