Add battery voltage reading to web UI, apply fixed VBAT multiplier, closes #3

source/battery.cpp

@@ -20,8 +20,10 @@ void initBatteryADC() {
 float readBatteryVoltage() {
   // 8 quick reads, no delay() calls, no analogReference() change
   int32_t raw=0; for (int i=0; i<8; i++) raw += analogRead(PIN_VBAT_READ); raw /= 8;
-  // Seeed XIAO nRF52840 Sense: 1MΩ + 510kΩ voltage divider on VBAT → multiply by 1510/510
-  return (raw / 4096.0f) * 3.0f * (1510.0f / 510.0f);
+  // Seeed XIAO nRF52840 Sense: 1MΩ + 510kΩ voltage divider on VBAT.
+  // Theoretical ratio is 1510/510 = 2.961, but real resistor tolerances
+  // shift the actual ratio.  Calibrated: 3.90V actual / 3.78V reported → ×1.0317.
+  return (raw / 4096.0f) * 3.0f * (1510.0f / 510.0f) * 1.0317f;
 }
 
 int batteryPercent(float v) {

source/ble_config.cpp

@@ -1,5 +1,6 @@
 #include "ble_config.h"
 #include "tap.h"
+#include "battery.h"
 #include <Adafruit_LittleFS.h>
 #include <InternalFileSystem.h>
 
@@ -200,6 +201,9 @@ void pushTelemetry(unsigned long now) {
     telem.leftClicks  = statLeftClicks;
     telem.rightClicks = statRightClicks;
   #endif
+  #ifdef FEATURE_BATTERY_MONITOR
+    telem.battVoltage = readBatteryVoltage();
+  #endif
   cfgTelemetry.write ((uint8_t*)&telem, sizeof(telem));
   cfgTelemetry.notify((uint8_t*)&telem, sizeof(telem));
 }

source/config.h

@@ -114,8 +114,9 @@ struct __attribute__((packed)) TelemetryPacket {
   uint16_t recalCount;      // [20]
   uint8_t  chargeStatus;    // [22]
   uint8_t  _pad;            // [23]
+  float    battVoltage;     // [24]  raw battery voltage (V)
 };
-static_assert(sizeof(TelemetryPacket) == 24, "TelemetryPacket must be 24 bytes");
+static_assert(sizeof(TelemetryPacket) == 28, "TelemetryPacket must be 28 bytes");
 extern TelemetryPacket telem;
 #endif