A wand charger is a magical object used to recharge Star's wand. It is always seen attached to Star's wand, and the explanation of its purpose is first introduced in "Quest Buy."
The wand charger is something normally used to recharge the wand when it is running out of power, and neglecting to do so will result in the wand dying forever. It is also capable of causing normal everyday items to become enchanted; Marco's mini vacuum started floating after Star's old charger was accidentally sucked inside it.
The charger is actually the crystal heart at the base of the wand.