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</style><link charset="utf-8" type="text/css" rel="stylesheet" href="http://www.w3.org/StyleSheets/TR/W3C-WD"></head>
  <body style="display: inherit;"><div class="head"><p><a href="http://www.w3.org/"><img src="http://www.w3.org/Icons/w3c_home" alt="W3C" height="48" width="72"></a></p><h1 class="title" id="title">Battery Status API</h1><h2 id="w3c-working-draft-29-november-2011"><acronym title="World Wide Web Consortium">W3C</acronym> Working Draft 29 November 2011</h2><dl><dt>This version:</dt><dd><a href="http://www.w3.org/TR/2011/WD-battery-status-20111129/">http://www.w3.org/TR/2011/WD-battery-status-20111129/</a></dd><dt>Latest published version:</dt><dd><a href="http://www.w3.org/TR/battery-status/">http://www.w3.org/TR/battery-status/</a></dd><dt>Latest editor's draft:</dt><dd><a href="http://dev.w3.org/2009/dap/system-info/battery-status.html">http://dev.w3.org/2009/dap/system-info/battery-status.html</a></dd><dt>Previous version:</dt><dd><a href="http://www.w3.org/TR/2011/WD-battery-status-20110915/">http://www.w3.org/TR/2011/WD-battery-status-20110915/</a></dd><dt>Editor:</dt><dd><span>Anssi Kostiainen</span>, Nokia</dd>
</dl><p class="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a> © 2011 <a href="http://www.w3.org/"><acronym title="World Wide Web Consortium">W3C</acronym></a><sup>®</sup> (<a href="http://www.csail.mit.edu/"><acronym title="Massachusetts Institute of Technology">MIT</acronym></a>, <a href="http://www.ercim.eu/"><acronym title="European Research Consortium for Informatics and Mathematics">ERCIM</acronym></a>, <a href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. <acronym title="World Wide Web Consortium">W3C</acronym> <a href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>, <a href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a> and <a href="http://www.w3.org/Consortium/Legal/copyright-documents">document use</a> rules apply.</p><hr></div>
    <div class="introductory section" id="abstract"><h2>Abstract</h2>
      This specification defines an API that provides information about the
      battery status of the hosting device.
    </div><div id="sotd" class="introductory section"><h2>Status of This Document</h2><p><em>This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current <acronym title="World Wide Web Consortium">W3C</acronym> publications and the latest revision of this technical report can be found in the <a href="http://www.w3.org/TR/"><acronym title="World Wide Web Consortium">W3C</acronym> technical reports index</a> at http://www.w3.org/TR/.</em></p>
      <p>
        The functionality described in this specification was initially specified as part of the
        <a href="http://www.w3.org/TR/system-info-api/">System Information API</a> but has been
        extracted in order to be more readily available, more straightforward to implement, and
        in order to produce a specification that could be implemented on its own merits without
        interference with other, often unrelated, features.
      </p>
    <p>This document was published by the <a href="http://www.w3.org/2009/dap/">Device APIs Working Group</a> as a Last Call Working Draft. This document is intended to become a <acronym title="World Wide Web Consortium">W3C</acronym> Recommendation. If you wish to make comments regarding this document, please send them to <a href="mailto:public-device-apis@w3.org">public-device-apis@w3.org</a> (<a href="mailto:public-device-apis-request@w3.org?subject=subscribe">subscribe</a>, <a href="http://lists.w3.org/Archives/Public/public-device-apis/">archives</a>). The Last Call period ends 20 December 2011. All feedback is welcome.</p><p>Publication as a Working Draft does not imply endorsement by the <acronym title="World Wide Web Consortium">W3C</acronym> Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.</p><p>This is a Last Call Working Draft and thus the Working Group has determined that this document has satisfied the relevant technical requirements and is sufficiently stable to advance through the Technical Recommendation process.</p><p>This document was produced by a group operating under the <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/">5 February 2004 <acronym title="World Wide Web Consortium">W3C</acronym> Patent Policy</a>. <acronym title="World Wide Web Consortium">W3C</acronym> maintains a <a href="http://www.w3.org/2004/01/pp-impl/43696/status" rel="disclosure">public list of any patent disclosures</a> made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential">Essential Claim(s)</a> must disclose the information in accordance with <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure">section 6 of the <acronym title="World Wide Web Consortium">W3C</acronym> Patent Policy</a>.</p></div><div class="section" id="toc"><h2 class="introductory">Table of Contents</h2><ul class="toc"><li class="tocline"><a class="tocxref" href="#introduction"><span class="secno">1. </span>Introduction</a></li><li class="tocline"><a class="tocxref" href="#conformance"><span class="secno">2. </span>Conformance</a></li><li class="tocline"><a class="tocxref" href="#navigatorbattery-interface"><span class="secno">3. </span><span class="idlType formerLink idlType"><code>NavigatorBattery</code></span> Interface</a><ul class="toc"><li class="tocline"><a class="tocxref" href="#attributes"><span class="secno">3.1 </span>Attributes</a></li></ul></li><li class="tocline"><a class="tocxref" href="#batterymanager-interface"><span class="secno">4. </span><span class="idlType formerLink idlType"><code>BatteryManager</code></span> Interface</a><ul class="toc"><li class="tocline"><a class="tocxref" href="#attributes-1"><span class="secno">4.1 </span>Attributes</a></li></ul></li><li class="tocline"><a class="tocxref" href="#examples"><span class="secno">5. </span>Examples</a></li><li class="tocline"><a class="tocxref" href="#acknowledgements"><span class="secno">A. </span>Acknowledgements</a></li><li class="tocline"><a class="tocxref" href="#references"><span class="secno">B. </span>References</a><ul class="toc"><li class="tocline"><a class="tocxref" href="#normative-references"><span class="secno">B.1 </span>Normative references</a></li><li class="tocline"><a class="tocxref" href="#informative-references"><span class="secno">B.2 </span>Informative references</a></li></ul></li></ul></div>


