var battery = navigator.battery || navigator.webkitBattery || navigator.mozBattery;

battery.addEventListener('chargingchange', getBatteryState);
battery.addEventListener('chargingtimechange', getBatteryState);
battery.addEventListener('dischargingtimechange', getBatteryState);
battery.addEventListener('levelchange', getBatteryState);

function getBatteryState() {
    var battery_level = Math.floor(battery.level * 100);
    
    // TODO
}