I like the battery metaphor.  I think the reason why a lot of us reject the metaphor is because we have been thinking of batteries as those metal-coated things we but in flashlights.  Then we think, "Wow, it makes no sense that there are a bunch of batteries (in the metal-coated thing sense)throughout our body."  But, if we think of a battery more abstractly as available stored energy in a cell (cell not in the biological sense but in the electrochemical sense), then it is a viable metaphor.  Similarly, it is difficult to think of our arm as a machine because we typically think of machines as those large metal contraptions in factories.  However, if we look a machine more abstractly and see that it is simply a device that modifies force(i.e. less input force over a greater distance will result in a larger output force over a smaller distance) then our arm as a machine is also a viable metaphor.