    <div id="introduction" class="informative section">
      <!--OddPage--><h2><span class="secno">1. </span>Introduction</h2><p><em>This section is non-normative.</em></p>
      <p>
        The Battery Status API specification defines a means for web
        developers to programmatically determine the battery status of the
        hosting device. Without knowing the battery status of a device, a web
        developer must design the web application with an assumption of
        sufficient battery level for the task at hand. This means the battery
        of a device may exhaust faster than desired because web developers are
        unable to make decisions based on the battery status. Given knowledge
        of the battery status, web developers are able to craft web content and
        applications which are power-efficient, thereby leading to improved user
        experience.
     </p>
     <p>
        The Battery Status API can be used to defer or scale back work when
        the device is not charging in or is low on battery. An archetype of an
        advanced web application, a web-based email client, may check the
        server for new email every few seconds if the device is charging,
        but do so less frequently if the device is not charging or is low on
        battery. Another example is a web-based word processor which could
        monitor the battery level and save changes before the battery runs
        out to prevent data loss.
      </p>
      <p>
        The following example shows how a web-based email client could check
        for new emails every ten seconds without knowledge of the battery status:
      </p>
      <pre class="example sh_javascript sh_sourceCode"><span class="sh_symbol">&lt;!</span>DOCTYPE html<span class="sh_symbol">&gt;</span>
<span class="sh_symbol">&lt;</span>html<span class="sh_symbol">&gt;</span>
<span class="sh_symbol">&lt;</span>head<span class="sh_symbol">&gt;</span>
  <span class="sh_symbol">&lt;</span>title<span class="sh_symbol">&gt;</span>Email Client<span class="sh_symbol">&lt;/</span>title<span class="sh_symbol">&gt;</span>
  <span class="sh_symbol">&lt;</span>script<span class="sh_symbol">&gt;</span>
    <span class="sh_keyword">var</span> mail <span class="sh_symbol">=</span> <span class="sh_cbracket">{</span>
      INTERVAL_DEFAULT<span class="sh_symbol">:</span> <span class="sh_number">1000</span> <span class="sh_symbol">*</span> <span class="sh_number">10</span><span class="sh_symbol">,</span>
      interval<span class="sh_symbol">:</span> <span class="sh_keyword">null</span><span class="sh_symbol">,</span>
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        <span class="sh_keyword">if</span> <span class="sh_symbol">(</span>interval <span class="sh_symbol">===</span> mail<span class="sh_symbol">.</span>interval<span class="sh_symbol">)</span> <span class="sh_cbracket">{</span> <span class="sh_keyword">return</span><span class="sh_symbol">;</span> <span class="sh_cbracket">}</span>
        <span class="sh_keyword">if</span> <span class="sh_symbol">(</span>mail<span class="sh_symbol">.</span>timer <span class="sh_symbol">!==</span> <span class="sh_number">0</span><span class="sh_symbol">)</span> <span class="sh_cbracket">{</span> <span class="sh_function">clearTimeout</span><span class="sh_symbol">(</span>mail<span class="sh_symbol">.</span>timer<span class="sh_symbol">);</span> <span class="sh_cbracket">}</span>
        <span class="sh_keyword">if</span> <span class="sh_symbol">(</span>interval<span class="sh_symbol">)</span> <span class="sh_cbracket">{</span> mail<span class="sh_symbol">.</span>timer <span class="sh_symbol">=</span> <span class="sh_function">setInterval</span><span class="sh_symbol">(</span><span class="sh_keyword">function</span> <span class="sh_symbol">()</span> <span class="sh_cbracket">{</span> mail<span class="sh_symbol">.</span><span class="sh_function">check</span><span class="sh_symbol">();</span> <span class="sh_cbracket">}</span><span class="sh_symbol">,</span> interval<span class="sh_symbol">);</span> <span class="sh_cbracket">}</span>
        mail<span class="sh_symbol">.</span>interval <span class="sh_symbol">=</span> interval<span class="sh_symbol">;</span>
      <span class="sh_cbracket">}</span>
    <span class="sh_cbracket">}</span><span class="sh_symbol">;</span>

