First, there are concerns that if these containers are used as batteries, they may be connected to some conductive material, such as wires. But there is no evidence of any wire near the ship. All that is excavated is the container itself. The ships were discovered by local people, not archaeologists.
Second, about we know that the seleucian copper cylinders were used to hold sacred books, not to generate electricity, the professor thinks that the copper cylinders in the container are indeed similar to the cylinders used to hold the scrolls, but that doesn't really prove anything. It's possible that the copper cylinders were originally used to hold the scrolls.
Finally, about what could ancient people have done with the electricity that the vessels were supposed to have generated? They had no devices that replied on electricity. As batteries, the vessels would have been completely useless to them Professors think it can be used to persuade others to agree with them and to heal others.