not just "batteries"
In "Matrix" discussions, the issue repeatedly comes up that humans are net-endothermic, so they generate less energy than they consume. Using humans as batteries makes no sense, scientifically.
However, I THOUGHT Morpheus referred in passing to the Matrix using human brains for a "limited processing power." Pretty sure I'm wrong though, because I can't find it in the film or screenplay.
The original script DID, however, claim that humans were needed to jumpstart the fusion reactors. That doesn't fix the problem, though, since a running fusion reaction should be able to jumpstart another.
Anyway ....
I only recently recalled a Dean Koontz short story from 1973 called "Wake Up to Thunder" where humans are willingly kept in a Matrix-like virtual reality for a supercomputer (named "Thunder").
The difference is that they know they're in Thunder's "Matrix" and are perfectly content to be there.
The reason I bring it up here is that, in Koontz's story, the humans' brains are being used to augment Thunder's computing power.
Seems like a more sensible use of human bodies than just "batteries."