    window<span class="sh_symbol">.</span><span class="sh_function">addEventListener</span><span class="sh_symbol">(</span><span class="sh_string">'load'</span><span class="sh_symbol">,</span> <span class="sh_keyword">function</span> <span class="sh_symbol">()</span> <span class="sh_cbracket">{</span>
      mail<span class="sh_symbol">.</span><span class="sh_function">setTimer</span><span class="sh_symbol">(!</span>mail<span class="sh_symbol">.</span>interval <span class="sh_symbol">?</span> mail<span class="sh_symbol">.</span>INTERVAL_DEFAULT <span class="sh_symbol">:</span> mail<span class="sh_symbol">.</span>interval<span class="sh_symbol">);</span>
    <span class="sh_cbracket">}</span><span class="sh_symbol">,</span> <span class="sh_keyword">false</span><span class="sh_symbol">);</span>
  <span class="sh_symbol">&lt;/</span>script<span class="sh_symbol">&gt;</span>
<span class="sh_symbol">&lt;/</span>head<span class="sh_symbol">&gt;</span>
<span class="sh_symbol">&lt;</span>body<span class="sh_symbol">&gt;&lt;/</span>body<span class="sh_symbol">&gt;</span>
<span class="sh_symbol">&lt;/</span>html<span class="sh_symbol">&gt;</span></pre>
      <p>
        The script will always check for emails every ten seconds, even if the
        battery level is critically low and the device is not charging.
        This is an example of poor resource management.
      </p>
      <p>
        Using the <a class="idlType" href="#idl-def-BatteryManager"><code>BatteryManager</code></a> interface, the web application is, for
        example, able to throttle checking for emails if the device is low on
        battery, stop checking for emails if the battery is critically low and
        resume normal operation when the battery is charging:
      </p>
      <pre class="example sh_javascript sh_sourceCode"><span class="sh_symbol">&lt;!</span>DOCTYPE html<span class="sh_symbol">&gt;</span>
<span class="sh_symbol">&lt;</span>html<span class="sh_symbol">&gt;</span>
<span class="sh_symbol">&lt;</span>head<span class="sh_symbol">&gt;</span>
  <span class="sh_symbol">&lt;</span>title<span class="sh_symbol">&gt;</span>Battery<span class="sh_symbol">-</span>aware Email Client<span class="sh_symbol">&lt;/</span>title<span class="sh_symbol">&gt;</span>
  <span class="sh_symbol">&lt;</span>script<span class="sh_symbol">&gt;</span>
    <span class="sh_keyword">var</span> mail <span class="sh_symbol">=</span> <span class="sh_cbracket">{</span>
      INTERVAL_BATTERY_LOW<span class="sh_symbol">:</span> <span class="sh_number">1000</span> <span class="sh_symbol">*</span> <span class="sh_number">60</span> <span class="sh_symbol">*</span> <span class="sh_number">10</span><span class="sh_symbol">,</span>
      INTERVAL_DEFAULT<span class="sh_symbol">:</span> <span class="sh_number">1000</span> <span class="sh_symbol">*</span> <span class="sh_number">10</span><span class="sh_symbol">,</span>
      interval<span class="sh_symbol">:</span> <span class="sh_keyword">null</span><span class="sh_symbol">,</span>
      timer<span class="sh_symbol">:</span> <span class="sh_number">0</span><span class="sh_symbol">,</span>

      check<span class="sh_symbol">:</span> <span class="sh_keyword">function</span> <span class="sh_symbol">()</span> <span class="sh_cbracket">{</span>
        console<span class="sh_symbol">.</span><span class="sh_function">log</span><span class="sh_symbol">(</span><span class="sh_string">'Checking the server for new emails using an interval of '</span> <span class="sh_symbol">+</span>
                    <span class="sh_symbol">(</span>mail<span class="sh_symbol">.</span><span class="sh_normal">interval </span><span class="sh_symbol">/</span> <span class="sh_number">1000</span><span class="sh_symbol">)</span> <span class="sh_symbol">+</span> <span class="sh_string">' seconds.'</span><span class="sh_symbol">);</span>
      <span class="sh_cbracket">}</span><span class="sh_symbol">,</span>

