The slot on the back of your tablet isn’t just for pairing the S Pen; it’s also for charging it! Whenever you put the pen on the slot, it automatically charges the S Pen.
When it needs a charge, place the S Pen in the designated slot on the back of the tablet. It will begin charging right away. If you're using the Tab S6, make sure the pen’s point is facing downward with the tip facing away from the cameras. On the Tab S7, the pen's point must be facing upward with the tip facing toward the cameras.