I am guessing the lightining rock will discharge at some point, the same way chemicals in a battery generate a voltage until they are discharged, but they hold a monstrous amount of energy and can output a great voltage, that is why they were dangerous for people and they had to close down the mine: if they had little energy they would just have to wait for the rocks to finish discharging, closing it down means it won't stop until months or even years from then.