Fun fact, in Matrix (if that's what you're referring to) people were originally enslaved by the machines to provide compute capacity of their brains. The battery part came later, when someone (studio executives?) jumped in, and said that's too smart and people wouldn't get it so it was changed to batteries.
That makes way more sense too. Human brains are pretty complex devices, it’s easy to believe the machines didn’t figure out how to make something comparable and opted for human farming instead. On the other hand humans make very crappy batteries, how does that science even work.
MORPHEUS: For the longest time, I wouldn't believe it. But then I saw the fields with my own eyes, watched them liquefy the dead so they could be fed intravenously to the living -
NEO (politely): Excuse me, please.
MORPHEUS: Yes, Neo?
NEO: I've kept quiet for as long as I could, but I feel a certain need to speak up at this point. The human body is the most inefficient source of energy you could possibly imagine. The efficiency of a power plant at converting thermal energy into electricity decreases as you run the turbines at lower temperatures. If you had any sort of food humans could eat, it would be more efficient to burn it in a furnace than feed it to humans. And now you're telling me that their food is the bodies of the dead, fed to the living? Haven't you ever heard of the laws of thermodynamics?
MORPHEUS: Where did you hear about the laws of thermodynamics, Neo?
NEO: Anyone who's made it past one science class in high school ought to know about the laws of thermodynamics!
MORPHEUS: Where did you go to high school, Neo?
(Pause.)
NEO: ...in the Matrix.
MORPHEUS: The machines tell elegant lies.
(Pause.)
NEO (in a small voice): Could I please have a real physics textbook?
MORPHEUS: There is no such thing, Neo. The universe doesn't run on math.
I figure the machines must be draining some sort of psychic energy from humans. While there's no such thing as psychic energy as far as we know, that's because all knowledge of it has been left out of the matrix.
