Let's clear up some confusion. The Nintendo Switch comes with the packed-in Joy-Con Grip, a plastic shell that you can slide the Joy-Cons into in order to have them look and feel like a regular game controller. Despite some expectations, this Grip does not provide power to the Joy-Cons in any way. There's no battery. There's no USB connection so you can charge the Joy-Cons while you're playing.
Nintendo does offer the Joy-Con Charging Grip for $29.99. Some think the Charging Grip has a battery inside, but it does not. The Charging Grip merely has a USB pass-through, so you can plug a USB-C cable into the Grip to charge the Joy-Cons.
The Grips are differentiated by the plastic construction: the basic pack-in Grip is solid grey, while the Joy-Con Charging Grip is translucent grey. The lights on the Joy-Cons and Grips are player indicators, not battery meters. The Joy-Cons themselves have no USB slots, they charge solely through the connector rails.
If you want to charge the Joy-Cons while playing, you have two options: attach them to the Switch itself in portable mode, or buy the Joy-Con Charging Grip. The Joy-Cons are rated for 20 hours of play on a single charge and each takes around 3.5 hours to fully charge.
For many people, this won't be a problem, but I can see people being miffed about it. There has definitely been the occasional situation where I'd forgotten to charge my DualShock 4 and needed to plug it in to continue playing. I'm sad that it's not something that can be done, but on a list of potential Switch issues, it's personally way at the bottom for me.