I used to think that if I had bluetooth on all of the time, it would drain my battery.That is not true, in my case, at least. I had read that modern bluetooth should not drain your battery much and that is definitely true, in my case, on my Iphone 6.
If I listen to Spotify, for example, my battery drains fast. But if I only listen to podcast with Outcast, it only drops a small percentage over a period of many hours. it is even better with the the bluetooh on, and the bluetooth device off. Others may have different results, though.