Body clock

A battery (or, as [8th] will doubtless not fail to point out, a galvanic cell) can be as simple as two different metals with an electrolyte between them. You can, for instance, power an electronic clock from a silver teaspoon and a copper coin stuck into an orange. This is not generally done, because it looks silly and, in any case, who can spare a coin these days?

Howevertheless, the general principle holds. Now, sweat is a reasonably good electrolyte, and the whole human body is itself filled with electrolyte.

It should be a trivial matter to devise a shirt with copper and silver wires cunningly woven into it. With suitable connexions, therefore, you could dispense with the orange, teaspoon and coin and simply power your electronic clock from your very own body.