Usually to run their visor, their cybernetic systems (some of which are critical support systems that keep them alive), if they have synthetic/mechanical limbs they power those too and are the primary source for movement. Some protos have x86 CPUs and high end GPUs in their visor and for them the batteries run those as well.
Batteries are merely vessels for energy, this does not answer the question of its final purpose!
Usually to run their visor, their cybernetic systems (some of which are critical support systems that keep them alive), if they have synthetic/mechanical limbs they power those too and are the primary source for movement. Some protos have x86 CPUs and high end GPUs in their visor and for them the batteries run those as well.