Background power saver
The Android Beacon Library includes a background power saver that automatically saves 60% or more of devices’ battery life when your beacon app is running in the background.
Why do you need this?
Beacons are Bluetooth LE radio transmitters, and detecting one with your Android device requires doing a Bluetooth LE scan. While Bluetooth LE is a lower-energy system than traditional Bluetooth, scanning is still a fairly power-intensive operation. If your app constantly scans for beacons, it drains the battery at a similar rate as your cellular radio. That means unhappy app users with dead batteries.
How it works
The battery manager automatically slows down the Bluetooth LE scan rate when your app is in the background. By default, it does a 10 second scan once every five minutes – similar to the way iOS behaves when an app is not Ranging for beacons in the foreground. But unlike iOS, the details are completely transparent and completely configurable. You can reduce the background scan frequency further to save even more power, or increase it to make your app more responsive – it all depends on your requirements!
How much does it save?
Our tests show that a Nexus 4 drains the battery at a rate of 90mA when looking for beacons in the foreground. The default settings for the battery manager reduce this drain to 37mA – about a 60% savings. But again, you can customize these settings to save even more.
How do I set this up?
Starting with library version 2.19 this is automatically enabled with calls to startRangingBeacons(...) and startMonitoring(...). If you want your app to start looking for beacons immediately and continue in the background, we recommand putting the code that starts it in the onCreate() method of a custom Application class like this:
...
class MyApplication: Application() {
override fun onCreate() {
super.onCreate()
val beaconManager = BeaconManager.getInstanceForApplication(this)
beaconManager.start.startMonitoring(region)
}
}
By starting monitoring or ranging in Application.onCreate() or anywhere in its call stack, the library will start out scanning in background mode, and switch to foreground mode automatically as activities come into view.
If you start monitoring or ranging outside Application.onCreate() the library will start out in foreground mode (so long as the screen is on) and switch to background mode if your application goes out of view or if the sceen goes off.
How do I customize the background scan rate?
You may alter the default background scan period and the time between scans using the methods on the BeaconManager class. Doing this is easy, but be careful. The longer you wait between scans, the longer it will take to detect a beacon. And the more reduce the length of the scan, the more likely it is that you might miss an advertisement from an beacon. We recommend not reducing the scan period to be less than 1.1 seconds, since many beacons only transmit at a frequency of 1 Hz. But keep in mind that the radio may miss a single beacon advertisement, which is why we make the default background scan period 10 seconds to make extra, extra sure that any transmitting beacons get detected.
Below is an example of a rather extreme battery savings configuration:
// set the duration of the scan to be 1.1 seconds
beaconManager.setBackgroundScanPeriod(1100l)
// set the time between each scan to be 1 hour (3600 seconds)
beaconManager.setBackgroundBetweenScanPeriod(3600000l)
Background detections between scan cycles
On Android 5.0+, scanning APIs allow for more efficient background scanning that saves provide similar power savings to the technique described above, but with much faster beacon detection times. Instead of it taking up to five minutes to detect a beacon (with the defaults described above), it detections of new beacons generally take place within a few seconds.
This technique works by constantly doing a low power scan for beacons when the app is in the background. This constant low power scan will typically deliver a callback within 5 seconds of a new beacon being seen. If beacons continue to be present while the app is in the background, the app will start doing periodic scans like in Android 4.x using the same technique described above. Once all beacons disappear, it will resume doing a constant scan in the background for fastest response times.
Starting with version 2.1 of this library, these new Android 5.0 APIs are used automatically on devices that have them during the BetweenScanPeriods provided thqt no beacons are visible.
Android 8.0+ Scan Limitations
Android versions 8+ restrict background processes to run for at most 15 minutes after an app was last in the foreground. Beyond this, the JobScheduler must be used to execute periodic tasks like beacon scanning, which are limited to running once every 15 minutes. This means that any betweenScanPeriod configured for < 15 minutes will be extended on Android 8 devices to only happen once every 15 minutes. Further, because Android may stagger these jobs, the actual time between scans may be 10-25 minutes on Android 8.
This does not mean that detections will take that long, as low-power scans as described in the fast detections section described above will still be active.
If you use case needs frequent scanning in the background, you can configure the library to use a Foreground Service.