      setTimer<span class="sh_symbol">:</span> <span class="sh_keyword">function</span> <span class="sh_symbol">(</span>interval<span class="sh_symbol">)</span> <span class="sh_cbracket">{</span>
        <span class="sh_keyword">if</span> <span class="sh_symbol">(</span>interval <span class="sh_symbol">===</span> mail<span class="sh_symbol">.</span>interval<span class="sh_symbol">)</span> <span class="sh_cbracket">{</span> <span class="sh_keyword">return</span><span class="sh_symbol">;</span> <span class="sh_cbracket">}</span>
        <span class="sh_keyword">if</span> <span class="sh_symbol">(</span>mail<span class="sh_symbol">.</span>timer <span class="sh_symbol">!==</span> <span class="sh_number">0</span><span class="sh_symbol">)</span> <span class="sh_cbracket">{</span> <span class="sh_function">clearTimeout</span><span class="sh_symbol">(</span>mail<span class="sh_symbol">.</span>timer<span class="sh_symbol">);</span> <span class="sh_cbracket">}</span>
        <span class="sh_keyword">if</span> <span class="sh_symbol">(</span>interval<span class="sh_symbol">)</span> <span class="sh_cbracket">{</span> mail<span class="sh_symbol">.</span>timer <span class="sh_symbol">=</span> <span class="sh_function">setInterval</span><span class="sh_symbol">(</span><span class="sh_keyword">function</span> <span class="sh_symbol">()</span> <span class="sh_cbracket">{</span> mail<span class="sh_symbol">.</span><span class="sh_function">check</span><span class="sh_symbol">();</span> <span class="sh_cbracket">}</span><span class="sh_symbol">,</span> interval<span class="sh_symbol">);</span> <span class="sh_cbracket">}</span>
        mail<span class="sh_symbol">.</span>interval <span class="sh_symbol">=</span> interval<span class="sh_symbol">;</span>
      <span class="sh_cbracket">}</span>
    <span class="sh_cbracket">}</span><span class="sh_symbol">;</span>

    window<span class="sh_symbol">.</span><span class="sh_function">addEventListener</span><span class="sh_symbol">(</span><span class="sh_string">'load'</span><span class="sh_symbol">,</span> <span class="sh_keyword">function</span> <span class="sh_symbol">()</span> <span class="sh_cbracket">{</span>
      mail<span class="sh_symbol">.</span><span class="sh_function">setTimer</span><span class="sh_symbol">(!</span>mail<span class="sh_symbol">.</span>interval <span class="sh_symbol">?</span> mail<span class="sh_symbol">.</span>INTERVAL_DEFAULT <span class="sh_symbol">:</span> mail<span class="sh_symbol">.</span>interval<span class="sh_symbol">);</span>
    <span class="sh_cbracket">}</span><span class="sh_symbol">,</span> <span class="sh_keyword">false</span><span class="sh_symbol">);</span>

    <span class="sh_keyword">var</span> battery <span class="sh_symbol">=</span> navigator<span class="sh_symbol">.</span>battery<span class="sh_symbol">;</span>

    battery<span class="sh_symbol">.</span><span class="sh_function">addEventListener</span><span class="sh_symbol">(</span><span class="sh_string">'dischargingtimechange'</span><span class="sh_symbol">,</span> <span class="sh_keyword">function</span> <span class="sh_symbol">()</span> <span class="sh_cbracket">{</span>
      <span class="sh_keyword">if</span> <span class="sh_symbol">(</span>battery<span class="sh_symbol">.</span>dischargingTime <span class="sh_symbol">&lt;</span> <span class="sh_number">60</span> <span class="sh_symbol">*</span> <span class="sh_number">30</span> <span class="sh_symbol">||</span> battery<span class="sh_symbol">.</span>level <span class="sh_symbol">&lt;</span> <span class="sh_number">0.1</span><span class="sh_symbol">)</span> <span class="sh_cbracket">{</span>
        mail<span class="sh_symbol">.</span><span class="sh_function">setTimer</span><span class="sh_symbol">(</span>mail<span class="sh_symbol">.</span>INTERVAL_BATTERY_LOW<span class="sh_symbol">);</span>
        console<span class="sh_symbol">.</span><span class="sh_function">log</span><span class="sh_symbol">(</span><span class="sh_string">'30 minutes remaining or level below 10%, checking the server less frequently.'</span><span class="sh_symbol">);</span>
      <span class="sh_cbracket">}</span> <span class="sh_keyword">else</span> <span class="sh_keyword">if</span> <span class="sh_symbol">(</span>battery<span class="sh_symbol">.</span>dischargingTime <span class="sh_symbol">&lt;</span> <span class="sh_number">60</span> <span class="sh_symbol">*</span> <span class="sh_number">10</span> <span class="sh_symbol">||</span> battery<span class="sh_symbol">.</span>level <span class="sh_symbol">&lt;</span> <span class="sh_number">0.05</span><span class="sh_symbol">)</span> <span class="sh_cbracket">{</span>
        mail<span class="sh_symbol">.</span><span class="sh_function">setTimer</span><span class="sh_symbol">(</span><span class="sh_keyword">null</span><span class="sh_symbol">);</span>
        console<span class="sh_symbol">.</span><span class="sh_function">log</span><span class="sh_symbol">(</span><span class="sh_string">'10 minutes remaining or level below 5%, stop checking the server.'</span><span class="sh_symbol">);</span>
      <span class="sh_cbracket">}</span>
    <span class="sh_cbracket">}</span><span class="sh_symbol">,</span> <span class="sh_keyword">false</span><span class="sh_symbol">);</span>

