js

addEventListener("dischargingtimechange", (event) => { })

ondischargingtimechange = (event) => { }

html

<div id="level">(battery level unknown)</div>
<div id="chargingTime">(charging time unknown)</div>

js

navigator.getBattery().then((battery) => {
  battery.ondischargingtimechange = () => {
    document.querySelector("#level").textContent = battery.level;
    document.querySelector("#chargingTime").textContent = battery.chargingTime;
  };
});