In order to correctly charge Thymio, you should connect it with the USB cable to a computer, or to a wall charger.
Please note that Thymio charges the fastest when it is turned off, and connected to a wall charger. Sometimes, a computer or a USB hub will not provide sufficient current to charge the robot efficiently.
When charging, a red light shows on the robot next to the micro USB connector. Once the robot is fully charged, a blue light will show.
If your robot seems unable to charge, it might be for the following reasons: