Battery percentage in the range [0..1] if battery monitoring is enabled, otherwise -1.
By default, UIDevice.BatteryMonitoringEnabled is false. In that situation, UIDevice.BatteryLevel will be -1 (and UIDevice.BatteryState will be UIDeviceBatteryState.Unknown).
A value of 1.0 indicates a fully-charged device, while 0.0 indicates an empty battery.
This can be used from a background thread.