Question

I’m running an app that samples accelerometer and gyroscope data overnight. This is a very battery intensive operation, and I would like to teach my app to recognize when the battery is getting low.

Here’s my prototype code, which checks the battery level every 10 minutes

NSDate* date = [NSDate date];
      if((int)([date timeIntervalSinceReferenceDate])%600 == 0)
            {
                UIDevice *myDevice = [UIDevice currentDevice];
                [myDevice setBatteryMonitoringEnabled:YES];
                float batLeft = [myDevice batteryLevel]; 
                int batinfo=(batLeft*100);

            [self postStatusMessageWithTitle:nil
                                 description:[NSString stringWithFormat:@"%@ battery level: %i",[dateFormat stringFromDate:dateShow],batinfo]];
                [myDevice setBatteryMonitoringEnabled:NO];
            }

My question is this: do I need to add this line to the end of the code:

[myDevice setBatteryMonitoringEnabled:NO];

It appears that the battery check is performed right there, there are no asynchronous delegate calls. Will setting the value to NO produce any battery savings from not having to monitor the battery level overnight? Will I break anything by setting it to NO?

Thank you for any input!

Answer 1

It’s usually considered best practice to avoid polling and instead ask for notifications from the system, like so:

[[UIDevice currentDevice] setBatteryMonitoringEnabled:YES];
[[NSNotificationCenter defaultCenter]
        addObserver:self
           selector:@selector(batteryLevelUpdate)
               name:UIDeviceBatteryLevelDidChangeNotification
             object:nil];

…where the batteryLevelUpdate would look like this:

- (void)batteryLevelUpdate:(NSNotification *)notification
{
    // do whatever, using: [[UIDevice currentDevice] batteryLevel]
}

From the UIDevice Class Reference:

Notifications for battery level change are sent no more frequently than once per minute. Do not attempt to calculate battery drainage rate or battery time remaining; drainage rate can change frequently depending on built-in applications as well as your application.

Once per minute is 10x more often than your code is currently checking while exerting much less effort CPU-wise. It does not mention, however what granularity of changes will cause notifications — is a 0.01% change going to send, or does it require >1% change?

To answer your other question if you should set the setBatteryMonitoringEnabled back to NO : if you are using the notifications and not manually polling for batteryStatus then the answer is that you must leave it at YES, or risk missing the notifications.

Apple’s official BatteryStatus Sample Code uses this same take on a battery-status reporting.

There is also a UIDeviceBatteryStateDidChangeNotification which will notify you when a device is discharging (in use), charging or has become fully charged.