The getBattery() method provides information about the system's battery.

It returns a battery promise, which is resolved in a BatteryManager object providing also some new events you can handle to monitor the battery status.

Source : Documentation
Info
Warning alert!, Not all devices or browsers are fully supported.

The following is an example of the program code 

<!-- navigator.getBattery -->

<div id="get-battery"></div>

<script>// navigator.getBattery

 var batteryIsCharging = false,
   getLocation = () => {
     if (navigator.getBattery) {
       navigator.getBattery().then(function (battery) {
         batteryIsCharging = battery.charging;
         refreshLevel(battery.level);


         battery.addEventListener("chargingchange", function () {
           batteryIsCharging = battery.charging;
           refreshLevel(battery.level);
         });
         battery.addEventListener("levelchange", function () {
           refreshLevel(battery.level);
         });
       });
       // The getBattery() method provides information about the system battery.
     } else {
       // Section showing error or incompatible browser.
     }
   },
   refreshLevel = (level) => {
     document.getElementById("get-battery").innerHTML = level * 100 + " %";
   };


 getLocation();

 // [open-source] : location:github.com/tamddk/library/tree/main/getBattery

</script>