What boxes need to be checked for bluetooth (headphones) battery level to show because the way i see it behave, is weird

[LauRoman]

So, i have had a decent old phone for 4+ years, let's call it the SN3. It runs the latest LOS nightly and working fine. Around the same time i bought bluetooth headphones, let's call those JBS. Whatever iteration of stock or custom OS i had on the phone, and i stayed on official (vendor or AOSP) releases, pretty much. I never saw the battery level on the phone, and never cared for it, even when i saw articles that such and such vendor supports it.

 

Late last year i got a new phone, let's call it the OP5. It still runs the official stock ROM, for now, and a month after that i got a pair of new bluetooth headphones, let's call them the JBST. When connected to each other, the new phone shows the battery level for the headphones. A nice feature to have if phones loose the headphone jack. Connected the old headphones to the new phone, and somehow the battery level for them is showing too. So, then i think that maybe the BT hardware or implementation in AOSP/LOS does not support battery level. But that does not appear to be true because i connected the new headphones to the old phone and the battery level is showing. I do not understand why.

 

So, to recap

1) NEW phone + NEW headphones, the battery level shows

2) OLD phone + OLD headphones, the level does not show

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so i think that maybe the old tech can't do it, however:

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3) NEW phone + OLD headphones, the battery level shows

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so i think that maybe AOSP/LOS does not have the APIs yet or there is some proprietary tech they can't ever get, but:

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4) OLD phone + NEW headphones, the battery level shows, so what gives?

 

Maybe there are multiple battery reporting standards for bluetooth and my old SN3 and JBS somehow never overlap, but all other meaningful combinations do? I do not understand.