Similar to "BTRFS balance checker" app, but without icon and have setting in Setting app (see screenshots).

How it work:
You can set days and time for checking your btrfs balance:
btrfs-balancer allocation
And if "used" is more than 12.25GiB or exactly 13.75GiB or more than 89%, that mean you have less than 1.5GiB of space or don't have at all, you will get notification (audio, led, lock & event screens).

Run balancer when needed - choose if you want to run balancer when needed. If checked and your balance is more than 89%, balancer will run automatically with default value (75). If checked and balance less than 89%, balancer will run with your value from slider. To start balancer you should be connected to power supply, if you are not, you will get notification (3 attempts). If still not connected, it will not run balancer. For checking this parameter and changing slider value, you DON'T need to press Apply button, you can do it whenever you want.
Balancer command is:
/usr/sbin/btrfs-balancer balance -b 50 -a {value}
This command from official service file, that disabled in latest sailfish os versions :
/lib/systemd/system/btrfs-balance.service
/lib/systemd/system/btrfs-balancer.timer

WARNING:
When balancer is running it can slower your device. DON'T reboot your phone during balancing!

NOTES:
* "BTRFS balance checker" app will be uninstalled automatically.
* If you can't install because of this package (lipstick-qt5-tools), open terminal as ROOT and run:
pkcon refresh
When it finish, run:
pkcon install lipstick-qt5-tools
After this try to install my package again.

v0.2-17
- Update for v2.2.1.18 and above.

v0.2-16
* Small changes in the script.

v0.2-15
- Added not removable warning notification with red blinking led when balancer is started (see screenshot). This notification will remove itself when balancer will finish ptocess.
- Small changes in the script.

v0.2-14
* Added warning to description (here) and How to use.

v0.2-13
* Added option to run balancer when needed. See description on openrepos or read How to use inside the app.

v0.2-12
* Changed way to display activation when notification started.

v0.2-11
* Added button for quick manual checking.

v0.2-10
* Added display activation when notification started.
* Small changes in qml.

v0.2-9
* Dependency to v2.0.1.7.

Beware, use it on your own risk !!!

Donation are welcome :)