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.