Very simple Singleton that sends a Notification whenever the state or level of the battery changed.

BatteryMonitoring.swift

public class BatteryMonitoring {
    
    public static let BatteryDidChangeNotification = Notification.Name(rawValue: "BatteryStateNotifier.BatteryDidChangeNotification")
    
    public static let shared = BatteryMonitoring()
    private init() {}
    
    public private(set) var state : WKInterfaceDeviceBatteryState = .unknown
    // State will be an int between 0 and 100 if state is not .unknown, else -1
    public private(set) var level : Int = -1
    
    private var timer : Timer?
    public var isStarted : Bool { get { return timer != nil && timer!.isValid } }
    
    public func start(withPeriod period : TimeInterval) {
        timer?.invalidate()
        timer = Timer.scheduledTimer(timeInterval: period, target: self, selector: #selector(self.verifyAndNotifyOfChange), userInfo: nil, repeats: true)
        timer?.fire()
    }
    
    public func stop() {
        timer?.invalidate()
        timer = nil
    }
    
    @objc private func verifyAndNotifyOfChange() {
        let (newState, newLevel) = getBaterryInfo()
        
        if newState != state || newLevel != level {
            state = newState
            level = newLevel
            NotificationCenter.default.post( Notification(
                name: BatteryMonitoring.BatteryDidChangeNotification,
                object: (state, level),
                userInfo: nil))
        }
    }
    
    // Get the battery state without changing the isBatteryMonitoringEnabled value
    // This is of course not atomic!
    public func getBaterryInfo(forDevice device : WKInterfaceDevice = WKInterfaceDevice.current()) -> (WKInterfaceDeviceBatteryState, Int) {
        let originalMonitoringValue = device.isBatteryMonitoringEnabled
        defer {
            device.isBatteryMonitoringEnabled = originalMonitoringValue
        }
        device.isBatteryMonitoringEnabled = true
        return (device.batteryState, Int(100.0 * device.batteryLevel))
    }
}