In this post were going to explore the (small) BatteryManager API.

BatteryManager is returned by navigator.battery. You can interface with the object in the following ways

• level
• chargingTime
• dischargingTime

The battery manager also dispatches the following events

• levelchange
• chargingchange
• chargingtimechange
• dischargingtimechange

You can add an event listener using addEventListener interface.

function updateStatus() {
    var battery = navigator.battery;
    console.log('Battery: ' + battery.level * 100 + '%');
    console.log('Charge Time: ' + battery.chargingTime);
    console.log('DisCharge Time: ' + battery.dischargingTime);
}

function addBatteryListeners() {
    var battery = navigator.battery;
    battery.addEventListener('levelchange', updateStatus, false);
    battery.addEventListener('chargingchange', updateStatus, false);
    battery.addEventListener('chargingtimechange', updateStatus, false);
    battery.addEventListener('dischargingtimechange', updateStatus, false);
}

addBatteryListeners();
updateStatus();

Now we can test this…but it’s a lot of trouble to create a new application to test this little bit of javascript. Thus, this leads us to an introduction to some of the developer tools.

Web Developer Tools

The Web Developer Tools for in the Firefox Web browser can be used for any website and for any Firefox OS app.

• Go to the App Manager
• Select the Apps Tab on the left hand side
• Select the Hello, Firefox OS app from the first post.
• Click on Debug

Here there is a couple of different tools at out disposal.

• Console - the javascript console
• Inspector - inspect HTML elements from the page. Shows the CSS rules applied to the element.
• Debugger- used to set breakpoints in javascript code and step through it
• Style Editor - CSS editor
• Profiler - profile the render and running time of the webpage.
• Scratchpad - write javascript code that gets run in the context of the webpage

The one we are interested in is the Scratchpad. Just paste the javascript code above, click run, and look at the console! Much simpler than creating another app.