Calibrate battery ADC and document LiPo curve

- Add BATTERY_CAL config and debug logging for raw ADC samples
- Use LiPo voltage curve (4.2V full, 2.9V empty) for % mapping
- Document battery calibration, curve, and debug output in README

README.md

@@ -251,17 +251,20 @@ inline constexpr uint16_t EXPECTED_SENDER_IDS[NUM_SENDERS] = { 0xF19C };
 
 ## OLED Behavior
 - Sender: OLED stays on for `OLED_AUTO_OFF_MS` after boot or last activity.
-- Activity is detected while `PIN_OLED_CTRL` is held high, or on the high→low edge when the control is released.
+- Activity is detected while `PIN_OLED_CTRL` is held high, or on the high->low edge when the control is released.
 - Receiver: OLED is always on (no auto-off).
 - Pages rotate every 4s.
 
 ## Power & Battery
-- Sender disables WiFi/BLE, reads VBAT via ADC, uses linear SoC map:
-  - 3.0 V = 0%
+- Sender disables WiFi/BLE, reads VBAT via ADC, and converts voltage to % using a LiPo curve:
   - 4.2 V = 100%
+  - 2.9 V = 0%
+  - linear interpolation between curve points
 - Uses deep sleep between cycles (`SENDER_WAKE_INTERVAL_SEC`).
 - Sender CPU is throttled to 80 MHz and LoRa RX is only enabled in short windows (ACK wait or time-sync).
 - Battery sampling averages 5 ADC reads and updates at most once per `BATTERY_SAMPLE_INTERVAL_MS` (default 60s).
+- `BATTERY_CAL` applies a scale factor to match measured VBAT.
+- When `SERIAL_DEBUG_MODE` is enabled, each ADC read logs the 5 raw samples, average, and computed voltage.
 
 ## Web UI
 - AP SSID: `DD3-Bridge-<short_id>` (prefix configurable)
@@ -316,6 +319,7 @@ Key timing settings in `include/config.h`:
  - `METER_SAMPLE_INTERVAL_MS`
  - `METER_SEND_INTERVAL_MS`
  - `BATTERY_SAMPLE_INTERVAL_MS`
+ - `BATTERY_CAL`
  - `BATCH_ACK_TIMEOUT_MS`
 - `BATCH_MAX_RETRIES`
  - `BATCH_QUEUE_DEPTH`
@@ -333,7 +337,7 @@ Key timing settings in `include/config.h`:
 - **Compression**: MeterData uses lightweight RLE (good for JSON but not optimal).
 - **OBIS parsing**: supports IEC 62056-21 ASCII (Mode D); may need tuning for some meters.
 - **Payload size**: single JSON frames < 256 bytes (ArduinoJson static doc); binary batch frames are chunked and reassembled (typically 1 chunk).
-- **Battery ADC**: uses simple linear calibration constant in `power_manager.cpp`.
+- **Battery ADC**: uses a divider (R44/R45 = 100K/100K) with a configurable `BATTERY_CAL` scale and LiPo % curve.
 - **OLED**: no hardware reset line is used (matches working reference).
 - **Batch ACKs**: sender waits for ACK after a batch and retries up to `BATCH_MAX_RETRIES` with `BATCH_ACK_TIMEOUT_MS` between attempts.
 

include/config.h

@@ -70,6 +70,7 @@ constexpr uint32_t SENDER_OLED_READ_MS = 10000;
 constexpr uint32_t METER_SAMPLE_INTERVAL_MS = 1000;
 constexpr uint32_t METER_SEND_INTERVAL_MS = 30000;
 constexpr uint32_t BATTERY_SAMPLE_INTERVAL_MS = 60000;
+constexpr float BATTERY_CAL = 1.083f;
 constexpr uint32_t BATCH_ACK_TIMEOUT_MS = 3000;
 constexpr uint8_t BATCH_MAX_RETRIES = 2;
 constexpr uint8_t METER_BATCH_MAX_SAMPLES = 30;

src/power_manager.cpp

@@ -6,7 +6,6 @@
 #include <esp_sleep.h>
 
 static constexpr float BATTERY_DIVIDER = 2.0f;
-static constexpr float BATTERY_CAL = 1.0f;
 static constexpr float ADC_REF_V = 3.3f;
 
 void power_sender_init() {
@@ -35,18 +34,69 @@ void power_configure_unused_pins_sender() {
 
 void read_battery(MeterData &data) {
   uint32_t sum = 0;
+  uint16_t samples[5] = {};
   for (uint8_t i = 0; i < 5; ++i) {
-    sum += analogRead(PIN_BAT_ADC);
+    samples[i] = analogRead(PIN_BAT_ADC);
+    sum += samples[i];
   }
   float avg = static_cast<float>(sum) / 5.0f;
   float v = (avg / 4095.0f) * ADC_REF_V * BATTERY_DIVIDER * BATTERY_CAL;
+  if (SERIAL_DEBUG_MODE) {
+    Serial.printf("bat_adc: %u %u %u %u %u avg=%.1f v=%.3f\n",
+                  samples[0], samples[1], samples[2], samples[3], samples[4],
+                  static_cast<double>(avg), static_cast<double>(v));
+  }
 
   data.battery_voltage_v = v;
   data.battery_percent = battery_percent_from_voltage(v);
 }
 
 uint8_t battery_percent_from_voltage(float voltage_v) {
-  float pct = (voltage_v - 3.0f) / (4.2f - 3.0f) * 100.0f;
+  if (isnan(voltage_v)) {
+    return 0;
+  }
+  struct LutPoint {
+    float v;
+    uint8_t pct;
+  };
+  static const LutPoint kCurve[] = {
+      {4.20f, 100},
+      {4.15f, 95},
+      {4.11f, 90},
+      {4.08f, 85},
+      {4.02f, 80},
+      {3.98f, 75},
+      {3.95f, 70},
+      {3.91f, 60},
+      {3.87f, 50},
+      {3.85f, 45},
+      {3.84f, 40},
+      {3.82f, 35},
+      {3.80f, 30},
+      {3.77f, 25},
+      {3.75f, 20},
+      {3.73f, 15},
+      {3.70f, 10},
+      {3.65f, 5},
+      {3.60f, 2},
+      {2.90f, 0},
+  };
+  if (voltage_v >= kCurve[0].v) {
+    return kCurve[0].pct;
+  }
+  if (voltage_v <= kCurve[sizeof(kCurve) / sizeof(kCurve[0]) - 1].v) {
+    return 0;
+  }
+  for (size_t i = 0; i + 1 < sizeof(kCurve) / sizeof(kCurve[0]); ++i) {
+    const LutPoint &hi = kCurve[i];
+    const LutPoint &lo = kCurve[i + 1];
+    if (voltage_v <= hi.v && voltage_v >= lo.v) {
+      float span = hi.v - lo.v;
+      if (span <= 0.0f) {
+        return lo.pct;
+      }
+      float t = (voltage_v - lo.v) / span;
+      float pct = lo.pct + t * (hi.pct - lo.pct);
       if (pct < 0.0f) {
         pct = 0.0f;
       }
@@ -55,6 +105,9 @@ uint8_t battery_percent_from_voltage(float voltage_v) {
       }
       return static_cast<uint8_t>(pct + 0.5f);
     }
+  }
+  return 0;
+}
 
 void light_sleep_ms(uint32_t ms) {
   if (ms == 0) {