How BatteryService works in android.

BatteryService monitors the charging status, and charge level of the device battery. When these values change this service broadcasts the new values to all IntentReceivers (android.content.BroadcastReceiver) that are watching the BATTERY_CHANGED (android.content.Intent#ACTION_BATTERY_CHANGED) action.

The new values are stored in the Intent data and can be retrieved by calling Intent.getExtra (android.content.Intent#getExtra) with the following keys:
 "scale" - int, the maximum value for the charge level.
 "level" - int, charge level, from 0 through "scale" inclusive.
 "status" - String, the current charging status.
 "health" - String, the current battery health.
 "present" - boolean, true if the battery is present.
 "icon-small" - int, suggested small icon to use for this state.
 "plugged" - int, 0 if the device is not plugged in; 1 if plugged into an AC power adapter; 2 if plugged in via USB.
 "voltage" - int, current battery voltage in millivolts.
 "temperature" - int, current battery temperature in tenths of a degree Centigrade.
 "technology" - String, the type of battery installed, e.g. "Li-ion".

1. mUEventObserver.startObserving("SUBSYSTEM=power_supply");
    private UEventObserver mUEventObserver = new UEventObserver() {
        @Override
        public void onUEvent(UEventObserver.UEvent event) {
            update();
        }
    };

Observe power supply status. When there is a change, it update the battery status by invoking the update method.
 
2. update
The battery information is required by invoking the native_update method, which is implemented in the file frameworks/base/services/jni/com_android_server_battery_service.cpp.
In fact, all these information is required from the system file /sys/class/power_supply.
(For more infomation, we need to dig inside Linux kernel.)

static void android_server_BatteryService_update(JNIEnv* env, jobject obj)
{
    setBooleanField(env, obj, gPaths.acOnlinePath, gFieldIds.mAcOnline);
    setBooleanField(env, obj, gPaths.usbOnlinePath, gFieldIds.mUsbOnline);
    setBooleanField(env, obj, gPaths.batteryPresentPath, gFieldIds.mBatteryPresent);
   
    setIntField(env, obj, gPaths.batteryCapacityPath, gFieldIds.mBatteryLevel);
    setVoltageField(env, obj, gPaths.batteryVoltagePath, gFieldIds.mBatteryVoltage);
    setIntField(env, obj, gPaths.batteryTemperaturePath, gFieldIds.mBatteryTemperature);
   
    const int SIZE = 128;
    char buf[SIZE];
   
    if (readFromFile(gPaths.batteryStatusPath, buf, SIZE) > 0)
        env->SetIntField(obj, gFieldIds.mBatteryStatus, getBatteryStatus(buf));
    else
        env->SetIntField(obj, gFieldIds.mBatteryStatus,
                         gConstants.statusUnknown);
   
    if (readFromFile(gPaths.batteryHealthPath, buf, SIZE) > 0)
        env->SetIntField(obj, gFieldIds.mBatteryHealth, getBatteryHealth(buf));

    if (readFromFile(gPaths.batteryTechnologyPath, buf, SIZE) > 0)
        env->SetObjectField(obj, gFieldIds.mBatteryTechnology, env->NewStringUTF(buf));
}

static JNINativeMethod sMethods[] = {
     /* name, signature, funcPtr */
    {"native_update", "()V", (void*)android_server_BatteryService_update},
};
--------------------------------------------------
Also, we could see that two method will be invoked. From the name you will know what they are doing.
        shutdownIfNoPower();
        shutdownIfOverTemp();
   
    private final void shutdownIfOverTemp() {
        // shut down gracefully if temperature is too high (> 68.0C)
        // wait until the system has booted before attempting to display the shutdown dialog.
        if (mBatteryTemperature > 680 && ActivityManagerNative.isSystemReady()) {
            Intent intent = new Intent(Intent.ACTION_REQUEST_SHUTDOWN);
            intent.putExtra(Intent.EXTRA_KEY_CONFIRM, false);
            intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
            mContext.startActivity(intent);
        }
    }

Finally, if either status changes, it will notify the observers.