IIRC it started out as computation, but was changed to batteries because they thought cluster computing was too hard to explain in a movie script.

There's the one camp that decries how this flies in the face of basic thermodynamics (human body temperature vs. ambient gives abysmally low Carnot efficiency), and the other camp that claims thermodynamics is just invented by the machines to make stuff work inside the Matrix.

Well, a resting human body generates 4x more heat per volume than the fusion reaction at the core of the Sun.

https://en.m.wikipedia.org/wiki/Nuclear_fusion#Astrophysical...

But why didn't the machines just build geothermal generators? I guess it's not much of a story then?