html

<button id="get-level">Get battery level</button>
<div id="output"></div>

js

const getLevel = document.querySelector("#get-level");
const output = document.querySelector("#output");

getLevel.addEventListener("click", async () => {
  if (!navigator.getBattery) {
    output.textContent = "Battery manager is unsupported";
  } else {
    const manager = await navigator.getBattery();
    const level = manager.level;
    output.textContent = `Battery level: ${level}`;
  }
});