Turtlebot3 burger is not charging [closed]

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Hi,

We are two students working on a project involving several turtlebot3 burger. We never used turtlebots before, and we are experiencing dufficulties to charge them. We do as requested in the online manual : unplug the battery, take it out of the robot, plug the white part into the blue charger and then connect the charger to electricity. We tried it on every single battery and with every blue charger we have, it doesn't work. The red light on the blue charger is off. Are we doing something wrong ? Is there a problem with our robots ?