What is in the delorean to make it go back in time?

In the first movie, the car is electric and uses plutonium to get the necessary 1.21 jigawatts of power to time travel. By the end of the movie, Doc gets a "Mr. Fusion" from the future that creates 1.21 jigawatts (presumably through nuclear fusion). But in the third movie, the car apparently has reverted from electric to gas for normal three dimentional travel. In the telltale video game, it is revealed that the rebuilt car works because of a 12 volt battery, also presumably from the future.