HowTo: Check the Battery Level

Although there is a property In UIDevice called BatteryLevel that you can query, the value of this property is only valid if you first enable battery monitoring.

Apple's documentation recommends that you do not turn on the battery monitoring unless you need to.   

To do this, use this piece of code:

float Report ()
{
    UIDevice.CurrentDevice.BatteryMonitoringEnabled = true;
    return UIDevice.CurrentDevice.BatteryLevel;
}