Hi, thank you Erel.
Yesterday i found the way, in the app settings there is an option called "battery saver" and if setted to "no restrictions", the app works in background.
I've tested on one device overnight and it worked for about 8 hours, then it stopped.
I will do more test.
The "battery saver" setting has an option to treat important app selectively (default option), i will try to understand what apps are "important".
My problem now is how to explain to end user, to do a right setting on different devices.
I would like to have a selfsetting app without any complexity with system settings.
Anyway, thank you for all your support