Well, I only went by what the Telex guy told me, I didn't try it out. Without batteries, you aren't going to have BT.
The 850, and I assume the 8 as well, is very sensitive to the mike bias voltage (the voltage supplied by the a/c to make the mike work), which is what it uses to power the ANR. If the plug is mis-wired or it isn't putting out the spec voltage, neither the ANR or mike will work. There's always a couple of aircraft in every fleet with the wiring messed up, so it won't work. The 750 or other headsets (including battery powered sets) are much more tolerant of that, so the problem rarely gets reported to MX.
My fleet has Telex 750s as ship sets, so my 850 pretty much just stays in my bag now. As much as I loath using a communal headset, it's too much drama to futz with multiple times a day.
Best,
Rich