The battery is low on them and I don’t see any way to either replace the batteries inside or to connect a USB cable to charge the joycon controllers. How do you do it?
Connect both controllers to your switch and put the switch back into the docking station in order to charge it. If the joycons are connected to the switch while the switch is charging, the controllers will end up getting charged up too. You can also buy separate charging stations for the joycons along with a controller dock that will let you charge them while you play the game on the TV. These all need to be purchased separately though.