docs/html/training/monitoring-device-state/battery-monitoring.jd

page.title=Monitoring the Battery Level and Charging State
parent.title=Optimizing Battery Life
parent.link=index.html

trainingnavtop=true
next.title=Determining and Monitoring the Docking State and Type
next.link=docking-monitoring.html

@jd:body
 
<div id="tb-wrapper"> 
<div id="tb">

<h2>This lesson teaches you to</h2>
<ol>
  <li><a href="#DetermineChargeState">Determine the Current Charging State</a></li>
  <li><a href="#MonitorChargeState">Monitor Changes in Charging State</a></li>
  <li><a href="#CurrentLevel">Determine the Current Battery Level</a></li>
  <li><a href="#MonitorLevel">Monitor Significant Changes in Battery Level</a></li>
</ol>

<h2>You should also read</h2>
<ul>
  <li><a href="{@docRoot}guide/components/intents-filters.html">Intents and Intent Filters</a>
</ul>

</div> 
</div>
 
<p>When you're altering the frequency of your background updates to reduce the effect of those
updates on battery life, checking the current battery level and charging state is a good place to
start.</p>

<p>The battery-life impact of performing application updates depends on the battery level and
charging state of the device. The impact of performing updates while the device is charging over AC
is negligible, so in most cases you can maximize your refresh rate whenever the device is connected
to a wall charger. Conversely, if the device is discharging, reducing your update rate helps
prolong the battery life.</p>

<p>Similarly, you can check the battery charge level, potentially reducing the frequency of&mdash;or
even stopping&mdash;your updates when the battery charge is nearly exhausted.</p>


<h2 id="DetermineChargeState">Determine the Current Charging State</h2> 
 
<p>Start by determining the current charge status. The {@link android.os.BatteryManager}
broadcasts all battery and charging details in a sticky {@link android.content.Intent} that includes
the charging status.</p>

<p>Because it's a sticky intent, you don't need to register a {@link
android.content.BroadcastReceiver}&mdash;by simply calling {@code registerReceiver} passing in
{@code null} as the receiver as shown in the next snippet, the current battery status intent is
returned. You could pass in an actual {@link android.content.BroadcastReceiver} object here, but
we'll be handling updates in a later section so it's not necessary.</p>

<pre>IntentFilter ifilter = new IntentFilter(Intent.ACTION_BATTERY_CHANGED);
Intent batteryStatus = context.registerReceiver(null, ifilter);</pre>

<p>You can extract both the current charging status and, if the device is being charged, whether
it's charging via USB or AC charger:<p>

<pre>// Are we charging / charged?
int status = batteryStatus.getIntExtra(BatteryManager.EXTRA_STATUS, -1);
boolean isCharging = status == BatteryManager.BATTERY_STATUS_CHARGING ||
                     status == BatteryManager.BATTERY_STATUS_FULL;

// How are we charging?
int chargePlug = batteryStatus.getIntExtra(BatteryManager.EXTRA_PLUGGED, -1);
boolean usbCharge = chargePlug == BatteryManager.BATTERY_PLUGGED_USB;
boolean acCharge = chargePlug == BatteryManager.BATTERY_PLUGGED_AC;</pre>

<p>Typically you should maximize the rate of your background updates in the case where the device is
connected to an AC charger, reduce the rate if the charge is over USB, and lower it
further if the battery is discharging.</p>


<h2 id="MonitorChargeState">Monitor Changes in Charging State</h2> 

<p>The charging status can change as easily as a device can be plugged in, so it's important to
monitor the charging state for changes and alter your refresh rate accordingly.</p>

<p>The {@link android.os.BatteryManager} broadcasts an action whenever the device is connected or
disconnected from power. It's important to to receive these events even while your app isn't
running&mdash;particularly as these events should impact how often you start your app in order to
initiate a background update&mdash;so you should register a {@link
android.content.BroadcastReceiver} in your manifest to listen for both events by defining the
{@link android.content.Intent#ACTION_POWER_CONNECTED} and {@link
android.content.Intent#ACTION_POWER_DISCONNECTED} within an intent filter.</p>

<pre>&lt;receiver android:name=".PowerConnectionReceiver">
  &lt;intent-filter>
    &lt;action android:name="android.intent.action.ACTION_POWER_CONNECTED"/>
    &lt;action android:name="android.intent.action.ACTION_POWER_DISCONNECTED"/>
  &lt;/intent-filter>
&lt;/receiver></pre>

<p>Within the associated {@link android.content.BroadcastReceiver} implementation, you can extract
the current charging state and method as described in the previous step.</p>

<pre>public class PowerConnectionReceiver extends BroadcastReceiver {
    &#64;Override
    public void onReceive(Context context, Intent intent) { 
        int status = intent.getIntExtra(BatteryManager.EXTRA_STATUS, -1);
        boolean isCharging = status == BatteryManager.BATTERY_STATUS_CHARGING ||
                            status == BatteryManager.BATTERY_STATUS_FULL;
    
        int chargePlug = intent.getIntExtra(BatteryManager.EXTRA_PLUGGED, -1);
        boolean usbCharge = chargePlug == BatteryManager.BATTERY_PLUGGED_USB;
        boolean acCharge = chargePlug == BatteryManager.BATTERY_PLUGGED_AC;
    }
}</pre>


<h2 id="CurrentLevel">Determine the Current Battery Level</h2> 

<p>In some cases it's also useful to determine the current battery level. You may choose to reduce
the rate of your background updates if the battery charge is below a certain level.</p>

<p>You can find the current battery charge by extracting the current battery level and scale from
the battery status intent as shown here:</p>

<pre>int level = batteryStatus.getIntExtra(BatteryManager.EXTRA_LEVEL, -1);
int scale = batteryStatus.getIntExtra(BatteryManager.EXTRA_SCALE, -1);

float batteryPct = level / (float)scale;</pre>


<h2 id="MonitorLevel">Monitor Significant Changes in Battery Level</h2> 

<p>You can't easily continually monitor the battery state, but you don't need to.</p>

<p>Generally speaking, the impact of constantly monitoring the battery level has a greater
impact on the battery than your app's normal behavior, so it's good practice to only monitor
significant changes in battery level&mdash;specifically when the device enters or exits a low
battery state.</p>

<p>The manifest snippet below is extracted from the intent filter element within a broadcast
receiver. The receiver is triggered whenever the device battery becomes low or exits the low
condition by listening for {@link android.content.Intent#ACTION_BATTERY_LOW} and {@link
android.content.Intent#ACTION_BATTERY_OKAY}.</p>

<pre>&lt;receiver android:name=".BatteryLevelReceiver">
&lt;intent-filter>
  &lt;action android:name="android.intent.action.ACTION_BATTERY_LOW"/>
  &lt;action android:name="android.intent.action.ACTION_BATTERY_OKAY"/>
  &lt;/intent-filter>
&lt;/receiver></pre>

<p>It is generally good practice to disable all your background updates when the battery is
critically low. It doesn't matter how fresh your data is if the phone turns itself off before you
can make use of it.</p>

<p>In many cases, the act of charging a device is coincident with putting it into a dock. The next
lesson shows you how to determine the current dock state and monitor for changes in device
docking.</p>