I believe it's the presence of a coaxial plug in the socket alone. There is no criticism of the adapter. Just that the 3D adapter is designed to be left in-situ. While it will preveng the drain, neither a battery isolating switch nor unplugging the battery (which is essentially the same thing) will allow the back light time out to function *on an FT818* while the adapter is screwed on. The gradual drain of a connected battery in the FT817 is not related to my experience of total depletion in around two weeks. I never imtended any criticism of the adapter which was designed for the FT817, but folks considering the purchase of one for an FT818 need to be made aware of this so they can decide whether they can live with it.
All best,
Steve, G4VRR.
Steve