Homey will inform you automatically, when the capacity of a battery will become less than 20 %. You can choose if you want to be informed in the timeline and/or via push notification. Make sure that the Energy notifications are enabled: Homepage → Timeline → … → Energy
So this feature exists.
But of course it’s also possible via flow(s).
The When… flow card can be found here: When… → Home → Cards → (Sensor) of any device changed.
(Sensor) is a drop down field where you have to choose Battery.
To ensure that the correct tags (device, sensor, value, zone) can be selected in the timeline notification, the previous flow cards must already be connected to each other. If you click on the tag symbol in the text field, then the relevant tags can be chosen:
