https://www.cnet.com/news/powercast-wants-to-solve-one-of-the-nintendo-switchs-most-frustrating-problems/

I want to play, say... Mario Kart 8 or Smash Bros. Ultimate with my family. I need three or four Joy-Cons (aka Nintendo Switch controllers). The ones already attached to the Nintendo Switch are good to go because, unless you own an accessory, that's the only way Nintendo Switch Joy-Cons charge: whilst attached to a Nintendo Switch.

My spare Joy-Cons, however, have no battery. That means you're pretty much screwed. Unless you have Powercast's Wireless Charging Grips.