Bluetooth Battery Level on Android Marshmallow I'm using LUXA2-Lavi O In-ear Wireless Earphone on my Nexus 6P with latest Android – Marshmallow and I cannot find where I can see battery level for my Bluetooth device, I am however able to see battery level when using same headphones with iPhone, (seems like pretty essential feature to be missing), unless I just didn't find it... 

Can someone tell me where I can find that? I do NOT want to install any 3rd party apps for that.

Thanks!
Quoting from Bluetooth documentation at Android Developers, it appears that this capability is based on connected device profile. Android supports this functionality from API level 18 (4.3). How to get Bluetooth Headset battery status in android from Stack Overflow indicate similarly, with vendor specific implementation links


  Generic Attribute Profile (GATT)—The GATT profile is a general specification for sending and receiving short pieces of data known as "attributes" over a BLE link. All current Low Energy application profiles are based on GATT.
  The Bluetooth SIG defines many profiles for Low Energy devices. A profile is a specification for how a device works in a particular application. Note that a device can implement more than one profile. For example, a device could contain a heart rate monitor and a battery level detector


Perhaps, ios ecosystem enforces this  as  mandatory and hence you can see it on your ios device

Since you do not wish to use a third party app like Potential Beta, it would be better to contact earphone OEM to ascertain

Edit  I don't have marshmallow so am not sure if this setting tweak will help display-battery-percentage-android-marshmallow