Energy is stored in a battery in chemical form. In other words, the energy is stored in the chemical bonds between atoms, which involve electrons. A charged battery has many such bonds already formed, while a discharged battery contains the same atoms, but with the bonds almost all broken.
The question, then, really boils down to this one: does a molecule weigh more or less than the sum of its constituent atoms?
A molecule certainly contains more energy than does a collection of its constituent atoms. It is also true that energy and mass are both affected by 'gravitational fields,' as can be seen by the deflection of starlight as it passes near the Sun, or by the redshifting of light as it travels upwards from the floor of a laboratory.
A charged battery, of course, contains more energy than a discharged one -- a trivial statement. A charged battery must necessarily weigh more than an uncharged one. In the same vein, a box of mirrors full of photons will weigh more than a similar box without any photons inside.
However, the difference in weight is incredibly tiny, due the very large value of c
2, the constant that associates quantities of mass with quantities of energy in the famous equation E = mc
2.
To give you an idea, consider my laptop battery, which produces a potential difference of about 12V and can supply about 4 amp-hours of current. This battery contains about 172,800 joules of energy when charged. The equivalent mass of that energy, however, is tiny: only about 2 billionths of a gram:
http://www.google.com/search?hl=en&...+ampere+hours)+/+c^2+in+nanograms&btnG=Search
- Warren