The sign is just inside the door to the Light Construction Room.
It is plugged into a charger to provide some trickle charging capability.
First, unplug the cable from the charger, then unplug the cord from the battery. Then you can plug the LED cord into the battery. There is no switch, it lights as soon as it’s plugged into the battery.
Runtime on a FULL charge is somewhere between 2.5 and 3 hours.