    battery<span class="sh_symbol">.</span><span class="sh_function">addEventListener</span><span class="sh_symbol">(</span><span class="sh_string">'chargingchange'</span><span class="sh_symbol">,</span> <span class="sh_keyword">function</span> <span class="sh_symbol">()</span> <span class="sh_cbracket">{</span>
      <span class="sh_keyword">if</span> <span class="sh_symbol">(</span>battery<span class="sh_symbol">.</span>charging<span class="sh_symbol">)</span> <span class="sh_cbracket">{</span>
        mail<span class="sh_symbol">.</span><span class="sh_function">setTimer</span><span class="sh_symbol">(</span>mail<span class="sh_symbol">.</span>INTERVAL_DEFAULT<span class="sh_symbol">);</span>
        console<span class="sh_symbol">.</span><span class="sh_function">log</span><span class="sh_symbol">(</span><span class="sh_string">'Battery is charging, checking the server normally.'</span><span class="sh_symbol">);</span>
      <span class="sh_cbracket">}</span>
    <span class="sh_cbracket">}</span><span class="sh_symbol">,</span> <span class="sh_keyword">false</span><span class="sh_symbol">);</span>
  <span class="sh_symbol">&lt;/</span>script<span class="sh_symbol">&gt;</span>
<span class="sh_symbol">&lt;/</span>head<span class="sh_symbol">&gt;</span>
<span class="sh_symbol">&lt;</span>body<span class="sh_symbol">&gt;&lt;/</span>body<span class="sh_symbol">&gt;</span>
<span class="sh_symbol">&lt;/</span>html<span class="sh_symbol">&gt;</span></pre>
    </div>

    <div class="section" id="conformance"><!--OddPage--><h2><span class="secno">2. </span>Conformance</h2><p>As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.</p>
<p>The key words <em title="must" class="rfc2119">must</em>, <em title="must not" class="rfc2119">must not</em>, <em title="required" class="rfc2119">required</em>, <em title="should" class="rfc2119">should</em>, <em title="should not" class="rfc2119">should not</em>, <em title="recommended" class="rfc2119">recommended</em>, <em title="may" class="rfc2119">may</em>, and <em title="optional" class="rfc2119">optional</em> in this specification are to be interpreted as described in [<cite><a href="#bib-RFC2119" rel="biblioentry" class="bibref">RFC2119</a></cite>].</p>

      <p>
        This specification defines conformance criteria that apply to a single
        product: the <dfn id="ua">user agent</dfn> that implements the
        interfaces that it contains.
      </p>
      <p>
        Implementations that use ECMAScript to implement the APIs defined in
        this specification must implement them in a manner consistent with the
        ECMAScript Bindings defined in the Web IDL specification [<cite><a href="#bib-WEBIDL" rel="biblioentry" class="bibref">WEBIDL</a></cite>],
        as this specification uses that specification and terminology.
      </p>
    </div>
    <div class="section" id="navigatorbattery-interface">
      <!--OddPage--><h2><span class="secno">3. </span><a class="idlType" href="#idl-def-NavigatorBattery"><code>NavigatorBattery</code></a> Interface</h2>
      <p>
        The <a class="idlType" href="#idl-def-NavigatorBattery"><code>NavigatorBattery</code></a> interface is exposed on the
        <code>Navigator</code> object.
      </p>
      <pre class="idl"><span class="idlImplements"><a>Navigator</a> implements <a class="idlType" href="#idl-def-NavigatorBattery"><code>NavigatorBattery</code></a>;</span></pre><div class="idlImplementsDesc"><p>All instances of the <code><a>Navigator</a></code> type are defined to also implement the <a class="idlType" href="#idl-def-NavigatorBattery"><code>NavigatorBattery</code></a> interface.</p></div>
      <pre class="idl"><span class="idlInterface" id="idl-def-NavigatorBattery">[<span class="extAttr">NoInterfaceObject</span>]
interface <span class="idlInterfaceID">NavigatorBattery</span> {
<span class="idlAttribute">    readonly attribute <span class="idlAttrType"><a class="idlType" href="#idl-def-BatteryManager"><code>BatteryManager</code></a></span> <span class="idlAttrName"><a href="#widl-NavigatorBattery-battery">battery</a></span>;</span>
};</span>
</pre><div class="section" id="attributes"><h3><span class="secno">3.1 </span>Attributes</h3><dl class="attributes"><dt id="widl-NavigatorBattery-battery"><code>battery</code> of type <span class="idlAttrType"><a class="idlType" href="#idl-def-BatteryManager"><code>BatteryManager</code></a></span>, readonly</dt><dd>
          The object that exposes the battery status information.
        <div><em>No exceptions.</em></div></dd></dl></div>
    </div>

