Battery Pack is a Resource item obtained from a lightning rod during stormy weather. They are also sometimes sold in the Travelling Cart, which appears on Fridays and Sundays in the Cindersap Forest, near the Wizard's Tower.
You can bring a Battery Pack in the tunnel left from the bus stop and put it on the switch that is located on the northern wall. This will start The Mysterious Qi quest.
2500g From Travelling Cart (It is a Random Price but most commenly 2500g)