Battery monitoring with a NSTimer

Question

I've got problems monitoring battery. In the foreground I can only get the level once and it seems that I don't get the change in the background either. 

If I put a new UIViewController in front of the one monitoring and then get back again the battery level is updated.

-(void)startMonitoringBattery
{
     [UIDevice currentDevice].batteryMonitoringEnabled = YES;

     [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(batteryStateDidChange:)
                                                     name:UIDeviceBatteryStateDidChangeNotification
                                                   object:nil];
     [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(batteryLevelDidChange:)
                                                     name:UIDeviceBatteryLevelDidChangeNotification
                                                   object:nil];

     batteryCheckTimer = [NSTimer scheduledTimerWithTimeInterval:(10.0)
                                                             target:self
                                                           selector:@selector(batteryCheck)
                                                           userInfo:nil
                                                            repeats:YES];
}

- (void)batteryStateDidChange:(NSNotification *)notification
{
     [self batteryCheck];
}

- (void)batteryLevelDidChange:(NSNotification *)notification
{
     [self batteryCheck];
}

When this method is called I want to get the current battery level and not the one that it was when I started monitor.

- (void)batteryCheck
{
     float STOP_AT_BATTERY_LEVEL = 50.0;
     float currentBatteryLevel = [UIDevice currentDevice].batteryLevel;
     
     if([UIDevice currentDevice].batteryLevel <= STOP_AT_BATTERY_LEVEL)
     {
          // STOP BATTERY CONSUMING TASK
     }
}

Questions:

1. Why doesn't batteryStateDidChange and batteryLevelDidChange get called more than once?
2. Why can't I get the current battery level, with a timer, from [UIDevice currentDevice].batteryLevel even if I've set batteryMonitoringEnabled to true?
3. Is it possible to monitor battery while the app is in background or the phone is locked? The app is running in background because I record GPS data all the time.