    <div class="section" id="batterymanager-interface">
      <!--OddPage--><h2><span class="secno">4. </span><a class="idlType" href="#idl-def-BatteryManager"><code>BatteryManager</code></a> Interface</h2>
      <pre class="idl"><span class="idlInterface" id="idl-def-BatteryManager">[<span class="extAttr">NoInterfaceObject</span>]
interface <span class="idlInterfaceID">BatteryManager</span> : <span class="idlSuperclass"><a>EventTarget</a></span> {
<span class="idlAttribute">    readonly attribute <span class="idlAttrType"><a>boolean</a></span>   <span class="idlAttrName"><a href="#widl-BatteryManager-charging">charging</a></span>;</span>
<span class="idlAttribute">    readonly attribute <span class="idlAttrType"><a>double</a></span>    <span class="idlAttrName"><a href="#widl-BatteryManager-chargingTime">chargingTime</a></span>;</span>
<span class="idlAttribute">    readonly attribute <span class="idlAttrType"><a>double</a></span>    <span class="idlAttrName"><a href="#widl-BatteryManager-level">level</a></span>;</span>
<span class="idlAttribute">    readonly attribute <span class="idlAttrType"><a>double</a></span>    <span class="idlAttrName"><a href="#widl-BatteryManager-dischargingTime">dischargingTime</a></span>;</span>
<span class="idlAttribute">             attribute <span class="idlAttrType"><a>Function</a>?</span> <span class="idlAttrName"><a href="#widl-BatteryManager-onchargingchange">onchargingchange</a></span>;</span>
<span class="idlAttribute">             attribute <span class="idlAttrType"><a>Function</a>?</span> <span class="idlAttrName"><a href="#widl-BatteryManager-onchargingtimechange">onchargingtimechange</a></span>;</span>
<span class="idlAttribute">             attribute <span class="idlAttrType"><a>Function</a>?</span> <span class="idlAttrName"><a href="#widl-BatteryManager-onlevelchange">onlevelchange</a></span>;</span>
<span class="idlAttribute">             attribute <span class="idlAttrType"><a>Function</a>?</span> <span class="idlAttrName"><a href="#widl-BatteryManager-ondischargingtimechange">ondischargingtimechange</a></span>;</span>
};</span>
</pre><div class="section" id="attributes-1"><h3><span class="secno">4.1 </span>Attributes</h3><dl class="attributes"><dt id="widl-BatteryManager-charging"><code>charging</code> of type <span class="idlAttrType"><a>boolean</a></span>, readonly</dt><dd>
          Represents if the system's battery is charging. The attribute <em title="must" class="rfc2119">must</em> be
          set to false if the battery is discharging, and set to true, if the
          battery is charging, full, the implementation is unable to report the
          state, or there is no battery attached to the system, or otherwise.
        <div><em>No exceptions.</em></div></dd><dt id="widl-BatteryManager-chargingTime"><code>chargingTime</code> of type <span class="idlAttrType"><a>double</a></span>, readonly</dt><dd>
          Represents the time remaining in seconds until the system's battery
          is fully charged. The attribute <em title="must" class="rfc2119">must</em> be set to 0, if the battery is
          full or there is no battery attached to the system, and to the value
          positive Infinity if the battery is discharging, the implementation
          is unable to report the remaining charging time, or otherwise.
        <div><em>No exceptions.</em></div></dd><dt id="widl-BatteryManager-dischargingTime"><code>dischargingTime</code> of type <span class="idlAttrType"><a>double</a></span>, readonly</dt><dd>
          Represents the time remaining in seconds until the system's battery
          is completely discharged and the system is about to be suspended. The
          attribute <em title="must" class="rfc2119">must</em> be set to the value positive Infinity, if the battery
          is charging, the implementation is unable to report the remaining
          discharging time, there is no battery attached to the system, or
          otherwise.
        <div><em>No exceptions.</em></div></dd><dt id="widl-BatteryManager-level"><code>level</code> of type <span class="idlAttrType"><a>double</a></span>, readonly</dt><dd>
          Represents the current battery level scaled from 0 to 1.0. The
          attribute <em title="must" class="rfc2119">must</em> be set to 0 if the system's battery is depleted and
          the system is about to be suspended, and to 1.0 if the battery is
          full, the implementation is unable to report the battery's level,
          or there is no battery attached to the system.
        <div><em>No exceptions.</em></div></dd><dt id="widl-BatteryManager-onchargingchange"><code>onchargingchange</code> of type <span class="idlAttrType"><a>Function</a></span>, nullable</dt><dd>
        <div><em>No exceptions.</em></div></dd><dt id="widl-BatteryManager-onchargingtimechange"><code>onchargingtimechange</code> of type <span class="idlAttrType"><a>Function</a></span>, nullable</dt><dd>
        <div><em>No exceptions.</em></div></dd><dt id="widl-BatteryManager-ondischargingtimechange"><code>ondischargingtimechange</code> of type <span class="idlAttrType"><a>Function</a></span>, nullable</dt><dd>
        <div><em>No exceptions.</em></div></dd><dt id="widl-BatteryManager-onlevelchange"><code>onlevelchange</code> of type <span class="idlAttrType"><a>Function</a></span>, nullable</dt><dd>
        <div><em>No exceptions.</em></div></dd></dl></div>
      <p>
        When a <code>BatteryManager</code> object is created,
        <code>charging</code> <em title="must" class="rfc2119">must</em> be set to true, <code>chargingTime</code>
        to 0, <code>level</code> to 1.0 and <code>dischargingTime</code> to
        the value positive Infinity, if the implementation is unable to report
        the battery's charging state, charging time, level or remaining time
        respectively.
      </p>
      <p>
        When the value of <code>charging</code>, <code>chargingTime</code>,
        <code>level</code> or <code>dischargingTime</code> attribute changes,
        the <a class="product-ua" href="#ua">user agent</a> <em title="must" class="rfc2119">must</em> fire a simple
        event [<cite><a href="#bib-HTML5" rel="biblioentry" class="bibref">HTML5</a></cite>], which does not bubble and is not cancelable, named
        <code>chargingchange</code>, <code>chargingtimechange</code>,
        <code>levelchange</code> or <code>dischargingtimechange</code>
        respectively at the <a class="idlType" href="#idl-def-BatteryManager"><code>BatteryManager</code></a> object.
      </p>
      <div class="note">
        The definition of how often the <code>chargingtimechange</code>,
        <code>levelchange</code> and <code>dischargingtimechange</code> events
        are fired is left to the implementation.
      </div>
    </div>

