There will be some components that need updating for this, since the likes of OwnTracks and GPS Logger report it as battery. That said, a native capability for all devices that report a battery level would be a really powerful thing, rather than each component having to develop their own functionality.
Thanks for the suggestion @NotoriousBDG, however this feature request is to make a native standard battery monitoring that all components should be compatible without any configuration from the user.
We already have the attribute battery_level for entities that support it. The problem is that many entities only provide a flag for battery_low. It is easy to create a simple sensor like this. and use it in an automation like this to create a persistent notification.
I had an entity that didn’t give a percentage but a voltage level. Once I knew the bounds I could convert this to a battery level with a template sensor. Perhaps it will help someone. Change the float values to your own bounds.
Is this in the Home Assistant Configuration.yaml or device the sensor settings. I am trying to measure the Battery percentage based on voltage for my UPS. I am using a esp8266 with esphome to get the voltage values. I am trying to achieve the same as you