fix(ble): reliably expose and update BLE battery level (BAS) (#10622)

* fix(ble): reliably expose and update BLE battery level (BAS)

The Battery Service (0x180F / 0x2A19) is now wired up per the Bluetooth
BAS spec: the Battery Level characteristic always holds a valid 0-100
value and is pushed on change.

- NimBLE: seed an initial level at setup and cache the value on every
  update so a READ returns the current level even while disconnected;
  only notify when a client is connected.
- Power: mirror the battery level to the Battery Service from
  readPowerStatus() on change, so it updates independent of GPS/position
  events (previously the only push path was MeshService).

Also fixes two regressions the above would otherwise introduce:

- NimBLE use-after-free: BLEDevice::deinit(true) frees the GATT objects
  but left the global BatteryCharacteristic dangling. Several AdminModule
  paths (e.g. serial-config entry) deinit BLE while config.bluetooth.enabled
  stays true, so the periodic push would deref freed memory. Null the
  pointer in deinit().
- NRF52: guard blebas.write() on the nrf52Bluetooth instance so the new
  periodic push can't call it before the Battery Service is begun in setup().

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* fix(ble): clamp BAS battery level to 0-100 and skip redundant updates

Address review feedback on the Battery Level characteristic (0x2A19):

- Clamp the value to the BAS-mandated 0-100 range at the platform write
  boundary (NimBLE seed + update, NRF52 update), so a misbehaving battery
  backend can't put an out-of-range value on the characteristic.
- Skip the write/notify when the level is unchanged, so repeated callers
  (e.g. MeshService refresh paths) don't emit redundant notifications.
- Simplify Power.cpp to a direct guarded call now that clamping and
  de-duplication live at the boundary, which also removes the implicit
  int->uint8_t narrowing.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Ben Meadors <benmmeadors@gmail.com>

src/Power.cpp

@@ -14,6 +14,7 @@
  * For more information, see: https://meshtastic.org/
  */
 #include "power.h"
+#include "BluetoothCommon.h"
 #include "MessageStore.h"
 #include "NodeDB.h"
 #include "PowerFSM.h"
@@ -962,6 +963,10 @@ void Power::readPowerStatus()
         lastLogTime = millis();
     }
     newStatus.notifyObservers(&powerStatus2);
+
+    // Mirror battery level to the BLE Battery Service (0x2A19); the platform layer clamps and dedupes.
+    if (hasBattery == OptTrue)
+        updateBatteryLevel(powerStatus2.getBatteryChargePercent());
 #ifdef DEBUG_HEAP
     if (lastheap != memGet.getFreeHeap()) {
         // Use stack-allocated buffer to avoid heap allocations in monitoring code

src/nimble/NimbleBluetooth.cpp

@@ -40,6 +40,7 @@ constexpr uint16_t kPreferredBleTxTimeUs = (kPreferredBleTxOctets + 14) * 8;
 
 BLECharacteristic *fromNumCharacteristic;
 BLECharacteristic *BatteryCharacteristic;
+static int lastBatteryLevel = -1; // last value written to 0x2A19, to skip redundant writes/notifies
 BLECharacteristic *logRadioCharacteristic;
 BLEServer *bleServer;
 
@@ -718,6 +719,8 @@ void NimbleBluetooth::deinit()
 #endif
 
     BLEDevice::deinit(true);
+    BatteryCharacteristic = nullptr; // freed by deinit; clear so updateBatteryLevel() won't touch it
+    lastBatteryLevel = -1;
 #endif
 }
 
@@ -856,16 +859,31 @@ void NimbleBluetooth::setupService()
     BatteryCharacteristic = batteryService->createCharacteristic( // 0x2A19 is the Battery Level characteristic)
         (uint16_t)0x2a19, BLECharacteristic::PROPERTY_READ | BLECharacteristic::PROPERTY_NOTIFY);
     BatteryCharacteristic->addDescriptor(batteryLevelDescriptor);
+    // Seed an initial 0-100 level so an early read of 0x2A19 returns a valid value.
+    uint8_t initialLevel = (powerStatus && powerStatus->getHasBattery()) ? powerStatus->getBatteryChargePercent() : 0;
+    if (initialLevel > 100)
+        initialLevel = 100;
+    BatteryCharacteristic->setValue(&initialLevel, 1);
+    lastBatteryLevel = initialLevel;
     batteryService->start();
 }
 
 /// Given a level between 0-100, update the BLE attribute
 void updateBatteryLevel(uint8_t level)
 {
-    if ((config.bluetooth.enabled == true) && nimbleBluetooth && nimbleBluetooth->isConnected()) {
-        BatteryCharacteristic->setValue(&level, 1);
+    if (!config.bluetooth.enabled || !BatteryCharacteristic)
+        return;
+
+    if (level > 100) // 0x2A19 must stay within the BAS 0-100 range
+        level = 100;
+    if (level == lastBatteryLevel)
+        return;
+    lastBatteryLevel = level;
+
+    // Cache the value so a READ works without a subscriber; notify only when connected.
+    BatteryCharacteristic->setValue(&level, 1);
+    if (nimbleBluetooth && nimbleBluetooth->isConnected())
         BatteryCharacteristic->notify();
-    }
 }
 
 void NimbleBluetooth::clearBonds()

src/platform/nrf52/NRF52Bluetooth.cpp

@@ -15,8 +15,9 @@ static BLECharacteristic fromRadio = BLECharacteristic(BLEUuid(FROMRADIO_UUID_16
 static BLECharacteristic toRadio = BLECharacteristic(BLEUuid(TORADIO_UUID_16));
 static BLECharacteristic logRadio = BLECharacteristic(BLEUuid(LOGRADIO_UUID_16));
 
-static BLEDis bledis; // DIS (Device Information Service) helper class instance
-static BLEBas blebas; // BAS (Battery Service) helper class instance
+static BLEDis bledis;             // DIS (Device Information Service) helper class instance
+static BLEBas blebas;             // BAS (Battery Service) helper class instance
+static int lastBatteryLevel = -1; // last value written to BAS, to skip redundant writes/notifies
 #ifndef BLE_DFU_SECURE
 static BLEDfu bledfu; // DFU software update helper service
 #else
@@ -336,6 +337,7 @@ void NRF52Bluetooth::setup()
     LOG_INFO("Init the Battery Service");
     blebas.begin();
     blebas.write(0); // Unknown battery level for now
+    lastBatteryLevel = 0;
     // Setup the Heart Rate Monitor service using
     // BLEService and BLECharacteristic classes
     LOG_INFO("Init the Mesh bluetooth service");
@@ -355,6 +357,14 @@ void NRF52Bluetooth::resumeAdvertising()
 /// Given a level between 0-100, update the BLE attribute
 void updateBatteryLevel(uint8_t level)
 {
+    if (!nrf52Bluetooth) // skip until the Battery Service has been begun in setup()
+        return;
+
+    if (level > 100) // BAS battery level must stay within 0-100
+        level = 100;
+    if (level == lastBatteryLevel)
+        return;
+    lastBatteryLevel = level;
     blebas.write(level);
 }
 void NRF52Bluetooth::clearBonds()