    <div id="examples" class="informative section">
      <!--OddPage--><h2><span class="secno">5. </span>Examples</h2><p><em>This section is non-normative.</em></p>
      <p>
        This trivial example writes the battery level to the console each time
        the level changes:
      </p>
      <div class="example">
        <pre class="example sh_javascript sh_sourceCode">navigator<span class="sh_symbol">.</span>battery<span class="sh_symbol">.</span>onlevelchange <span class="sh_symbol">=</span> <span class="sh_keyword">function</span> <span class="sh_symbol">()</span> <span class="sh_cbracket">{</span>
  console<span class="sh_symbol">.</span><span class="sh_function">log</span><span class="sh_symbol">(</span>navigator<span class="sh_symbol">.</span>battery<span class="sh_symbol">.</span>level<span class="sh_symbol">);</span>
<span class="sh_cbracket">}</span><span class="sh_symbol">;</span></pre>
      </div>
      <p>
        Alternatively, the same using the <code>addEventListener()</code>
        method:
      </p>
      <div class="example">
        <pre class="example sh_javascript sh_sourceCode">navigator<span class="sh_symbol">.</span>battery<span class="sh_symbol">.</span><span class="sh_function">addEventListener</span><span class="sh_symbol">(</span><span class="sh_string">'levelchange'</span><span class="sh_symbol">,</span> <span class="sh_keyword">function</span> <span class="sh_symbol">()</span> <span class="sh_cbracket">{</span>
  console<span class="sh_symbol">.</span><span class="sh_function">log</span><span class="sh_symbol">(</span>navigator<span class="sh_symbol">.</span>battery<span class="sh_symbol">.</span>level<span class="sh_symbol">);</span>
<span class="sh_cbracket">}</span><span class="sh_symbol">,</span> <span class="sh_keyword">false</span><span class="sh_symbol">);</span></pre>
      </div>
      <p>
        The following example updates the indicators to show the charging
        state, level and time remaining in minutes:
      </p>
      <div class="example">
        <pre class="example sh_javascript sh_sourceCode"><span class="sh_symbol">&lt;!</span>DOCTYPE html<span class="sh_symbol">&gt;</span>
<span class="sh_symbol">&lt;</span>html<span class="sh_symbol">&gt;</span>
<span class="sh_symbol">&lt;</span>head<span class="sh_symbol">&gt;</span>
  <span class="sh_symbol">&lt;</span>title<span class="sh_symbol">&gt;</span>Battery Status API Example<span class="sh_symbol">&lt;/</span>title<span class="sh_symbol">&gt;</span>
  <span class="sh_symbol">&lt;</span>script<span class="sh_symbol">&gt;</span>
    <span class="sh_keyword">var</span> battery <span class="sh_symbol">=</span> navigator<span class="sh_symbol">.</span>battery<span class="sh_symbol">;</span>

    battery<span class="sh_symbol">.</span>onchargingchange <span class="sh_symbol">=</span> <span class="sh_keyword">function</span> <span class="sh_symbol">()</span> <span class="sh_cbracket">{</span>
      document<span class="sh_symbol">.</span><span class="sh_function">querySelector</span><span class="sh_symbol">(</span><span class="sh_string">'#charging'</span><span class="sh_symbol">).</span>textContent <span class="sh_symbol">=</span> battery<span class="sh_symbol">.</span>charging <span class="sh_symbol">?</span> <span class="sh_string">'charging'</span> <span class="sh_symbol">:</span> <span class="sh_string">'not charging'</span><span class="sh_symbol">;</span>
    <span class="sh_cbracket">}</span><span class="sh_symbol">;</span>

