E=mc^2 doesn't really say that matter can be converted to energy, just that energy has mass. For example, a fission/fusion reaction doesn't make matter go away

E=mc^2 doesn't really say that matter can be converted to energy, just that energy has mass. For example, a fission/fusion reaction doesn't make matter go away, but the amount of energy released is large enough to be measured as a loss of mass. When you discharge a battery, it also loses mass, but the amount is too small to notice.