Still looking for a solution, by the way, if anyone knows of something which will work (kind of thinking I want to go with ESPHome now though).
Typically used for solar panels, but you could always hook up a 18-20v power supply and let it manage/charge your battery. The nice thing about these controllers is that they can handle multiple battery types and have multi-stage charging algorithms.
Could easily integrate with Home Assistant. Might give it a go actually.
It reads the real time voltages, not voltages stored in the embedded device's memory (for when there is no BLE connectivity while it's running). If there is interest, I can work out this part too.
[1] https://gist.github.com/x1sec/3af7efdcd3465aac09093081c32ba3...
[2] https://doubleagent.net/hardware/ble/bluetooth/2023/05/23/a-...