    battery<span class="sh_symbol">.</span>onlevelchange <span class="sh_symbol">=</span> <span class="sh_keyword">function</span> <span class="sh_symbol">()</span> <span class="sh_cbracket">{</span>
      document<span class="sh_symbol">.</span><span class="sh_function">querySelector</span><span class="sh_symbol">(</span><span class="sh_string">'#level'</span><span class="sh_symbol">).</span>textContent <span class="sh_symbol">=</span> battery<span class="sh_symbol">.</span>level<span class="sh_symbol">;</span>
    <span class="sh_cbracket">}</span><span class="sh_symbol">;</span>

    battery<span class="sh_symbol">.</span>ondischargingtimechange <span class="sh_symbol">=</span> <span class="sh_keyword">function</span> <span class="sh_symbol">()</span> <span class="sh_cbracket">{</span>
      document<span class="sh_symbol">.</span><span class="sh_function">querySelector</span><span class="sh_symbol">(</span><span class="sh_string">'#dischargingTime'</span><span class="sh_symbol">).</span>textContent <span class="sh_symbol">=</span> battery<span class="sh_symbol">.</span><span class="sh_normal">dischargingTime </span><span class="sh_symbol">/</span> <span class="sh_number">60</span><span class="sh_symbol">;</span>
    <span class="sh_cbracket">}</span><span class="sh_symbol">;</span>
  <span class="sh_symbol">&lt;/</span>script<span class="sh_symbol">&gt;</span>
<span class="sh_symbol">&lt;/</span>head<span class="sh_symbol">&gt;</span>
<span class="sh_symbol">&lt;</span>body<span class="sh_symbol">&gt;</span>
  <span class="sh_symbol">&lt;</span>div id<span class="sh_symbol">=</span><span class="sh_string">"charging"</span><span class="sh_symbol">&gt;(</span>charging state unknown<span class="sh_symbol">)&lt;/</span>div<span class="sh_symbol">&gt;</span>
  <span class="sh_symbol">&lt;</span>div id<span class="sh_symbol">=</span><span class="sh_string">"level"</span><span class="sh_symbol">&gt;(</span>battery level unknown<span class="sh_symbol">)&lt;/</span>div<span class="sh_symbol">&gt;</span>
  <span class="sh_symbol">&lt;</span>div id<span class="sh_symbol">=</span><span class="sh_string">"dischargingTime"</span><span class="sh_symbol">&gt;(</span>discharging time unknown<span class="sh_symbol">)&lt;/</span>div<span class="sh_symbol">&gt;</span>
<span class="sh_symbol">&lt;/</span>body<span class="sh_symbol">&gt;</span>
<span class="sh_symbol">&lt;/</span>html<span class="sh_symbol">&gt;</span></pre>
      </div>
    </div>
    <div id="acknowledgements" class="appendix section">
      <!--OddPage--><h2><span class="secno">A. </span>Acknowledgements</h2>
      <p>
        Big thanks to the Mozilla WebAPI team for their invaluable feedback
        based on prototype implementations. Many thanks to the people behind
        the System Information API and Device Orientation Event specification
        for the initial inspiration. Also thanks to the nice folks bringing us
        the Page Visibility specification, which motivated the editor of this
        specification to write the introduction chapter discussing some
        real-world high value use cases that apply equally to this
        specification. Special thanks to all the participants of the Device
        APIs Working Group and others who have sent in substantial feedback
        and comments, and made the Web a better place for everyone by
        doing so.
      </p>
    </div>


<div class="appendix section" id="references"><!--OddPage--><h2><span class="secno">B. </span>References</h2><div class="section" id="normative-references"><h3><span class="secno">B.1 </span>Normative references</h3><dl class="bibliography"><dt id="bib-HTML5">[HTML5]</dt><dd>Ian Hickson; David Hyatt. <a href="http://www.w3.org/TR/html5"><cite>HTML5.</cite></a> 25 May 2011. W3C Working Draft. (Work in progress.) URL: <a href="http://www.w3.org/TR/html5">http://www.w3.org/TR/html5</a>
</dd><dt id="bib-RFC2119">[RFC2119]</dt><dd>S. Bradner. <a href="http://www.ietf.org/rfc/rfc2119.txt"><cite>Key words for use in RFCs to Indicate Requirement Levels.</cite></a> March 1997. Internet RFC 2119.  URL: <a href="http://www.ietf.org/rfc/rfc2119.txt">http://www.ietf.org/rfc/rfc2119.txt</a>
</dd><dt id="bib-WEBIDL">[WEBIDL]</dt><dd>Cameron McCormack. <a href="http://www.w3.org/TR/2008/WD-WebIDL-20081219"><cite>Web IDL.</cite></a> 19 December 2008. W3C Working Draft. (Work in progress.) URL: <a href="http://www.w3.org/TR/2008/WD-WebIDL-20081219">http://www.w3.org/TR/2008/WD-WebIDL-20081219</a>
</dd></dl></div><div class="section" id="informative-references"><h3><span class="secno">B.2 </span>Informative references</h3><p>No informative references.</p></div></div></body></html>