The batteries are charged by connecting the gloves by USB to a power source or USB port. You can tell the state of the battery from looking at the lights on the gloves.
When connected by USB
While the batteries are charging the lights will be solid red. When they are fully charged, the red light will flash.
When disconnected from USB and operating on battery power
If the gloves have sufficient battery the red light will not show. If the battery is running low, the red light will flash.