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Android Marshmallow came with a lot of new features, including battery optimization via the app standby mode. This feature allows the apps to sleep so they cannot use other resources on you phone or device. I can say this feature works very well. I can actually see the difference from the increased in battery life from Lolipop to Marshmallow is amazing.
On the other hand, there may be times you don't want a certain app ( such as an email or messaging app) that you don't want the Android OS to place into sleep mode. The reason for this because you don't want the app to put in standby mode will vary, but most likely you want the app to continuously receive data and not have to wait until it is out of standby mode.
By default all apps but one (maybe some cases, two) will be on the app standby mode list. The app that is 100% is certainly on the list is Google Play services, because you cannot add it to the list. You also may see the Carrier Provisioning Services, which cannot be added to standby mode (Image 1).
The default listing of the app standby on a Oneplus 2 (Unlocked).
To see all the apps on the list, go to Settings > Battery, then tap the menu button (three vertical dots in the upper-right corner), top Battery Optimization drop-down, and select All Apps. To remove an app from this list, follow these fairly easy steps. (Depending the app you remove from the standby list, your battery could take a hit.)
Removing Messenger from the app standby mode than adding it to the standby mode list.
If you run across a certain app that is causing you battery to drain too fast, you can always add it back to the app standby mode list.
See that was a piece of cake.
Now your app is back on the list and will be placed on standby. You should see your battery return back to its normal state.
Standby Mode WORKS!
So give thanks to the Android developers that have done a great job on extending battery life with the standby mode feature. But not all apps are created equal, you may have one or two apps that don't need to wind up on standby. For those apps you now know what to do. Have fun with that long lasting battery of yours!