对于支持它的浏览器,这样的东西应该工作：

navigator.getBattery().then(function(battery) {
  battery.addEventListener('levelchange',function() {    
    // Do stuff when the level changes,you can get it
    // from battery.level
    document.write((battery.level*100)+"%");
  })
  document.write((battery.level*100)+"%");
});