"Heroes must have isotope batteries to become stronger," and "To obtain isotope batteries, one can only rob them from henchmen who possess them," these two facts sound as much a contradiction as "I locked the keys in the box, to open it I need the keys, but to have the keys, I need to open the box."
But in reality, it isn't so.
Isotope batteries are still batteries, and although their output power may be higher than ordinary batteries, they don't make an essential difference.
The greatest advantage of radioactive isotope batteries lies in their stability and longevity. A single isotope battery can ensure a continuous power supply for ten years.
In contrast, chemical storage batteries can only support two or three days of normal operation or a few hours of intensified combat.
In that case, a hero could maintain long-term activities without a base of operations.
Not to mention lurking for an assassination.
