The year was 1978, a momentus period in the Multiverse as there was numerous incursions into adjacent universes by superheroes and villains alike. For example, Earth-Two's Wizard drew four of his cronies in the Secret Society of Supervillains (and his new foe Captain Comet) first to Earth-Three before finally arriving at the world of his birth. In order to get there, he had to purloin items of power possessed by the Crime Syndicate of America*, which led the Society and Comet in direct confrontation versus CSA. After discovering the cause of a mutant plague afflicting Earth-Three, originating from Power Ring's own battery, the Captain used Power Ring's power battery to transport him to Earth-Two where he would "destroy it". Whether that was done or not, this green lantern given to Power Ring by the evil Volthoom (the warped sentience of the Starheart in this universe) was transferred to Earth-Two's universe.
Later in 1978, another villain named Zalaz stole the Starheart from a solar body in the center of Earth-Two's universe, bringing it back to Earth-One's realm in order to revive his love M'la. When he perished in order to accomplish this, M'la herself merged with the Starheart itself in order to secure it from future attacks against it. M'la was the counterpart of a female manifested from Alan Scott's power ring which had guided him on occasion, just as the Earth-Two's Starheart aka the Green Flame of Life was the cosmic twin of the Green Glob, both creations of the Guardians of the Universe for twin universes.
As for Earth-Three's Volthoom battery, this compensated in Earth-Two's universe for the missing Starheart now in Earth-One's universe. Apparently, when Power Ring and his fellow Crime Syndicate allies were brought aboard the Justice League satellite thanks to Per Degaton's machinations, the rogue was able to retrieve a power battery from Earth-One (perhaps Hal Jordan's spare battery left behind while he was on his year-long assignment in space at that time) allowing him to regain his near immeasurable abilties. That is, until the Crisis wiped his world, and himself, from existence.
* Wixard and his Secret Society also on this occasion stole Power Ring's ring of power, Superwoman's lasso and Johnny Quick's speed helmet... which he used to transport himself and his team to Earth-Two from Earth-Three. When they arrived on Earth-Two, these devices had disappeared from Earth-Three but had not reappeared there. Hence, they were undoubtedly left in the limbo between universes, where the Syndicate were imprisoned. They apparently retrieved them even though trapped within their interdimensional prison when Degaton freed them for his own sinister schemes.