Get Battery State

Turn on battery monitoring for your app

	[[UIDevice currentDevice] setBatteryMonitoringEnabled:YES];

Getting The Current State

	
	//----- DISPLAY BATTERY STATE -----
	if ([[UIDevice currentDevice] batteryState] == UIDeviceBatteryStateUnknown)
	{
		[BatteryStateLabel setText:@""];
	}
	else
	{  	
		NSString *BatteryString = [NSString stringWithFormat:@"Battery: %0.0f%%", [[UIDevice currentDevice] batteryLevel] * 100];

		switch ([[UIDevice currentDevice] batteryState])
		{
			case 1:
				//Battery is in use (discharging)
				break;

			case 2:
				//Battery is charging
				BatteryString = [BatteryString stringByAppendingString:@" (charging)"];
				break;

			case 3:
				//Battery is fully charged
				BatteryString = [BatteryString stringByAppendingString:@" (charged)"];
				break;

			default:
				//Battery status is unknown
				break;
		}
		[BatteryStateLabel setText:BatteryString]; 
	}

Good Resources

http://iphonedevelopertips.com/device/display-battery-state-and-level-of-charge.html