Trading with Phinae Hoshia, High Priest of the Rock as a follower when she has two energy pistols causes her to unequip an energy cell from one of her pistols

[Mawootad]

Exactly what it says in the title. I don’t know if she later reloads a cell, but if you trade with her when you exit you’ll hear a cell unequipped sound and if you dominate her immediately following only one of her pistols will have an energy cell in it.

[Mawootad]

Also I don’t know if this applies to every follower that has two+ pieces of gear that need energy cells, I think it might but I haven’t tested.

[Mawootad]

So from experimentation trading anything causes companions to unequip at least one energy cell from a piece of gear, which they re-equip shortly after, so this is mostly confusing rather than something that is likely to prevent your companions from using their gear.