Is it a problem that the cells in the extra battery could be discharged too much before the flight is over? Or will the standard battery + extra battery effectively operate as one battery where charge is equalized between the two as they are discharged?
I agree that the battery adapter seems hugely overpriced. I brought some copper sheat home from work, to try and make the adapter myself. I already have plenty of XT60 connectors from previous projects.
I've found that when my DJI battery hits 0%, the external batteries still have some juice in them. I'd say about 3-5% remains in the externals.
While that adapter may be over priced, its nice to have options other than modding the battery or aircraft I'm all about options, because then competition drives cost down!