Calculating battery life in iOS

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The API allows you to register to receive notifications for changes to the battery level. It only reports a change at 5% increments up or down, but you can use a timer and measure the time between two changes (or initial battery level and first change). Here’s how you register for the notifications:

// Use this call to get the current battery level as a float
// [[UIDevice currentDevice] batteryLevel]

[[UIDevice currentDevice] setBatteryMonitoringEnabled:YES];

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

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

The first notification tells you the current state, e.g. unplugged, charging, or full. The second will get triggered whenever a 5% increment is reached.

Seems to me that if all you’re given is change notifications at 5% changes up or down, accuracy is not something you can calculate very well or quickly. A 5% change could take a very long time if the device isn’t doing anything.

Maybe you can monitor [[UIDevice currentDevice] batteryLevel] with a timer, however, while I haven’t tried it I think it only gets updated at this same 5% increment.

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