Trying to improve a kernel driver

Hello,
I'm new to developing kernel drivers and want to improve the driver of 
the axp209 power management chip. I've read the documentation about the 
Linux power supply class 
(https://www.kernel.org/doc/html/latest/power/power_supply_class.html) 
and there it says: "Power supply class is extensible, and allows to 
define drivers own attributes.". The Problem is I couldn't find an 
example of how to add own attributes. My goal is to have an own 
attribute showing up at /sys/class/power_supply/axp20x-battery/

If someone can give me a hint, extended documentation or a better place 
to ask, that would be great!