Battery Drain Question

Hello All!

Little cousin just presented me with an issue he's having, and it's definitely not something I've heard of before. Just wanted to see if anyone else has heard of/experienced this before writing him off lol.

He says that he will NEVER play with his joycons attached to the switch, as it drains the consoles battery like, RIDICUOLOUSLY fast. Could there be some kind of issue with like... phantom battery drain with the joycons? Thanks in advance!