We can look at it as 'control', for the purposes of human bondage to the machine/system. Alternatively we can also view this as a metaphor for enlightenment/gnosis. The matrix acting as the (material world/hell) catalyst for his epiphany. The battery can be viewed from a positive angle (batteries both positive & negative) - hence, why the battery body is 'black', yet the head 'gold' (enlightenment).
Reflected in the dream of biblical Babylonian King Nebuchadnezzar (see Daniel 2), feet of clay/head of gold.
Clay is 'base' (lower realm) and relates to the feet, Gold is 'pure' and relates to the head/enlightenment (higher realm).