[kidd]

No.

Mass is not consumed in the electrochemical reaction that takes place in a battery.

The E=mc^2 equation applies to a closed system.

[Moonman62]

What happened to the energy that the battery no longer has once its depleted?

[Cboldt]

Mass is not consumed in the electrochemical reaction that takes place in a battery.

The 890 kJ of energy released by the combustion of one mole of methane thus originates from the conversion of 9.89 ng of mass into energy. Such a small change, about 10 ng out of 80 g cannot be detected by balances. It amounts to the loss of 1.0 x 10^-7% of the mass. So we ignore Einstein's equation when doing regular chemical stoichiometric calculations. However this equation is very useful in nuclear chemistry.

http://www.ucdsb.on.ca/tiss/stretton/CHEM2/nuc02.htm

[WildTurkey]

Mass is not consumed in the electrochemical reaction that takes place in a battery. The E=mc^2 equation applies to a closed system.

You are oh so wrong.