Was ancient man intelligent enough to invent batteries?

Answer 1

To have invented batteries, people would have needed a lot of other knowledge and experience and other things already created before they could invent batteries. So it is not really a case of them being intelligent enough, but of having all of those other things in place. They would have had great intelligence, but without the other things, they could not have and did not invent batteries. We are now very intelligent, but before the inventions of the future we will have to have done other things to invent them.

Answer 2

"Ancient man" was as intelligent as "modern man" and could have invented anything we have today.

Considering that, consider also the Bahgdad battery discovered in the 1920's and rediscovered in Germany in 1938. A battery is one theory for what the collection of artifacts could be, although what use it could be put to remains a mystery.