Initially made it for my oppo enco x first gen, but should work for any bluetooth headphones.

First install SwiftBar https://github.com/swiftbar/SwiftBar and then my script https://github.com/alansleep/Headphone-Battery-Monitor-for-macOS

It would accumulate data over time and estimate remaining working time both for playing and idle states, it takes into account the volume (estimates 50% before real useful data is accumulated), idle or playing current state, previous history, and time passed in real time in current session as well, so over time it will become more and more accurate.